Narrative Review of Sex Differences in Muscle Strength, Endurance, Activation, Size, Fiber Type, and Strength Training Participation Rates, Preferences, Motivations, Injuries, and Neuromuscular Adaptations

Abstract

Nuzzo, JL. Narrative review of sex differences in muscle strength, endurance, activation, size, fiber type, and strength training participation rates, preferences, motivations, injuries, and neuromuscular adaptations. J Strength Cond Res 37(2): 494–536, 2023—Biological sex and its relation with exercise participation and sports performance continue to be discussed. Here, the purpose was to inform such discussions by summarizing the literature on sex differences in numerous strength training–related variables and outcomes—muscle strength and endurance, muscle mass and size, muscle fiber type, muscle twitch forces, and voluntary activation; strength training participation rates, motivations, preferences, and practices; and injuries and changes in muscle size and strength with strength training. Male subjects become notably stronger than female subjects around age 15 years. In adults, sex differences in strength are more pronounced in upper-body than lower-body muscles and in concentric than eccentric contractions. Greater male than female strength is not because of higher voluntary activation but to greater muscle mass and type II fiber areas. Men participate in strength training more frequently than women. Men are motivated more by challenge, competition, social recognition, and a desire to increase muscle size and strength. Men also have greater preference for competitive, high-intensity, and upper-body exercise. Women are motivated more by improved attractiveness, muscle “toning,” and body mass management. Women have greater preference for supervised and lower-body exercise. Intrasexual competition, mate selection, and the drive for muscularity are likely fundamental causes of exercise behaviors in men and women. Men and women increase muscle size and strength after weeks of strength training, but women experience greater relative strength improvements depending on age and muscle group. Men exhibit higher strength training injury rates. No sex difference exists in strength loss and muscle soreness after muscle-damaging exercise.

Introduction

Strength training involves repeated muscle contractions against resistance with the intent of increasing muscle strength. Professional organizations such as the National Strength and Conditioning Association (¹³⁹) and American College of Sports Medicine (^18,19,91) recommend participation in strength training across the lifespan. Calls exist for strength training to be featured more heavily in public health initiatives and chronic disease prevention strategies (^42,435). Thus, with researchers, practitioners, and health officials encouraging greater uptake of strength training, factors that correlate with muscle strength and other strength training–related outcomes should be understood. One of these factors is sex or gender (¹⁴⁰).

Men and women differ on various physical and psychological traits. They differ on body height and mass (¹⁵⁴), dietary habits and preferences (^362,418), vocational interests (^418,448), sociopolitical beliefs (^43,363), personality characteristics (^{13,21,62,76,96,143,279,302,352,405,430,479}), and rates of various mental and physical health conditions (^185,328). Men and women also probably differ on various strength training–related outcomes. Some outcomes have been examined in previous reviews—for example, sex differences in changes in muscle size and strength after weeks of training (^230,387). Nevertheless, other outcomes have not been examined, and updated or more nuanced presentations of previously reviewed outcomes align with calls for more sex-segregated information in exercise and sports science to advance personalized and “precision medicine” (⁴⁰⁴).

Sex differences in muscle strength are well known. However, revisiting this topic is important for a couple of reasons. First, a general update on sex differences in muscle strength would be timely given ongoing discussions about (a) biological male subjects competing in the female category of sport (^{44,191,218,226,227,385}) and (b) sex-specific physical fitness standards within the military (¹²⁴), law enforcement (²⁸¹), and firefighting (⁴¹⁴). Second, the degree to which sex differences in muscle strength might differ by muscle group and muscle contraction type is not obvious. Early research suggested that the magnitude of the sex difference in muscle strength is not the same for eccentric, isometric, and concentric muscle contractions (⁴²⁴). If true, this would mean men and women might perform differently or respond differently to strength training programs with certain types of muscle contractions (e.g., eccentric-only exercise). Such information could then have implications for exercise prescriptions on connective adaptive resistance exercise (CARE) machines, which are new exercise technology with potential to make eccentric-only exercise and accentuated eccentrics more accessible to the general population (³³³).

Sex differences in muscle endurance (i.e., fatigability) have also been discussed in previous reviews (^47,209–211). Such reviews have generally focused on isometric exercise. From a practical standpoint, information on potential sex differences in muscle fatigability during eccentric-concentric contractions would be more instructive, given that eccentric-concentric muscle contractions are the types of repetitions usually performed in strength training programs. If men and women differ in the number of contractions they can complete at equal relative loads, practitioners might then need to consider how this might affect exercise volume and load prescriptions.

Discussions of sex differences in muscle strength and endurance often give rise to an interest in understanding the biological mechanisms that underlie such differences. Muscle strength is determined by muscle architecture and neural drive to the muscle (i.e., voluntary activation) (³³⁴). Sex differences in muscle mass and size are well known, but whether a sex difference in voluntary activation exists is unclear. Moreover, the degree to which sex differences in muscle size might differ by muscle group is not obvious and in need of clearer presentation. Muscle fiber type differences might also exist between men and women and could help to explain sex differences in muscle endurance. Nevertheless, few attempts have been made to collate or review the literature on sex differences in muscle fiber type (²¹⁰) or consider whether such differences might depend on muscle group.

Various psychological and sociological aspects of the strength training experience also likely to differ between men and women, given sex differences in interests, preferences, and personality traits. Motivation for exercise is one these aspects. A clear and comprehensive review of the factors that drive men and women to exercise could be useful, for example, in designing strength training programs aimed at maximizing personal enjoyment and adherence. Preferences for exercise program variables also warrant consideration for the same reason. Exercise programs can be performed in different environments, with varying levels of interpersonal contact, and with or without competition or supervision. Exercise programs can also differ by equipment used, muscles targeted, and “intensity” of exercise. Such preferences and practices warrant consideration when delivering exercise programs. However, reviews on sex differences in preferences and practices related to strength training programs seem to be absent from the literature.

Recently, over 4 decades of research on potential sex differences in changes in muscle size and strength after weeks of strength training were reviewed (^230,387). Findings from these reviews are summarized in the current article and provide practitioners with insight into whether men and women can be expected to respond similarly to weeks of strength training. Moreover, although strength training is generally beneficial for health and function, injuries occur during strength training. Currently, it is not obvious whether men and women experience the same rates and types of such injuries. Knowledge of such information could be useful in tailoring educational content to men and women about proper exercise technique and safe use of exercise equipment. Finally, muscle damage, which is characterized by force loss, muscle swelling, and loss of joint range of motion after exercise, is 1 potential side effect of eccentric contractions during strength training. Whether men and women experience similar levels of muscle damage after eccentric exercise warrants consideration, given the potential for CARE machines to increase participation in eccentric-only and accentuated eccentric exercise (³³³).

In summary, men and women differ on various physiological and psychological characteristics. Some of these differences are well known, whereas others are not fully realized. The purpose of the current review was to collate findings on sex differences on a range of strength training variables and outcomes into 1 source. Variables and outcomes included muscle strength, muscle endurance (repetitions to failure), voluntary activation, muscle twitch forces, muscle mass, muscle size, muscle fiber type, strength training participation rates, preferences for strength training program variables (i.e., location, interpersonal contact, supervision, competition, type of equipment, muscles exercised, and “intensity”), changes in muscle size and strength after weeks of training, and injuries and muscle soreness from training.

The literature review was thorough but not necessarily exhaustive. For example, in the tables and figures, results from tests of isometric and isoinertial muscle strength are summarized, but results from isokinetic machines and hand-held dynamometry are not. In addition, not all possible differences in male and female neuromuscular anatomy and physiology have been discussed. Finally, in the current article, sex differences in muscle strength and other outcomes have been expressed as the mean female value as a percent of the mean male value. Other researchers have used this metric as a way to communicate sex differences in strength training–related outcomes (^{49,269,313,359,424}). Percent differences can be computed by the reader using mean values presented in Supplemental Digital Content, https://links.lww.com/JSCR/A339.

Muscle Strength

Development and Aging

Quetelet, in his 1842 Treatise on Man and the Development of His Faculties (³⁶⁷), reported on grip strength and lumbar extension strength in male and female subjects across the lifespan (6–60 years old). The study by Quetelet might have been the first attempt to compare muscle strength in male and female subjects and explore the effects of development and aging on strength. Quetelet observed (a) male subjects have higher grip and lumbar extensor strength than female subjects at all ages, (b) strength is markedly greater in male subjects than female subjects at age 14 years (e.g., female grip strength was 64% of male grip strength at age 14 years), and (c) peak strength occurs at age 25–30 years in both sexes and decreases progressively thereafter. In 1899, Carman (⁶⁶) reported higher grip strength in male subjects than female subjects at most ages between the ages of 10 and 18 years, with female grip strength 55% of male grip strength by age 18 years (⁶⁶). In 1941, Methany (³¹¹) presented the results from 11 studies that included measurements of grip strength in male and female subjects aged 4–18 years. The results illustrated that male subjects are stronger than female subjects at most ages, with the difference becoming more pronounced with age (³¹¹).

Since these early works, numerous researchers have examined grip strength in children and adolescents (^{61,66,70,107,146,150,173,187,235,294,306,311,317,319,322,349,355,367,466,476,492}) (Table 1) and adults (^{28,55,75,77,85,115,116,144,150,156,181,194,275,277,291,293,319,344,346,348,349,370,383,408,417,447,451,476,483,486,492,502}) (Figure 1; Table 1, Supplemental Digital Content, https://links.lww.com/JSCR/A339). The results have generally confirmed earlier findings (^66,311,367). At 10 years old, female grip strength is usually 95% of male grip strength. This prepubertal sex difference in grip strength is not statistically significant, but it is present in most studies, with earlier studies reporting larger differences than studies published since 2009.

Table 1 - Differences in grip strength between boys and girls during development.^*

Reference	Year	Country	M (n)	F (n)	Age (y)	Age 10 years, F% of M strength†	Age 15 years, F% of M strength†	Age when F % of M strength ≤75%	M > F mean at most ages?
Quetelet (367)	1842	BEL	∼300	∼300	6–60	57%	54%	12	Yes
Carman (66)	1899	USA	756	751	10–18	69%	70%	15	Yes
Methany (311)	1941	Review	∼36,000	∼36,000	3–18	M > F	M > F	16	Yes
Finlayson and Reitan (146)	1976	NR	60	60	6–14	n/a	n/a	None	Yes
Montoye and Lamphiear (319)	1977	USA	3,163	3,345	10–69	84%	60%	14	Yes
Newman et al. (322)	1984	AUS	736	678	5–18	∼85%	∼74%	15	Yes
Mathiowetz et al. (294)	1986	USA	231	240	6–19	93%	75%	14–15	Yes
Katzmarzyk et al. (235)	1997	USA	377	363	7–12	∼85%	n/a	None	Yes
De Smet and Vercammen (107)	2001	BEL	268	219	5–15	81%	76%	None	Yes
Fortier et al.(150)	2001	CAN	951	958	11–69	n/a	65%	15–16	Yes
Häger-Ross and Rösblad (173)	2002	SWE	267	263	4–16	72%	70%	15	Yes
Butterfield et al. (61)	2009	USA	383	353	5–19	96%	70%	14	Yes
Molenaar et al. (317)	2009	NLD	100	115	4–12	95%	n/a	n/a	Yes
Tremblay et al. (466)	2010	CAN	889	870	8–19	89%	n/a	15–19	Yes
Catley and Tomkinson (70)	2013	AUS	NR	NR	9–15	90%	74%	15	Yes
Ploegmakers et al. (355)	2013	NLD	1,112	1,129	4–14	97%	n/a	None	Yes
Hepping et al. (187)	2015	NLD	1,130	1,154	4–14	98%	n/a	None	Yes
McQuiddy et al. (306)	2015	USA	741	767	6–19	96%	81%	16	Yes
Perna et al. (349)	2016	USA	3,175	3,400	6–70	95%	71%	15	Yes
Wang et al. (476)	2018	USA	6,783	6,893	6–80	97%	72%	15	No
Wong (492)	2016	CAN	5,438	5,670	6–79	95%	74%	15	Yes

^*AUS = Australia; BEL = Belgium; CAN = Canada; F = female; M = male; NLD = the Netherlands; NR = not reported; SWE = Sweden; USA = United States of America.

^†Computed by dividing female group mean by the male group mean then multiplying by 100.

Figure 1.: Adult sex differences in grip strength. Data are presented as the mean female strength value as a percent of the mean male strength value. See Supplemental Digital Content (Table 1, https://links.lww.com/JSCR/A339) for mean values and details on study samples.

Around 15 years old, a marked increase in grip strength occurs in male subjects. At this age, female grip strength is ∼75% of male grip strength. The results from longitudinal studies confirm that the ages of 14 and 15 years are when marked sex differences in grip strength arise. Neu et al. (³²¹) assessed grip strength annually in 25 male subjects and 25 female subjects from ages 6 to 23 years. By age 14 years, female grip strength was 78% of male grip strength, and by age 18 years, it was 62% (³²¹). Janz et al. (²²¹) assessed grip strength in 63 boys and 62 girls at age 10.5 years and then annually for 5 years. At age 10.5 years, female grip strength was 79% of male grip strength. At age 14.5 years, female grip strength was 76% of male grip strength.

Peak grip strength occurs in both sexes between age 30 and 39 years (^349,476). However, the magnitude of the strength increase from puberty to the fourth decade of life is higher in male subjects than female subjects. In 1 study of 11,000 residents of Canada, female grip strength increased by 11% from ages 12–19 to 20–39 years (25.6–28.4 kg), whereas male grip strength increased by 32% (36.7–48.5 kg) (⁴⁹²). In another study, grip strength for female subjects increased by 9.5% from ages 16–19 to 30–39 years (124.7–136.5 lb), whereas male grip strength increased by 15.3% (187.7–216.4 lb) (³⁴⁹). In another study, female grip strength increased by 7% from age 18 to 30–34 years (29.5–31.6 kg), whereas male grip strength increased by 11.7% (45–50.3 kg) (⁴⁷⁶). At age 30–39 years, female grip strength is ∼60% of male grip strength. This magnitude of sex difference is also observed in adults aged 60–69 years (Figure 1).

Muscle Group

Numerous articles have included sex-segregated data for muscle groups other than those used for hand gripping ^{(5,10,12,14–16,20,22,26–29,31,33–35,45,48,49,53,55,63,64,68,71,75,77,80,81,82,85,93–95,100,101,109–112,114–116,126,129,142,144,150,156,160,164,166,168,172,174–176,178,180–182,190,192,194,195,199,202,207,208,212–214,219,224,231,236,238–240,246,247,250,252,253,256–259,261,262,267–269,273–277,286,289,291,293,295–301,305,310,313–315,318,319,323,325,326,335,338–340,342–344,346–349,351,353,354,365,367,369,370,376,377,379–384,386,390,392,394,396,397,400,401,408,410,411,413–415,417,424,425,427–429,442,443,445,447,449–451,453–455,457,460,462–464,468,470,474–476,481,483,485,486,488–490,492,494–496,498,499,502}).

A consistent result from this literature is that men are significantly stronger than women on all tests of muscle strength, but the difference is greater for tests of upper-body than lower-body muscle strength. In 1976, Laubach (²⁶⁹) was perhaps the first to review sex differences in upper-body and lower-body muscle strength. With data available from 9 studies, Laubach (²⁶⁹) computed female upper-body strength to be 55.8% of male upper-body strength, female lower-body strength to be 71.9% of male lower-body strength, and female trunk strength to be 63.8% of male trunk strength (²⁶⁹). The findings by Laubach (²⁶⁹) are fairly consistent with 45 years of subsequent research, which has shown that female upper-body strength is roughly 50–60% of male upper-body strength; female lower-body strength is roughly 60–70% of male lower-body strength; and female trunk strength is roughly 60% of male trunk strength (Figure 2).

Figure 2.: Summary of adult sex differences in muscle strength for various muscle groups. The values, which are presented as the female strength value as a percent of the male strength value, are based on the results presented in the figures and Supplemental Digital Content (see tables, https://links.lww.com/JSCR/A339) of the current review.

Before discussing various muscle groups and strength tests in more detail, the study by Feeler et al. (¹⁴²) warrants attention. Published in the ergonomics and manual handling literature, this study might be unfamiliar to exercise scientists. This study included a sample of 107,755 men and 23,078 women and thus should be weighed heavily when discussing the magnitude of sex differences in muscle strength. Feeler et al. (¹⁴²) measured strength during the isometric biceps curl, isometric back extension, and isometric back and leg extension (i.e., isometric deadlift or midthigh pull) (¹⁴²). For the isometric biceps curl, female strength was 55% of male strength (¹⁴²). For the isometric back extension, female strength was 56% of male strength (¹⁴²). For the isometric deadlift, female strength was 53% of male strength (¹⁴²). The results, particularly for upper-body strength, are generally similar to those computed by Laubach (²⁶⁹) which indicates consistency in adult sex differences in muscle strength over time.

Other studies have included measures of muscle strength in men and women in smaller samples. Figure 3 and Supplemental Digital Content (see Table 2, https://links.lww.com/JSCR/A339) present data on sex differences in upper-body muscle strength measured in multijoint movements. For the bench and chest press, female strength is usually 40–55% of male strength. For the shoulder press (overhead press), female strength is roughly 50–55% of male strength. For the lat pulldown, row, and upright row, the results have been mixed, but female strength is usually 50–60% of male strength.

Figure 3.: Adult sex differences in upper-body muscle strength measured in multijoint exercises: Bench or chest press (A), shoulder press (B), lat pulldown (C), row (D), and upright row (E). Data are presented as the mean female strength value as a percent of the mean male strength value. See Table 2 for bench press data in male and female powerlifters. See Supplemental Digital Content (Table 2, https://links.lww.com/JSCR/A339) for mean values and details on study samples. 1RM = 1 repetition maximum.

For the elbow flexors and extensors, female strength is roughly 55 and 50% of male strength, respectively (Figure 4, Tables 3 and 4, Supplemental Digital Content, https://links.lww.com/JSCR/A339). For the wrist flexors and extensors, female strength is roughly 60 and 50% of male strength, respectively (Figure 4 and Table 5, Supplemental Digital Content, https://links.lww.com/JSCR/A339). For muscles about the shoulder, female strength is roughly 50–55% of male strength (Figure 5, Table 6, Supplemental Digital Content, https://links.lww.com/JSCR/A339).

Figure 4.: Adult sex differences in strength of the elbow flexors (A), elbow extensors (B), wrist flexors (C), and wrist extensors (D). Data are presented as the mean female strength value as a percent of the mean male strength value. See Supplemental Digital Content (Tables 3–5, https://links.lww.com/JSCR/A339) for mean values and details on study samples. 1RM = 1 repetition maximum.

Figure 5.: Adult sex differences in strength of the shoulder flexors (A), shoulder extensors (B), shoulder abductors (C), shoulder adductors (D), shoulder internal rotators (E), and shoulder external rotators (F). Data are presented as the mean female strength value as a percent of the mean male strength value. See Supplemental Digital Content (Table 6, https://links.lww.com/JSCR/A339) for mean values and details on study samples.

Figure 6 and Supplemental Digital Content (see Table 7, https://links.lww.com/JSCR/A339) present the results from multijoint tests of lower-body muscle strength in men and women. For the leg press, female strength is roughly 65% of male strength. For the back squat 1RM, female strength is roughly 60% of male strength. Few studies have included comparisons of deadlift 1RMs between men and women who are not competitive powerlifters, but multiple articles have presented sex-segregated data on an isometric variant of the deadlift—the back and leg dynamometer test. On this test, female strength is 53–58% of male strength.

Figure 6.: Adult sex differences in lower-body muscle strength measured in multijoint exercises: Leg press (A), squat (B), and deadlift (C). Data are presented as the mean female strength value as a percent of the mean male strength value. See Table 2 for squat and deadlift data in male and female powerlifters. See Supplemental Digital Content (Table 7, https://links.lww.com/JSCR/A339) for mean values and details on study samples. 1RM = 1 repetition maximum.

For the knee extensors and flexors, female strength is roughly 65 and 60% of male strength (Figure 7, Tables 8 and 9, Supplemental Digital Content, https://links.lww.com/JSCR/A339). However, the sex difference in strength of the knee flexors is greater during 1RM than isometric assessments. For muscles about the hip, female strength is roughly 60% of male strength (Figure 8, Table 10, Supplemental Digital Content, https://links.lww.com/JSCR/A339). For the ankle plantarflexors and dorsiflexors, female strength is roughly 70 and 65% of male strength, respectively (Figure 9, Tables 11 and 12, Supplemental Digital Content, https://links.lww.com/JSCR/A339).

Figure 7.: Adult sex differences in muscle strength of the knee extensors (A) and knee flexors (B). Data are presented as the mean female strength value as a percent of the mean male strength value. See Supplemental Digital Content (Tables 8 and 9, https://links.lww.com/JSCR/A339) for mean values and details on study samples.

Figure 8.: Adult sex differences in strength of the hip extensors (A), hip flexors (B), hip abductors (C), hip adductors (D), hip internal rotators (E), and hip external rotators (F). Data are presented as the mean female strength value as a percent of the mean male strength value. See Supplemental Digital Content (Table 10, https://links.lww.com/JSCR/A339) for mean values and details on study samples.

Figure 9.: Adult sex differences in strength of the ankle plantarflexors (A) and dorsiflexors (B). Data are presented as the mean female strength value as a percent of the mean male strength value. See Supplemental Digital Content (Tables 11 and 12, https://links.lww.com/JSCR/A339) for mean values and details on study samples.

Figure 10 and Supplemental Digital Content (see Tables 13 and 14, https://links.lww.com/JSCR/A339) present the results from tests of neck and trunk muscle strength in men and women. For the neck extensors and flexors, female strength is roughly 70% of male strength. For the trunk extensors, female strength is roughly 55–60% of male strength. For the trunk flexors, female strength is roughly 60% of male strength.

Figure 10.: Adult sex differences in strength of the neck extensors (A), neck flexors (B), trunk extensors (C), and trunk flexors (D). Data are presented as the mean female strength value as a percent of the mean male strength value. See Supplemental Digital Content (Tables 13 and 14, https://links.lww.com/JSCR/A339) for mean values and details on study samples.

The above results illustrate greater sex differences in muscle strength of upper-body than lower-body muscles. Strength tests performed in competitive powerlifting events—the 1RM bench press, back squat, and deadlift—confirm this finding (^{27,267,268,443}) (Table 2). Because thousands of men and women compete in powerlifting events and data are often publicly available, such results provide a unique opportunity to explore sex differences in upper-body and lower-body strength. However, comparisons between men and women of similar weight categories are inappropriate because men in the general population weigh more than women in the general population (¹⁵⁴). The 63 kg female and 74 kg male body mass categories in powerlifting permit an appropriate comparison between the sexes because 63 kg is roughly the 25th percentile for female body mass in the United States of America (USA) and 74 kg is roughly the 25th percentile for male body mass (i.e., similar sex-relative body masses) (¹⁵⁴). When mean 1RMs from male and female powerlifters from these 2 body mass categories are compared, the findings are generally similar to those found when men and women from the general population are compared on the same or similar tests. Female powerlifter bench press, back squat, and deadlift 1RMs are equal to 46–55%, 56–61%, and 58–64% of male powerlifter 1RMs, respectively. Moreover, in some (²⁶⁸) but not all (²⁷) powerlifting data sets, men in the lightest body mass categories are stronger than women in the heaviest body mass categories. In addition, Table 2 presents the results from a more random sample of 51 male and 53 female powerlifters who reported their best 1RMs as part of a study on injuries among powerlifters (⁴⁴³). The mean body masses of the male and female subjects were 91 and 70 kg, respectively (⁴⁴³). Female strength as a percent of male strength was 46, 56, and 61% for the bench press, squat, and deadlift 1RMs, respectively (⁴⁴³).

Table 2 - Sex difference in 1RM from competitive powerlifters in the 74 kg (male) and 63 kg (female) body mass categories and from a random sample of powerlifters, who lifted without assistive equipment (i.e., “raw”).^*†

Test Reference	Year	Country	M (n)	F (n)	M body mass (kg)	F body mass (kg)	M mean strength (kg)	F mean strength (kg)	F% of M strength‡ (%)
Bench press—1RM
Stromback et al. (443)	2018	SWE	51	53	∼91	∼70	140.0	64.0	46
Latella et al. (268)	2019	AUS (N)	NR	NR	74	63	135.0	63.3	47
Ball and Weidman (27)	2018	USA	2,501	2,505	74	63	108.0	55.0	51
Latella et al. (267)	2018	AUS	NR	NR	74	63	113.9	59.4	52
Latella et al. (268)	2019	AUS (L)	NR	NR	74	63	111.3	58.3	52
Latella et al. (268)	2019	INT	NR	NR	74	63	129.3	70.9	55
Squat—1RM
Stromback et al. (443)	2018	SWE	51	53	∼91	∼70	196.5	111.0	56
Latella et al. (268)	2019	AUS (N)	NR	NR	74	63	206.7	116.5	56
Latella et al. (267)	2018	AUS	NR	NR	74	63	178.7	105.4	59
Latella et al. (268)	2019	AUS (L)	NR	NR	74	63	172.7	104.0	60
Ball and Weidman (27)	2018	USA	2,501	2,505	74	63	159.0	95.0	60
Latella et al. (268)	2019	INT	NR	NR	74	63	200.2	123.0	61
Deadlift—1RM
Latella et al. (268)	2019	AUS (N)	NR	NR	74	63	238.8	139.0	58
Stromback et al. (443)	2018	SWE	51	53	∼91	∼70	229.0	138.8	61
Latella et al. (267)	2018	AUS	NR	NR	74	63	209.4	129.4	62
Latella et al. (268)	2019	AUS (L)	NR	NR	74	63	206.6	129.0	62
Latella et al. (268)	2019	INT	NR	NR	74	63	230.0	146.1	64
Ball and Weidman (27)	2018	USA	2,501	2,505	74	63	193.0	123.0	64

^*1RM = 1 repetition maximum; AUS = Australia; INT = international competitions; L = local competitions; F = female; M = male; N = national competitions; NR = not reported; SWE = Sweden; USA = United States of America.

^†The 74 (male) and 63 kg (female) body mass categories were selected because they represent the sex-specific 25th percentile for body mass in the USA and thus allow for comparison of male subjects and female subjects of similar relative body masses (¹⁵⁴).

^‡Computed by dividing female group mean by male group mean then multiplying by 100.

Finally, men also have greater strength-to-body mass ratios than women for most exercises. This finding has been observed in single-joint tests of upper-body and lower-body strength in the general population (^92,489), as well as in the bench press (^299,462), while the leg press exhibits the smallest sex difference in the strength-to-body mass ratio (^314,489). Greater strength-to-body mass ratios have also been observed in male than female powerlifters (^27,267). Strength-to-body mass ratios in male vs. female powerlifters are approximately 1.5 vs. 1.0 for the bench press, 2.1 vs. 1.6 for the squat, and 2.5 vs. 2.0 for the deadlift (^27,267). The above results suggest that body mass is not the sole contributor to sex differences in muscle strength.

Muscle Contraction Type and Muscle Length

The sex difference in muscle strength is affected by muscle contraction type and muscle length. In 1968, Singh and Karpovich (⁴²⁴) reported female elbow flexor concentric strength was 37% of male concentric strength, whereas female eccentric and isometric strengths were both 43% of male strength. However, no effect of contraction type on the sex difference in muscle strength of the elbow extensors was observed, with female subjects exhibiting ∼47% of male elbow extensor strength for all contraction types.

Researchers have also used isokinetic dynamometry to examine sex differences in muscle strength by contraction type (^{92,171,286,359}). In 1989, Colliander and Tesch (⁹²) reported female concentric knee extensor and flexor strength were 62 and 63% of male strength, respectively, whereas eccentric knee extensor and flexor strength were 74 and 68% of male strength, respectively. Thus, eccentric:concentric torque ratios were greater in women than men for the knee extensors (1.35–2.01 vs. 1.26–1.59) and flexors (1.10–1.50 vs. 1.09–1.31). In the 1990s, Porter et al. (³⁵⁹), who studied the knee extensors, and Griffin et al. (¹⁷¹), who studied the knee extensors, knee flexors, and elbow flexors, reported that female eccentric:concentric torque ratios were greater than male eccentric:concentric torque ratios for all muscle groups, again indicating that the sex difference in muscle strength is greater in concentric than eccentric contractions. Marcell et al. (²⁸⁶) also showed that the sex difference in muscle strength is contraction type-specific, as concentric and eccentric strength of the knee extensors in women were 57 and 73% of that in men, respectively.

Studies of sex differences in eccentric and concentric 1RMs have been unocommon. In 1 study, Hollander et al. (¹⁹⁵) measured eccentric and concentric 1RMs for numerous upper-body and lower-body exercises in 10 men and 10 women. Upper-body exercises included bench press, seated military press, and lat pulldown. Lower-body exercises included leg press, leg extension, and leg curl. Sex differences in muscle strength were typically less in the eccentric than concentric 1RMs. Thus, the eccentric:concentric strength ratios were higher in women than men. The lat pulldown was the only exercise in which the ratio was not higher in women than men.

The magnitude of the sex difference in muscle strength is often greater in concentric strength tests (i.e., 1RMs) than isometric strength tests. This has been observed in the elbow flexors (Figure 4), elbow extensors (Figure 4), knee extensors (Figure 7), and knee flexors (Figure 7). However, during isometric contractions, the magnitude of the sex difference in muscle strength depends on the length of the muscle during the contraction. Sex differences in isometric strength are greatest when assessments occur at longer muscle lengths (^{111,257,286,336,424,491}). This has been observed in the elbow flexors (^336,424,491), knee extensors (²⁸⁶), knee flexors (²⁸⁶), and trunk flexors (^111,257). In some cases, test angle substantially impacts sex differences in isometric strength. In isometric tests of the trunk flexors, female strength is nearly equal to male strength at 20° of trunk flexion but it is approximately 60% of male strength at −40° (¹¹¹).

Muscle Endurance

Women are less fatigable than men in tests of isometric muscle endurance (^209–211). However, this sex difference is less obvious in nonisometric contractions and might depend on factors such as the muscle group assessed (^209–211). In the current review, the results from 6 studies on the number of repetitions men and women completed with equal relative loads are presented (^{148,193,295,299,347,445}) (Figure 11 and Table 15, Supplemental Digital Content, https://links.lww.com/JSCR/A339). The results are mixed, and consistent evidence of a sex difference in muscle fatigability as measured by repetitions-to-failure at relative loads is lacking.

Figure 11.: Maximum number of repetitions completed by men and women at equal relative loads for various exercises. Values exceeding 50 repetitions have been truncated. See Supplemental Digital Content (Table 15, https://links.lww.com/JSCR/A339) for mean values and details on study samples.

For the bench press, no consistent sex differences exist in the number of repetitions performed with loads between 60 and 90% 1RM. For the lat pulldown, data from 1 study suggest that women perform more repetitions than men at most loads between 40 and 80% 1RM. For the biceps curl, the results have varied. Hoeger et al. (¹⁹³) reported men performed approximately 2–5 more repetitions than women with a load equal to 60% 1RM, whereas other researchers have reported women perform approximately 13–16 more repetitions than men with a load equal to 60% 1RM (^295,313). For the leg press, Hoeger et al. (¹⁹³) reported women performed more repetitions than men with loads equal to 40 and 60% 1RM, whereas Peiffer et al. (³⁴⁷) reported men performed more repetitions than women with loads equal to 70% 1RM. For the knee extension exercise, either no sex difference exists or men are able to perform more repetitions than women. Finally, for the trunk extensors, Stuart et al. (⁴⁴⁵) found that when repetition duration was controlled, women performed muscle shortening and lengthening contractions for longer durations than men when exercising with loads equal to 50 and 80% of maximum torque. The sex difference in fatigability was more pronounced at the 50% load condition.

How Are Men Stronger Than Women? Anatomy and Physiology

Development

Various factors have been proposed to explain how male subjects become stronger than female subjects during and after puberty. These factors include sex differences in body height, body mass, bone length, moment arm length, testosterone levels, muscle mass amount, voluntary activation of agonist muscles, coactivation of antagonist muscles, participation in physical activities in childhood, and “additional factors” (^{56,188,229,321,345,391}). In the current review, the focus is on voluntary activation, muscle mass and size, and muscle fiber type. The role of testosterone will not be discussed other than to mention that before puberty, circulating testosterone is equal between male and female subjects (¹⁷⁹). After puberty, healthy men exhibit circulating testosterone levels 15 times greater than levels exhibited by healthy women (¹⁷⁹). This difference in circulating testosterone causes sex differences in muscle size, bone size, and circulating hemoglobin and is believed to explain “most, if not all, the sex differences in sporting performance” (¹⁷⁹). Reviews on testosterone and its impact on development and performance (¹⁷⁹) as well as the influence of resistance training on testosterone (^196,484) can be found elsewhere.

Voluntary Activation

Voluntary activation is the ability of the nervous system to drive the muscle to create its maximal force (³³⁴). Voluntary activation is assessed by delivering an external pulse to the motor nerves during a maximal voluntary contraction (MVC) and also at rest immediately after the MVC (i.e., potentiated resting twitch). If, during the MVC, the external pulse creates a twitch force, this means the individual was not driving the muscle maximally (i.e., not all motoneurons were recruited or motoneurons were discharging at subtetanic rates) (³³⁴). Sizes of evoked twitch forces during the MVC and at rest are then used to compute voluntary activation. Voluntary activation equals 100% when no superimposed twitch occurs during the MVC (³³⁴).

Sex-segregated data on voluntary activation have been reported in at least 11 articles (^{212,236,239,289,305,318,339,369,400,495,498}) (Figure 12, Table 16, Supplemental Digital Content, https://links.lww.com/JSCR/A339). No evidence exists for a sex difference in voluntary activation. For both sexes, voluntary activation during isometric contractions of the elbow flexors, knee extensors, and ankle plantarflexor is roughly 95, 90, and 90%, respectively. In children, O'Brien et al. (³³⁹) discovered boys had higher voluntary activation of the knee extensors than girls, but girls had greater voluntary muscle strength.

Figure 12.: Voluntary activation of upper-limb and lower-limb muscles in men and women. Data are presented as the mean female voluntary activation value as a percent of the mean male voluntary activation value. See Supplemental Digital Content (Table 16, https://links.lww.com/JSCR/A339) for mean values and details on study samples.

Muscle Twitch Forces

The lack of sex difference in voluntary activation suggests sex differences in muscle strength are due primarily to characteristics of muscle, such as muscle mass, size, and fiber type. One way to explore this possibility is to examine sex differences in muscle forces when volitional drive is absent—that is, through muscle twitch forces evoked from electrical stimulation of peripheral nerves. The amplitudes of twitch forces will be larger when more muscles fibers or more type II muscle fibers are stimulated (^487,501).

Sex-segregated data of muscle twitch forces have been reported in at least 11 articles. Muscle twitch forces are greater in men than women (^{31,32,178,236,240,313,336,394,400,470,499}) (Figure 13, Table 17, Supplemental Digital Content, https://links.lww.com/JSCR/A339). However, the magnitude of the sex difference is not the same for all muscle groups. For the elbow flexors, ankle dorsiflexors, and ankle plantarflexors, evoked twitch forces in women are approximately 40–50, 65, and 80% of evoked twitch forces in men, respectively. These muscle-specific sex differences in evoked twitch forces are comparable with the muscle-specific sex differences observed for volitional muscle strength.

Figure 13.: Adult sex differences in muscle twitch forces of the elbow flexors (A), ankle plantarflexors (B), and ankle dorsiflexors (C) from peripheral nerve stimulation. Data are presented as the mean female twitch force value as a percent of the mean male twitch force value. See Supplemental Digital Content (Table 17, https://links.lww.com/JSCR/A339) for mean values and details on study samples.

Muscle Mass

Many researchers have used imaging techniques to measure muscle mass in men and women (^{6,90,112,158,216,220,223,237,245,248,255,260,263,264,270–274,278,341,347,365,412,419,420,428,471,474,497}). Figure 14 and Supplemental Digital Content (see Table 18, https://links.lww.com/JSCR/A339) present the results of total lean body mass (LBM), total fat-free mass (FFM), total skeletal muscle mass (SMM), and total appendicular SMM. Figure 15 and Supplemental Digital Content (see Table 19, https://links.lww.com/JSCR/A339) present the results of upper-limb lean mass (LM) and SMM. Figure 16 and Supplemental Digital Content (see Table 20, https://links.lww.com/JSCR/A339) present the results of lower-limb LM and SMM.

Figure 14.: Adult sex differences in total LBM (A), FFM (B), SMM (C), and appendicular SMM (D). Data are presented as the mean female value as a percent of the mean male value. See Supplemental Digital Content (Table 18, https://links.lww.com/JSCR/A339) for mean values and details on study samples. ASMM = appendicular skeletal muscle mass; DEXA = dual-energy x-ray absorptiometry; FFM = fat-free mass; LBM = body lean body mass; MRI = magnetic resonance imaging; SMM = skeletal muscle mass.

Figure 15.: Adult sex differences in lean mass or skeletal mass of the upper limbs (A) and lower limbs (B). Data are presented as the mean female value as a percent of the mean male value. See Supplemental Digital Content (Tables 19 and 20, https://links.lww.com/JSCR/A339) for mean values and details on study samples and the outcome measured. DEXA = dual-energy x-ray absorptiometry; MRI = magnetic resonance imaging.

Figure 16.: Adult sex differences in upper-body muscle size. Data for pectoralis major (A), biceps brachii (B), biceps brachii plus other elbow flexors of the upper arm (C), triceps brachii (D), and forearm (E) are presented as the mean female value as a percent of the mean male value. See Supplemental Digital Content (Table 21, https://links.lww.com/JSCR/A339) for mean values and details on study samples. CSA = cross-sectional area; CT = computed tomography; MRI = magnetic resonance imaging; US = ultrasonography.

The results reveal that men have more muscle mass than women—both in absolute terms and as a proportion of total body mass (^{49,50,157,198,220,339,403}). Moreover, the sex difference in muscle mass as a proportion of total body mass is greater in the upper body than lower body (^{50,157,220,313,403}). For example, Janssen et al. (²²⁰) reported men had an average of 33 kg of muscle mass, which was 38% of their average body mass, whereas women had an average of 21 kg of muscle mass, which was 31% of their average body mass. Female upper-body muscle mass was 60% of male upper-body mass, whereas female lower-body muscle mass was 67% of male lower-body muscle mass (²²⁰). A summary of the reviewed data on sex differences in muscle mass is as follows: (a) female total LBM and FFM is roughly 70% of male LBM and FFM; (b) female total body SMM and appendicular SMM is roughly 65% of male SMM and appendicular SMM; (c) female upper-limb LM and SMM is roughly 55–60% of male upper-limb LM and SMM; and (d) female lower-limb LM and SMM is roughly 68–72% of male LM and SMM.

Muscle Size

Researchers have also used imaging techniques to measure sizes of muscles of the upper limbs (Figure 16 and Table 21, Supplemental Digital Content, https://links.lww.com/JSCR/A339), lower limbs (Figure 17 and Table 22, Supplemental Digital Content, https://links.lww.com/JSCR/A339), and trunk (Figure 18 and Table 23, Supplemental Digital Content, https://links.lww.com/JSCR/A339) in men and women (^{2–5,7–10,15,17,24,26,30,51,79,99,153,155,176,189,202,217,219,232,234,254,273,284,296,308,309,313,325,335,337,339,368,369,390,395,399,403,409,428,432,437,438,452,455,456,465,467,470,474,478,480,481,494,495,497}). Sex differences are obvious in all muscle groups but are larger for upper-body than lower-body muscles. The magnitude of the sex difference also depends on the method used to take the measurement. Sex differences are largest when volume and cross-sectional area (CSA) are measured and smallest when thicknesses are measured. This likely occurs because thicknesses do not account for multiple spatial dimensions of muscle.

Figure 17.: Adult sex differences in lower-body muscle size. Data for all quadriceps muscles combined (A), VL (B), VM (C), RF plus VI (D), multiple or all combined hamstrings muscles (E), tibialis anterior (F), medial gastrocnemius (G), and multiple or all combined triceps surae muscles (H) are presented as the mean female value as a percent of the mean male value. See Supplemental Digital Content (Table 22, https://links.lww.com/JSCR/A339) for mean values and details on study samples. CSA = cross-sectional area; CT = computed tomography; MRI = magnetic resonance imaging; RF = rectus femoris; US = ultrasonography; VI = vastus intermedius; VL = vastus lateralis; VM = vastus medialis.

Figure 18.: Adult sex differences in trunk muscle size. Data for rectus abdominis (A), combined lateral abdominal wall muscles (B), and multifidus (C) are presented as the mean female value as a percent of the mean male value. See Supplemental Digital Content (Table 23, https://links.lww.com/JSCR/A339) for mean values and details on study samples. CSA = cross-sectional area; MRI = magnetic resonance imaging; US = ultrasonography.

For biceps brachii and other muscles of the anterior upper limb, CSA in women is approximately 50–60% of CSA in men, whereas thicknesses of these muscles in women are approximately 69–79% of thicknesses in men. For triceps brachii, CSA in women is approximately 61–63% of CSA in men, whereas thicknesses of triceps brachii in women are approximately 70–80% of thicknesses in men. For muscles of the forearm, female muscle thicknesses are approximately 75–80% of muscle thicknesses in men.

For the quadriceps muscle group, female muscle volume is approximately 65% of male muscle volume and female CSA is approximately 70% of male CSA. When the vastus lateralis (VL) and medialis are considered separately, female CSA of these muscles is usually 70–80% of male CSA. Thicknesses of VL in women are usually 83–93% of VL thicknesses in men. Combined rectus femoris and vastus intermedius thicknesses in women are usually 80–95% of thicknesses in men. For the whole hamstrings muscle group, female muscle volume and CSA are usually 70–73% of male muscle volume and CSA. Hamstrings muscle thicknesses in women are usually 90–95% of thickness in men. For both tibialis anterior and triceps surae, muscle thicknesses in women are usually 85–93% of muscle thicknesses in men.

Sex differences in muscle size also exist for trunk muscles. For rectus abdominis, muscle thicknesses in women are usually 70–80% of muscle thicknesses in men. For multifidus, muscle thicknesses in women are usually 70–85% of muscle thicknesses in men.

Muscle Fiber Types

Histological analyses of muscle biopsies have been used to compare relative numbers and areas of type I and II muscles fibers in living men and women in 34 studies (^{17,54,67,86,87,97,113,133–138,161,165,177,197,222,250,263,285,288,313,335,360,388,395,403,421,422,434,458,461,497}) (Figure 19, Table 24, Supplemental Digital Content, https://links.lww.com/JSCR/A339). VL has been investigated most frequently, followed by biceps brachii, gastrocnemius, and tibialis anterior. No consistent evidence exists for a sex difference in the relative number of type I and II fibers in these muscles. In both sexes, the relative numbers of type I and IIA muscle fibers in VL are roughly 50 and 35%, respectively. However, there is a sex difference in the relative area of whole muscle size occupied by type I and II fibers. A greater area of female than male muscle is occupied by type I fibers, whereas a greater area of male than female muscle is occupied by type II fibers. Thus, the type IIA/type I muscle fiber area ratio is greater in men than women. This has been observed in VL, biceps brachii, triceps brachii, and lateral gastrocnemius. Because type II muscle fibers create greater forces than type I fibers (^487,501), the greater relative areas of male muscle occupied by type II fibers likely contribute to greater muscle twitch forces and voluntary muscle strength in men than women.

Figure 19.: Relative numbers of VL type I (A) and type IIA muscle fibers (B), relative distribution areas of VL type I (C) and type IIA muscle fibers (D), and type I/IIA muscle fiber area ratios (E) for various muscles in living men and women as determined by muscle biopsy. See Supplemental Digital Content (Table 24, https://links.lww.com/JSCR/A339) for details on study samples and for data on distributions and distribution areas for other muscles. VL = vastus lateralis.

Strength Training Participation

Participation Rates

In 2020, Nuzzo (³²⁹) reviewed male and female participation rates in muscle-strengthening activities. The review included the results from 16 population-level surveys conducted in 6 countries. The review was undertaken to assess the conclusion by Rhodes et al. (³⁷⁸) that there is no reliable sex difference in resistance training participation rates. Their conclusion was problematic because it was based on a method that mixed participation rates from nationally representative surveys with adherence rates from clinical trials (³⁷⁸). When Nuzzo (³²⁹) limited the analysis to participation rates from nationally representative samples, participation rates were higher in men than women in all countries except Australia where the results were mixed.

Table 3 presents an updated list of male and female participation rates in muscle-strengthening activities from 27 population-level surveys: Australia (^{40,41,205,206}), Brazil (¹⁰⁵), Canada (⁴⁰²), England (⁴⁰⁶), Finland (³⁹), Germany (³⁷), Ireland (^280,473), Libya (¹²⁸), Scotland (^58,441), South Korea (⁹⁸), United States (^{38,72–74,78,125,147,159,282,361,375}), and Europe (³⁶). The magnitude of the sex difference in participation differs by country and how participation is measured. However, the results illustrate higher male than female participation rates in Brazil, Canada, England, Finland, Ireland, Libya, Scotland, South Korea, and USA. One of the most comprehensive studies found that across 28 countries in Europe (n = 280,605), 19.8% of men participated in muscle-strengthening activities ≥2 times per week compared with 15.0% of women (³⁶). In addition, in the USA, approximately 70% of persons engaged in “weightlifting” on a given day are male (^108,493). Finally, participation rates in competitive forms of muscle-strengthening activities, such as competitive powerlifting and weightlifting, are also higher in men than women (^27,204,266).

Table 3 - Rates of participation in muscle-strengthening activities between men and women in various countries.^*

Reference	Country	Year	Sample (n)	Outcome assessed	Participation rate (%)
					M	F
Humphries et al. (205)	AUS	2010	1,230	Gym-based resistance training past week	12.6	14.8
Bennie et al. (40)	AUS	2016	21,603	MSA ≥ 2 times/wk	11.8	12.1
Bennie et al. (41)	AUS	2016	9,434	Strengthening or toning activities ≥2 times/wk	20.7	16.6
Humphries et al. (206)	AUS	2018	1,237	Strength-based training to build or maintain muscle	22.5	17.7
de Lima et al. (105)	BZL	2017	40,853	Practiced “bodybuilding” in past 3 mo	3.7–7.5	2.0–5.2
Scarapicchia et al. (402)	CAN	2015	2,812	“Exercise to strengthen muscles” (8–10 reps) ≥2 times/wk	36	20
Scholes and Mindell (406)	ENG	2012	8,291	MSA ≥ 2 times/wk	34	24
Bennie et al. (36)	EUR	2020	280,605	MSA ≥ 2 times/wk	19.8	15.0
Bennie et al. (39)	FIN	2017	64,380	MSA ≥ 2 times/wk	18.1	16.4
Bennie et al. (37)	GER	2021	24,016	MSA ≥ 2 times/wk	31.2	27.6
Livingstone et al. (280)	IRE	2001	1,379	“Exercise with weights” ≥1 time/wk	9.7	5.7
Walker et al. (473)	IRE	2014	4,509	MSA ≥ 2 times/wk	25	14
Curtin et al. (98)	KOR	2018	4,592	“Resistance training” ≥2 times/mo	15.1	7.6
El Ansari et al. (128)	LIB	2014	1,300	“Strengthening exercise” ≥2 times/wk	62.9	33.9
Brown et al. (58)	SCO	2015	5,000	MSA ≥ 2 times/wk	30	25
Strain et al. (441)	SCO	2016	14,366	≥1 gym workout (resistance training or bike) in past 4 wk	18	12
Firebaugh (147)	USA	1989	33,360	Participated in “weightlifting” past 2 wk	3.2–30.8	0.7–10
Eaton et al. (125)	USA	1994	33,428	“Weightlifting” that worked up sweat ≥1 time/wk	0.2–5.8	0.0–0.7
Centers for Disease Control and Prevention (72)	USA	1996	∼35,000	Weightlifting or other to increase strength in past 2 wk	20	14.1
Powell et al. (361)	USA	1998	5,236	Participated in weightlifting in past 30 d	30.2	12.4
Galuska et al. (159)	USA	2002	16,697	“Lift weights” ≥2 times/wk in past mo	12.7	5.0
Centers for Disease Control and Prevention (73)	USA	2006	∼30,000	Weights or calisthenics to strengthen muscle ≥2 times/wk	21.9	17.5
Chevan (78)	USA	2008	29,783	Weights or calisthenics to strengthen muscle ≥2 times/wk	22.3	17.4
Redwood et al. (375)	USA	2009	10,372	NR	45.5	19.6
Centers for Disease Control and Prevention (74)	USA	2013	469,312	MSA ≥ 2 times/wk	34.4	24.5
Loustalot et al. (282)	USA	2013	4,271	MSA ≥ 2 times/wk	34.0	29.5
Bennie et al. (38)	USA	2018	397,423	MSA ≥ 2 times/wk	34.8	25.8

^*AUS = Australia; BZL = Brazil; CAN = Canada; CDC = Centers for Disease Control and Prevention; ENG = England; EUR = Europe (28 countries with samples ranging from 3,774 to 24,016); F = female; FIN = Finland; GER = Germany; IRE = Ireland; KOR = South Korea; LIB = Libya; M = male; MSA = muscle-strengthening activities; NR = not reported or unclear; SCO = Scotland; USA = United States of America.

Men also spend more time within a given week participating in strength training. In a study of bodybuilders, 29% of male bodybuilders and 18% of female bodybuilders reported 6–9 hours per week of strength training and 6% of male bodybuilders and 1% of female bodybuilders reported ≥10 hours per week of strength training (³⁶⁴). Female bodybuilders spent more time on cardiovascular training than the male bodybuilders (³⁶⁴). In addition, among athletes, men spend more time strength training than women. Male university athletes reported participating in strength training 2.6–3.8 days per week for a total of 49–70 minutes per week, whereas female athletes reported participating in strength training 1.9–3.0 days per week for a total of 26–44 minutes per week (³⁵⁶).

Preference for Strength Training Vs. Other Exercise

Table 4 summarizes the results from surveys that have asked men and women about their preferences for strength training compared with other exercise modes (^{11,59,122,439,482}). Sex differences in self-reported preferences for exercise mode are evident and correspond to sex differences in participation. Each study summarized in Table 4 found that men were more likely than women to prefer strength training. Men prefer strength training over yoga, stretching, dancing, and group aerobics, whereas women tend to prefer all the latter activities over strength training. Walking is the preferred activity for both sexes, but the difference in percentage of women who prefer walking vs. the percentage of women who prefer strength training is consistently greater than the percentage of men who prefer walking vs. the percentage of men who prefer strength training.

Table 4 - Preferences for strength training and other exercise modes among men and women.^*

Reference	Year	Country	M (n)	F (n)	Outcome assessed	Result (%)
						M	F
Busch et al. (59)	2016	USA	47 (D)	51 (D)	Prefer weightlifting	23	10
					Prefer walking	33	28
					Prefer yoga	2	16
					Prefer dance	4	14
Abrantes et al. (11)	2011	USA	43 (SA)	54 (SA)	Prefer resistance training	50.0	20.9
					Prefer walking	64.8	88.4
					Prefer sports	44.4	16.3
					Prefer running/jogging	25.9	9.3
					Prefer yoga/stretching	9.3	37.2
					Prefer exercise videos	0	20.9
					Prefer swimming	∼24	∼37
					Prefer cycling	∼25	∼22
Doyle et al. (122)	2019	UAE	194	434	Prefer “fitness/weights”	49.7	26.0
					Prefer walking	56.0	71.4
					Prefer jogging	50.8	39.6
					Prefer aerobics	16.6	40.6
					Prefer yoga	16.1	34.3
					Prefer swimming	56.5	64.1
					Prefer cycling	32.1	41.5
Stoutenberg et al. (439)	2015	USA	62 (AD)	57 (AD)	Prefer strength training	∼44	∼33
					Prefer walking/running	∼45	∼45
					Prefer yoga/stretching	∼20	∼40
					Prefer swimming	∼30	∼30
					Prefer cycling	∼29	∼25
Weller et al. (482)	2019	CAN	103 (PC)	0	Prefer resistance exercise	59.2	n/a
					Prefer walking	54.4	n/a
					Prefer flexibility exercise	45.6	n/a
					Prefer yoga or Pilates	25.2	n/a
					Prefer bicycling	41.7	n/a
					Prefer swimming	22.3	n/a

^*AD = alcohol use disorder; CAN = Canada; D = depression; F = female; M = male; NR = not reported; PC = prostate cancer; SA = substance abuse; UAB = United Arab Emirates; USA = United States of America. Bold entries are data on preferences for strength-training related activities.

Greater preference for strength training among men than women was documented as early as the 1930s. In 1935, Waggoner (⁴⁷²) asked 289 female university students in the USA about preferences for 38 different sports and physical activities. Dumbbell and Indian club exercise were included on the list. Women ranked dumbbell exercise as their number 1 most disliked activity, with 34% stating they disliked the activity (⁴⁷²). Indian club exercise was ranked as the eighth most disliked activity by women, with 19% stating they disliked it (⁴⁷²). For dumbbell exercise, 1% of women said they participated in the activity during its season, 7% said they would enroll in a dumbbell class if it were organized, and 1% said they would like specific coaching on dumbbell exercise (⁴⁷²). For Indian club exercise, 3% of women said they participated in the activity during its season, 12% said they would enroll in an Indian club class if it were organized, and 2% said they would like specific coaching on Indian club exercise (⁴⁷²).

In 1935, Cameron (⁶⁵) also conducted a survey on interests in sports and exercise. The sample by Cameron consisted of men who were 20 years or older and were professional workers. In the age 20–29 years cohort, 32% of men said they disliked dumbbell exercise and 26% said they disliked Indian club exercise (⁶⁵)—similar to the findings by Waggoner (⁴⁷²). However, when the entire sample of 20–60-year-old men was considered, only 16% said they disliked dumbbell exercise, indicating that older men preferred dumbbell exercise more than the younger men. To the extent findings from Waggoner (⁴⁷²) and Cameron (⁶⁵) are representative of views toward strength training with dumbbells in the 1930s, they indicate preference for and participation in strength training in both sexes has increased in the past 90 years.

Program Variables: Preferences and Practices

Introduction

For men and women who participate in strength training, the way they participate might not be the same. Program variables, such as the exercises performed, the type of equipment used, or loads or “intensities” used, might differ between men and women. To investigate this possibility, the current review includes summaries of 2 source types: (a) surveys asking individuals about their preferences for certain program variables and (b) descriptive accounts of program variables in strength training programs. Owing to the lack of information available on preferences for strength training specifically, the following discussion is informed by studies on preferences for general physical exercise programs. Preferences for general exercise programs should not be considered identical to preferences for strength training programs. Nevertheless, they should also not be considered completely dissimilar because strength training is a form of physical exercise that some survey respondents will have in mind when answering questions about exercise preferences, and one study found little difference between preferences for program variables for strength training and general exercise programs (¹⁸⁴).

Location

Strength training can be performed at home, at a gym or fitness center, at a treatment center, or outdoors. Table 5 presents the results from studies that have asked men and women, usually patients, about their preferences for location for physical exercise (^{11,59,122,184,389,482}). No obvious sex differences exist. Only 1 survey, which was administered to male patients with prostate cancer, asked specifically about muscle-strengthening exercise (¹⁸⁴). Of the patients, 55% said they prefer performing muscle-strengthening exercise at home, 21% said they prefer exercise at a gym, and 14% said they prefer exercise at a treatment center (¹⁸⁴).

Table 5 - Preferences for exercise location among men and women.^*

Reference	Year	Country	M (n)	F (n)	Patient group	Exercise mode	Prefer home (%)		Prefer gym (%)		Prefer outdoors (%)		Prefer treatment center (%)
							M	F	M	F	M	F	M	F
Rogers et al. (389)	2009	NR	0	483	Yes (BC)	General	n/a	38	n/a	19	n/a	25	n/a	3
Abrantes et al. (11)	2011	USA	43	54	Yes (SA)	General	37	31	48	33	35	50	4	17
Harrington et al. (184)	2013	USA	0	135	Yes (PC)	Strength	55	n/a	21	n/a	n/a	n/a	14	n/a
Busch et al. (59)	2016	USA	47	51	Yes (D)	General	62	67	21	14	n/a	n/a	4	2
Weller et al. (482)	2019	CAN	103	0	Yes (PC)	General	43	n/a	56	n/a	n/a	n/a	7	n/a
Doyle et al. (122)	2019	UAE	194	434	No	General	43	58	n/a	n/a	62	65	n/a	n/a

^*BC = breast cancer; CAN = Canada; D = depression; F = female; M = male; NR = not reported; PC = prostate cancer; SA = substance abuse; UAB = United Arab Emirates; USA = United States of America.

Interpersonal Contact

Strength training can be performed in various social contexts. It can be performed alone, with a training partner, or within a group. Table 6 presents the results from studies that have asked men and women about their preferences for interpersonal contact during exercise (^{11,122,184,389,482}). No consistent sex differences exist. A study of substance abuse patients found that men were more likely than women to prefer exercise alone (¹¹), but a study in a larger, nonpatient group found no sex difference in preference for interpersonal contact during exercise (¹²²). Women, however, are more likely to prefer to exercise with other women than with men (^122,469). In older adults, 33% of women and 11% of men say they prefer to exercise with individuals of their own sex (⁴⁶⁹). Finally, in the survey of patients with prostate cancer, 43% said they prefer to perform muscle-strengthening activities alone, 44% said they prefer performing these activities in a group or with family and friends, and 8% had no preference (¹⁸⁴).

Table 6 - Preference for interpersonal contact in exercise programs among men and women.^*

Reference	Year	Country	M (n)	F (n)	Patient group	Exercise mode	Prefer to exercise alone (%)		Prefer exercise with others (%)		No preference (%)
							M	F	M	F	M	F
Weller et al. (482)	2019	CAN	103	0	Yes (PC)	General	14	n/a	44	n/a	39	n/a
Rogers et al. (389)	2009	NR	0	483	Yes (BC)	General	n/a	41	n/a	38	n/a	21
Abrantes et al. (11)	2011	USA	43	54	Yes (SA)	General	36	6	34	43	30	51
Harrington et al. (184)	2013	USA	135	0	Yes (PC)	Strength	43	n/a	44	n/a	8	n/a
Doyle et al. (122)	2019	UAE	194	434	No	General	72	74	68	66	∼20	∼20

^*AUS = Australia; BC = breast cancer; F = female; M = male; NR = not reported; PC = prostate cancer; SA = substance abuse; UAE = United Arab Emirates; USA = United States of America.

Supervision

Strength training can be performed with or without supervision from an exercise trainer or coach. Table 7 presents the results from studies that have asked men and women about their preferences for supervision during exercise (^{11,89,122,184,389,469}). Women are more likely than men to prefer supervision. This finding was observed in all studies that included male and female subjects. In the study with the largest sample, 26% of women and 12% of men expressed a preference for supervision during exercise (⁴⁶⁹). Finally, in the survey of patients with prostate cancer, 59% of patients said they prefer executing muscle-strengthening exercise without supervision, 37% said they prefer supervision, and 2% said they had no preference (¹⁸⁴).

Table 7 - Preference for supervision in exercise programs among men and women.^*

Reference	Year	Country	M (n)	F (n)	Patient group	Exercise mode	Prefer supervision (%)		Prefer no supervision (%)		No preference (%)
							M	F	M	F	M	F
Cohen-Mansfield et al. (89)	2004	USA	138	186	No	General	1.67†	1.95†	n/a	n/a	n/a	n/a
Rogers et al. (389)	2009	NR	0	483	Yes (BC)	General	n/a	22	n/a	47	n/a	31
Abrantes et al. (11)	2011	USA	43	54	Yes (SA)	General	26	51	74	49	n/a	n/a
Harrington et al. (184)	2013	USA	135	0	Yes (PC)	Strength	37	n/a	59	n/a	2	n/a
van Uffelen et al. (469)	2017	AUS	756	1,089	No	General	12	26	63	48	n/a	n/a
Doyle et al. (122)	2019	UAE	194	434	No	General	59	72	17	8	24	23

^*AUS = Australia; BC = breast cancer; F = female; M = male; NR = not reported; PC = prostate cancer; SA = substance abuse; UAE = United Arab Emirates; USA = United States of America.

^†Likert scale of 0–4.

The greater female preference for supervision likely stems from a lack of knowledge and comfort with strength training equipment (^215,398,433) and perhaps a feeling of uneasiness with exercising alone. Peters et al. (³⁵⁰) found university women ranked “I do not feel comfortable resistance training alone” as the highest of all “barriers” to their participation in resistance training. Nevertheless, women do not seem to have a preference for the sex of the personal trainer who supervises their exercise, whereas men prefer male personal trainers (⁵²). Finally, women seem to be more likely than men to prefer that strength training be prescribed to them. Huebner et al. (²⁰³) found a greater proportion of female (88%) than male (61%) Olympic lifters had their programs designed by a coach, whereas a greater proportion of male lifters (30%) than female lifters (8%) created their own programs.

Competition

Male subjects are more competitive than female subjects in various domains of human activity (^302,324), particularly in exercise and sport (^{46,57,69,145,162,163,244,290,292,316,356,477}). Male subjects are more likely than female subjects to report more positive attitudes toward competition (¹³), enjoy competition (²⁰⁰), and respond favorably to competition (^167,444). Gneezy and Rustichini (¹⁶⁷) found that boys' sprint times improve more than girls' sprint times when both sexes go from sprinting alone to sprinting against competitors of their own sex (¹⁶⁷). Strong (⁴⁴⁴) reported a similar finding in 1963. In a study of university athletes, 78% of men said they believed competition added to the sport experience, whereas 58% of women said they believe competition detracted from the sport experience (⁴⁷⁷). Thus, perhaps unsurprisingly, sex differences are apparent when men and women are asked about their preference for competition in physical exercise programs (^122,469). In a study of 1,845 older adults, 22% of men and 12% of women said they prefer competition in their exercise programs, whereas 52% of men and 73% of women said they prefer no competition (⁴⁶⁹). In a study of 628 university students in the United Arab Emirates, 69% of men and 56% of women said they prefer competition in their exercise programs, whereas 5% of men and 17% of women said they prefer no competition, and 27% of both men and women said they have no preference (¹²²). Finally, greater male preference for competition within strength training is also evident by the fact that more men than women participate in competitive forms of muscle-strengthening exercise such as powerlifting and weightlifting (^27,204,266).

Equipment Type (Use of, Comfort With, and Knowledge of)

Men and women do not have the same preferences for or knowledge of strength training equipment (^{65,184,215,371,373,398,433,472}) (Table 8). Women are less comfortable with strength training equipment, and women are more comfortable using cardiovascular than strength training equipment (³⁹⁸). This helps to explain why greater proportions of women use cardiovascular equipment (86%) than strength training machines (47%) and free weights (34%) (³⁷¹) and also why women perform strength training less frequently than men. Women also say they have less knowledge of free weights and plate-loaded machines than body weight exercises (²¹⁵). Ratamess et al. (³⁷³) asked women, who had been participating consistently in strength training over the past 3 months, about the type of strength training equipment they used. Women who were coached by a personal trainer were more likely to use barbells (79%) than women not coached by a personal trainer (54%). Thus, lack of knowledge and comfort in using strength training equipment likely contributes to the greater female than male preference for exercise supervision. Finally, greater comfort among men in use of strength training equipment might also correlate with a psychological propensity for such equipment. Men have a long-standing interest in designing strength training equipment. Approximately 99% of patents for strength training equipment before 1980 were obtained by men (³³⁰).

Table 8 - Use of, preference for, comfort with, and knowledge of strength training equipment among men and women.^*

Reference	Year	Country	M (n)	F (n)	Outcome	Result (% yes or Likert scale)
						M	F
Waggoner (472)	1935	USA	0	289	Dislike dumbbell exercise	n/a	34%
					Dislike Indian club exercise	n/a	19%
Cameron (65)	1935	USA	335	0	Dislike dumbbell exercise	16%	n/a
Harrington et al. (184)	2013	USA	135 (PC)	0	Prefer weight machines	43%	n/a
					Prefer free weights	35%	n/a
					Prefer bands	13%	n/a
					Prefer calisthenics	20%	n/a
Ratamess et al. (373)	2008	USA	0	19 (PT)	Use barbells	n/a	79%
				27 (NPT)	Use barbells	n/a	54%
Rapport et al. (371)	2018	United Kingdom	463	533	Use weight machines	72%	47%
					Use free weights	70%	34%
					Use aerobic equipment	67%	86%
Salvatore and Marecek (398)	2010	USA	26	35	Use weight machines	96%	79%
					Use free weights	81%	66%
					Use aerobic equipment	77%	82%
					Comfort—weight machines	5.0	4.0†
					Comfort—free weights	4.5	3.0†
					Comfort—aerobic equipment	3.6	4.2
Stankowski et al. (433)	2017	USA	291	429	I would use free weights more if I knew how to lift safely	2.68	3.40†
Hurley et al. (215)	2018	USA	0	116	Knowledge of body resistance	n/a	3.97
					Knowledge of dumbbells	n/a	3.80
					Knowledge of selectorized	n/a	3.74
					Knowledge of free weights	n/a	3.45
					Knowledge of plate-loaded	n/a	3.39
					Knowledge of bands	n/a	3.22

^*DB = dumbbell; F = female; M = male; NPT = no personal trainer (i.e., not currently training with a personal trainer); PC = prostate cancer; PT = personal trainer (i.e., currently training with a personal trainer); USA = United States of America.

^†Statistically significant difference between male and female means.

Muscle Group Exercised

Men and women do not necessarily focus on the same muscle groups or perform the same exercises when they participate in strength training. The most consistent finding from this literature is that men spend more time exercising upper-body muscles than women do (^140,225,307). Using a 5-point Likert scale (1—not at all; 5—very much), Jonason (²²⁵) asked male and female undergraduates “How much do you work out your [body part]?” Men indicated greater emphasis on upper-body muscles than women (men: 2.90, women: 2.06, d = 0.59), women indicated greater emphasis on lower-body muscles (men: 2.43, women: 2.74, d = 0.20), and there was no sex difference for emphasis on abdominal muscles (men: 2.78, women: 2.94, d = 0.10). In the study by Fairchild Saidi and Branscum (¹⁴⁰), 272 female and 120 male university students were surveyed about how many days per week they performed muscle-strengthening activities for various body parts. Men reported spending more days than women exercising their chest and arms (d ≥ 0.20), whereas women reported spending more days exercising their legs (d = 0.29) (¹⁴⁰). No statistically significant sex differences were observed for the back, shoulders, hips, and abdominals, although mean values for days spent exercising the back and shoulders were higher in men, and mean values for days spent exercising the hips and abdominals were higher in women (¹⁴⁰). Finally, Mealey (³⁰⁷) found that men spend more time exercising upper-body muscles more than women, but no sex differences existed for time spent exercising muscles of the lower-body and torso.

As mentioned earlier, the sex difference in muscle strength is more pronounced in upper-body than lower-body muscles. This might be, in part, attributed to more frequent completion of upper-body exercises among men than women. Bishop et al. (⁴⁹) compared sex differences in upper-body and lower-body muscle strength in competitive swimmers and controls. The sex differences in upper-body strength and muscle mass were smaller between male and female swimmers than between male and female controls, indicating frequent use of upper-body muscles in swimming among female swimmers reduced the sex difference in upper-body strength (⁵⁰).

Exercise “Intensity” and Self-Selected Training Loads

Table 9 summarizes the results from surveys that have asked men and women about their preferred exercise “intensity” (^{11,59,184,374,389,439,469}), with 2 studies on strength training intensity (^184,374). Most studies have found that men are more likely than women to prefer exercise at high intensities, but both sexes prefer moderate intensities. Reading and LaRose (³⁷⁴) asked overweight and nonoverweight men and women about their preferred strength training intensities. Preference for higher-intensity strength training (e.g., “muscle building”) was greater among obese men (15%) than obese women (0%), overweight men (14.3%) than overweight women (1.9%), and healthy body mass men (16.0%) than healthy body mass women (0%) (³⁷⁴). Sex differences were smaller for the proportion of respondents who preferred light or moderate strength training (e.g., “toning” and “light weights”) (³⁷⁴).

Table 9 - Preferences for exercise “intensity” among men and women.^*

Reference	Year	Country	M (n)	F (n)	Exercise mode	Low intensity		Moderate intensity		High intensity		No preference
						M	F	M	F	M	F	M	F
Reading and LaRose (374)	2020	USA	75	112	Strength	8% (HW) 11% (OW) 5% (OB)	8% (HW) 6% (OW) 3% (OB)	n/a (HW) n/a (OW) n/a (OB)	n/a (HW) n/a (OW) n/a (OB	16% (HW) 14% (OW) 15% (OB)	0% (HW) 2% (OW) 0% (OB)	n/a (HW) n/a (OW) n/a (OB)	n/a (HW) n/a (OW) n/a (OB)
Harrington et al. (184)	2013	USA	135	0	Strength	27%	n/a	64%	n/a	5%	n/a	n/a	n/a
Abrantes et al. (11)	2011	USA	43 (SA)	54 (SA)	General	13%	10%	53%	76%	21%	5%	14%	10%
Rogers et al. (389)	2009	NR	0	483 (BC)	General	n/a	26%	n/a	65%	n/a	4%	n/a	5%
Stoutenberg et al. (439)	2015	USA	62 (AD)	57 (AD)	General	15%	11%	42%	58%	34%	32%	10%	0%
Busch et al. (59)	2016	USA	47 (D)	51 (D)	General	17%	24%	45%	26%	13%	6%	n/a	n/a
van Uffelen et al. (469)	2017	AUS	1,089	756	General	n/a	n/a	n/a	n/a	36%	24%	n/a	n/a

^*AD = alcohol use disorder; AUS = Australia; BC = breast cancer; D = depression; F = female; HW = healthy weight; M = male; OB = obese; OW = overweight; SA = substance abuse; USA = United States of America.

Table 10 summarizes the results from studies that have examined loads that men and women choose to lift in their strength training programs (^{106,130,141,149,164,373}). Investigators have asked men and women to select loads that they would use in a strength training session for 10 repetitions. Both men and women tend to select loads between 40 and 60% 1RM. Nevertheless, sex differences are somewhat uncertain because only 2 studies have included both men and women (^121,164). Glass and Stanton (¹⁶⁴) found that when men and women, who had not participated in strength training experience in the past 6 months, were asked to choose loads sufficient to improve muscle strength for the seated chest press, seated back row, military press, biceps curl, and knee extension, both sexes selected loads equal to 40–60% 1RM across the 5 exercises. No statistically significant sex differences were observed. However, on all 5 exercises, men, on average, selected higher relative intensities than women. Dos Santos et al. (¹²¹) also observed small sex differences in their study. They asked men and women, who had experience with strength training, to select loads that they use in their bench press, biceps curl, and leg press routines for 10 repetitions (¹²¹). They then asked subjects to perform as many repetitions as possible. A greater proportion of men than women selected loads in which they could perform only 8–12 repetitions before failure on the bench press (46.5 vs. 12%), biceps curl (28.5 vs. 12%), and leg press (14.3 vs. 4%). The loads were not reported as %1RM. Men presumably completed fewer repetitions because they chose heavier loads, although potential sex differences in muscle fatigability also warrant consideration. No studies have discovered women select higher relative loads than men.

Table 10 - Self-selected loads for strength training among men and women.^*

Reference	Year	M (n)	F (n)	Age (y)	Self-selected load instruction	Result (%1RM)
						M	F
Summed exercises
Focht (149)	2007	0	19	∼21	Comfortable yet good workout	n/a	∼56
Faries and Lutz (141)	2016	0	39	∼20	Comfortable load	n/a	57
Faries and Lutz (141)	2016	0	39	∼20	To improve strength	n/a	60
Bench, chest press
Glass and Stanton (164)	2004	13	17	∼19	To improve strength	∼50	∼41
Ratamess et al. (373)	2008	0	27 (NPT)	∼25	Used in workouts for 10 reps	n/a	∼48
Ratamess et al. (373)	2008	0	19 (PT)	∼28	Used in workouts for 10 reps	n/a	∼57
Elsangedy et al. (130)	2013	0	20	60–70	For 3 × 10–15 reps	n/a	41
de Oliveira Segundo et al. (106)	2016	0	16	∼71	Comfortable for 3 × 10 reps	n/a	52
Elsangedy et al. (131)	2016	12	0	∼36	For 3 × 10 reps	51	n/a
Overhead press
Glass and Stanton (164)	2004	13	17	∼19	To improve strength	∼50	∼45
Elsangedy et al. (131)	2016	12	0	∼36	For 3 × 10 reps	53	n/a
Lat pulldown
Elsangedy et al. (130)	2013	0	20	60–70	For 3 × 10–15 reps	n/a	47
de Oliveira Segundo et al. (106)	2016	0	16	∼71	Comfortable for 3 × 10 reps	n/a	57
Seated row
Glass and Stanton (164)	2004	13	17	∼19	To improve strength	∼49	∼43
Ratamess et al. (373)	2008	0	27 (NPT)	∼25	Used in workouts for 10 reps	n/a	∼42
Ratamess et al. (373)	2008	0	19 (PT)	∼28	Used in workouts for 10 reps	n/a	∼55
Elsangedy et al. (131)	2016	12	0	∼36	For 3 × 10 reps	57	n/a
Lateral raise
Elsangedy et al. (130)	2013	0	20	60–70	For 3 × 10–15 reps	n/a	44
Biceps curl
Glass and Stanton (164)	2004	13	17	∼19	To improve strength	∼58	∼52
Elsangedy et al. (130)	2013	0	20	60–70	For 3 × 10–15 reps	n/a	48
Elsangedy et al. (131)	2016	12	0	∼36	For 3 × 10 reps	59	n/a
Triceps pushdown
Elsangedy et al. (130)	2013	0	20	60–70	For 3 × 10–15 reps	n/a	52
Elsangedy et al. (131)	2016	12	0	∼36	For 3 × 10 reps	57	n/a
Leg press
Ratamess et al. (373)	2008	0	27 (NPT)	∼25	Used in workouts for 10 reps	n/a	∼42
Ratamess et al. (373)	2008	0	19 (PT)	∼28	Used in workouts for 10 reps	n/a	∼50
Elsangedy et al. (130)	2013	0	20	60–70	For 3 × 10–15 reps	n/a	43
Elsangedy et al. (131)	2016	12	0	∼36	For 3 × 10 reps	58	n/a
Knee extension
Glass and Stanton (164)	2004	13	17	∼19	To improve strength	∼54	∼50
Ratamess et al. (373)	2008	0	27 (NPT)	∼25	Used in workouts for 10 reps	n/a	∼37
Ratamess et al. (373)	2008	0	19 (PT)	∼28	Used in workouts for 10 reps	n/a	∼43
Elsangedy et al. (130)	2013	0	20	60–70	For 3 × 10–15 reps	n/a	33
Elsangedy et al. (131)	2016	12	0	∼36	For 3 × 10 reps	52	n/a
de Oliveira Segundo et al. (106)	2016	0	16	∼71	Comfortable for 3 × 10 reps	n/a	38
Knee flexion
de Oliveira Segundo et al. (106)	2016	0	16	∼71	Comfortable for 3 × 10 reps	n/a	55

^*1RM = 1 repetition maximum; F = female; M = male; NPT = no personal trainer (i.e., not currently training with a personal trainer); PT = personal trainer (i.e., currently training with a personal trainer).

Experience with a personal trainer seems to affect load selection in women. Ratamess et al. (³⁷³) found that women who were currently coached by a personal trainer selected higher relative loads for 10 repetitions on leg press (50% 1RM), chest press (57% 1RM), knee extension (42% 1RM), and seated row (55% 1RM) than women who were not currently coached by a personal trainer (41, 48, 38, and 41% 1RM, respectively). Other factors that might affect load selection are training goals or exercise instructions. Faries and Lutz (¹⁴¹) found that women who were told to select a load that would improve their strength chose, on average, a load equal to 59.6% 1RM, which they completed, on average, for 15 repetitions (¹⁴¹). Women who were told to select a load “that is comfortable” chose a lighter load (56.8% 1RM), which they completed fewer repetition with (13 repetitions).

Reasons for Strength Training Participation

Perceived Importance of Strength Training

A likely contributor to greater male than female participation in strength training is that male subjects perceive strength training to be more important for their lives. Poiss et al. (³⁵⁶) surveyed 139 male and 164 female university athletes about the importance of resistance training for general and sport-specific training. Male subjects were more likely than female subjects to agree resistance training was essential for their general and sport-specific training (³⁵⁶). Consequently, male athletes also participated in resistance training more frequently and for longer durations (³⁵⁶).

Self-Reported Reasons for Physical Exercise

Table 11 summarizes the results from 20 studies that have surveyed men and women about their reasons or motivations for participation in physical exercise (^{1,25,103,104,127,244,249,287,290,292,303,312,316,358,371,426,446,459,469,500}). Desires to improve health and physical fitness are the 2 most commonly reported reasons for both sexes. Other reasons for exercise participation that are rated equally between men and women are enjoyment/fun, stress or mood management, and socializing. However, sex differences exist for other motivators. Men are generally more motivated by challenge, competition, social recognition, status attainment, and a desire to improve muscle size, strength, and endurance. Women are generally motivated more by improving physical appearance and attractiveness, muscle “toning,” and weight loss or weight management. Notably, the greater female interest in weight management and muscle “toning” dates back to early research on the effects of strength training on the human body. The first studies on “spot reduction” in the 1960s were conducted by female investigators who studied female subjects (³³¹).

Table 11 - Reasons or motivations for participation in physical exercise among men and women.^*

Reason	Year	Country	M	F	Age	Result†
Reference			n	n	y	M	F
Affiliation
Ashford et al. (25)	1993	United Kingdom	93	243	≥16	3.88	3.72
Markland and Hardy (287)	1993	Wales	57	74	∼21	2.37	2.75
Aaltonen et al. (1)	2004	FIN	602	600	32–37	3.72	3.45
Aaltonen et al. (1)	2004	FIN	473	633	32–38	2.99	2.92
Kilpatrick et al. (244)	2005	USA	101	132	18–47	2.44	2.38
Egli et al. (127)	2011	USA	1,081	1,182	NR	2.79	2.32
Molanorouzi et al. (316)	2015	MAL	703	657	20–64	19.7	18.0
Portela-Pino et al. (358)	2019	SPA	417	435	12–17	37.1	32.5‡
Appearance (physical), attractiveness
Ashford et al. (25)	1993	United Kingdom	93	243	≥16	4.02	4.07
Davis et al. (103)	1995	USA	106	105	18–60	5.0–5.1	5.3–5.6
Smith et al. (426)	1998	USA	78	100	18–25	4.5	5.4‡
Koivula (249)	1999	SWE	148	124	∼24	3.65	4.05
Tiggemann and Williamson (459)	2000	AUS	48	70	16–60	10.0–12.5	11.3–12.5‡
Aaltonen et al. (1)	2004	FIN	602	600	32–37	3.36	3.96
Aaltonen et al. (1)	2004	FIN	473	633	32–38	3.06	3.68
Kilpatrick et al. (244)	2005	USA	101	132	18–47	3.70	3.93
De Andrade Bastos et al. (104)	2006	BZL	100	138	16–60	10.3	10.9
Egli et al. (127)	2011	USA	1,081	1,182	NR	3.32	3.51
Stults-Kolehmainen et al. (446)	2013	USA	1,803 (A)	497 (A)	18–64	6.8%	15.1%
Martinez et al. (290)	2014	USA	52	60	∼30	3.29	3.77‡
Michaelidou and Moraes (312)	2014	United Kingdom	87	72	≥18	4.26	5.14‡
Molanorouzi et al. (316)	2015	MAL	703	657	20–64	18.3	20.4‡
You and Shin (500)	2017	KOR	228	174	40–59	3.10	3.20
van Uffelen et al. (469)	2017	AUS	756	1,089	60–67	60%	81%
Challenge
Kilpatrick et al. (244)	2005	USA	101	132	18–47	3.38	3.01‡
Egli et al. (127)	2011	USA	1,081	1,182	NR	3.22	2.88‡
Competition
Mathes and Battista (292)	1985	USA	100	100	19.7	5.29	4.53
Ashford et al. (25)	1993	United Kingdom	93	243	≥16	3.65	2.49
Markland and Hardy (287)	1993	Wales	57	74	∼21	3.39	2.25
Koivula (249)	1999	SWE	148	124	∼24	3.34	2.64
Aaltonen et al. (1)	2004	FIN	473	633	32–38	3.07	3.11
Aaltonen et al. (1)	2004	FIN	602	600	32–37	3.50	3.38
Kilpatrick et al. (244)	2005	USA	101	132	18–47	3.52	2.13‡
Egli et al. (127)	2011	USA	1,081	1,182	NR	3.31	2.27‡
Martinez et al. (290)	2014	USA	52	60	∼30	2.64	1.64‡
Molanorouzi et al. (316)	2015	MAL	703	657	20–64	19.3	15.5‡
Portela-Pino et al. (358)	2019	SPA	417	435	12–17	48.0	41.0‡
Enjoyment, fun
Markland and Hardy (287)	1993	Wales	57	74	∼21	3.09	3.40
Davis et al. (103)	1995	USA	106	105	18–60	3.8	3.7
Koivula (249)	1999	SWE	148	124	∼24	5.14	5.38
Tiggemann and Williamson (459)	2000	AUS	48	70	16–60	10.5–12.6	10.3–12.0‡
Aaltonen et al. (1)	2004	FIN	602	600	32–37	4.57	4.65
Aaltonen et al. (1)	2004	FIN	473	633	32–38	3.63	3.66
Kilpatrick et al. (244)	2005	USA	101	132	18–47	3.47	3.36
De Andrade Bastos et al. (104)	2006	BZL	100	138	16–60	6.8	7.1
Egli et al. (127)	2011	USA	1,081	1,182	NR	3.33	3.07‡
Michaelidou and Moraes (312)	2014	United Kingdom	87	72	≥18	4.11	4.01
Stults-Kolehmainen et al. (446)	2013	USA	1,803 (A)	497 (A)	18–64	57.3%	57.7%
Molanorouzi et al. (316)	2015	MAL	703	657	20–64	20.9	19.8‡
Fitness
Markland and Hardy (287)	1993	Wales	57	74	∼21	3.37	3.83
Ashford et al. (25)	1993	United Kingdom	93	243	≥16	4.28	3.93
Tiggemann and Williamson (459)	2000	AUS	48	70	16–60	16.6–18.4	18.2–18.3
Aaltonen et al. (1)	2004	FIN	602	600	32–37	4.69	4.83
Aaltonen et al. (1)	2004	FIN	473	633	32–38	4.31	4.51
McCabe and James (303)	2009	AUS/NZ	245	275	18–65	36.1%	37.0%
Rapport et al. (371)	2018	United Kingdom	1,325	2,071	≥18	77.1%	81.8%
Stults-Kolehmainen et al. (446)	2013	USA	1,803 (A)	497 (A)	18–64	40.5%	39.3%
Molanorouzi et al. (316)	2015	MAL	703	657	20–64	19.9	20.8‡
Health
Mathes and Battista (292)	1985	USA	100	100	17–29	5.18	5.35
Ashford et al. (25)	1993	United Kingdom	93	243	≥16	4.45	4.31
Davis et al. (103)	1995	USA	106	105	18–60	6.0	6.1
Smith et al. (426)	1998	USA	78	100	18–25	5.6	5.6
Koivula (249)	1999	SWE	148	124	∼24	5.23	5.60
Tiggemann and Williamson (459)	2000	AUS	48	70	16–60	18.9–19.6	19.6–20.9
Kilpatrick et al. (244)	2005	USA	101	132	18–47	4.22	4.52
De Andrade Bastos et al. (104)	2006	BZL	100	138	16–60	10.6	11.3
Egli et al. (127)	2011	USA	1,081	1,182	NR	3.79	3.93‡
Michaelidou and Moraes (312)	2014	United Kingdom	87	72	≥18	5.24	4.88
Rapport et al. (371)	2018	United Kingdom	1,325	2,071	≥18	40.0%	45.8%
Portela-Pino et al. (358)	2019	SPA	417	435	12–17	32.2	34.2
Muscle
Koivula (249)	1999	SWE	148	124	∼24	4.94	5.31
McCabe and James (303)	2009	AUS/NZ	245	275	18–65	27.8%	4.6%
Rapport et al. (371)	2018	United Kingdom	1,325	2,071	≥18	67.6%	27.4%‡
Mental health; stress, mood management; relaxation
Ashford et al. (25)	1993	United Kingdom	93	243	≥16	4.11	4.21
Markland and Hardy (287)	1993	Wales	57	74	∼21	2.36	2.65
Davis et al. (103)	1995	USA	106	105	18–60	4.5	5.2
Smith et al. (426)	1998	USA	78	100	18–25	4.8	5.2
Koivula (249)	1999	SWE	148	124	∼24	3.75	4.48
Tiggemann and Williamson (459)	2000	AUS	48	70	16–60	15.2–15.4	17.1–18.1‡
Aaltonen et al. (1)	2004	FIN	602	600	32–37	4.63	4.80
Aaltonen et al. (1)	2004	FIN	473	633	32–38	4.17	4.48
Kilpatrick et al. (244)	2005	USA	101	132	18–47	3.28	3.50‡
Stults-Kolehmainen et al. (446)	2013	USA	1,803 (A)	497 (A)	18–64	19.8%	20.6%
Michaelidou and Moraes (312)	2014	United Kingdom	87	72	≥18	3.78	3.80
Molanorouzi et al. (316)	2015	MAL	703	657	20–64	19.9	20.8‡
van Uffelen et al. (469)	2017	AUS	756	1,089	60–67	80%	85%
Rapport et al. (371)	2018	United Kingdom	1,325	2,071	≥18	23.1%	22.7%
You and Shin (500)	2017	KOR	228	174	40–59	2.89	3.41
Portela-Pino et al. (358)	2019	SPA	417	435	12–17	11.2	10.9
Skill mastery; performance improvement
Ashford et al. (25)	1993	United Kingdom	93	243	≥16	3.98	3.69
Koivula (249)	1999	SWE	148	124	∼24	4.10	4.08
Aaltonen et al. (1)	2004	FIN	602	600	32–37	4.23	4.09
Aaltonen et al. (1)	2004	FIN	473	633	32–38	3.46	3.12
Stults-Kolehmainen et al. (446)	2013	USA	1,803 (A)	497 (A)	18–64	55.2%	47.6%
Molanorouzi et al. (316)	2015	MAL	703	657	20–64	20.6	18.3‡
Socializing with others
Mathes and Battista (292)	1985	USA	100	100	17–29	4.65	5.08
Smith et al. (426)	1998	USA	78	100	18–25	3.8	3.7
Koivula (249)	1999	SWE	148	124	∼24	3.61	3.08
De Andrade Bastos et al. (104)	2006	BZL	100	138	16–60	8.1	7.1‡
Stults-Kolehmainen et al. (446)	2013	USA	1,803 (A)	497 (A)	18–64	6.2%	11.3%
Michaelidou and Moraes (312)	2014	United Kingdom	87	72	≥18	2.35	2.23
van Uffelen et al. (469)	2017	AUS	756	1,089	60–67	52%	68%
Social recognition, gain status
Markland and Hardy (287)	1993	Wales	57	74	∼21	1.52	0.93
Ashford et al. (25)	1993	United Kingdom	93	243	≥16	3.27	2.76
Kilpatrick et al. (244)	2005	USA	101	132	18–47	2.84	2.14‡
Egli et al. (127)	2011	USA	1,081	1,182	NR	2.66	2.07‡
Strength, endurance
Kilpatrick et al. (244)	2005	USA	101	132	18–47	4.41	3.82‡
Egli et al. (127)	2011	USA	1,081	1,182	NR	3.89	3.50‡
Portela-Pino et al. (358)	2019	SPA	417	435	12–17	18.2	15.6‡
“Tone” muscle
Tiggemann and Williamson (459)	2000	AUS	48	70	16–60	9.1–11.9	12.0–12.1‡
McCabe and James (303)	2009	AUS/NZ	245	275	18–65	9.1%	19.1%
Weight management
Markland and Hardy (287)	1993	Wales	57	74	∼21	1.68	3.10
Davis et al. (103)	1995	USA	106	105	18–60	3.8	5.3‡
Koivula (249)	1999	SWE	148	124	∼24	2.72	3.86
Tiggemann and Williamson (459)	2000	AUS	48	70	16–60	8.8–9.0	12.3–12.5‡
Kilpatrick et al. (244)	2005	USA	101	132	18–47	3.30	4.14‡
McCabe and James (303)	2009	AUS/NZ	245	275	18–65	13.0%	37.4%
Egli et al. (127)	2011	USA	1,081	1,182	NR	2.96	3.73‡
Stults-Kolehmainen et al. (446)	2013	USA	1,803 (A)	497 (A)	18–64	17.5%	27.0%
Martinez et al. (290)	2014	USA	52	60	∼30	3.07	4.22‡
Michaelidou and Moraes (312)	2014	United Kingdom	87	72	≥18	4.39	4.84
van Uffelen et al. (469)	2017	AUS	756	1,089	60–67	84%	88%
Rapport et al. (371)	2018	United Kingdom	1,325	2,071	≥18	31.3%	52.3%
Portela-Pino et al. (358)	2019	SPA	417	435	12–17	25.9	27.3‡

^*A = athletes; AUS = Australia; BZL = Brazil; F = female; FIN = Finland; KOR = South Korea; M = male; MAL = Malaysia; NR = not reported; NZ = New Zealand; SPA = Spain; SWE = Sweden; USA = United States of America.

^†Results are reported in the units of scale used to measure the outcome, typically Likert scales, but sometimes percent agreement scales.

^‡Statistically significant difference, as reported by the authors. Not all authors conducted a statistical significance test for main effect of sex.

Intrasexual Competition, Mate Selection, and the Drive for Muscularity

Evolutionary psychologists hypothesize intrasexual competition influences exercise motivations and behaviors, such that men undertake behaviors that will cause their bodies to look larger and that women undertake activities that will cause their bodies to look smaller (²²⁵). Such hypotheses stem from evidence that women are typically more physically attracted to men who have bigger muscles, look stronger, and have mesomorphic builds (^{117,118,120,152,169,283,407}); age of first sexual intercourse and the number of sexual partners in the past year for male subjects correlates with their amount of fat-free mass and limb muscle volume (²⁶⁵); and individuals who are not married rate “appearance” higher as a motive for their exercise participation than individuals who are married (³¹²).

Mate attraction and intrasexual competition likely explain why the drive for muscularity is more pronounced in men than women (^233,304,436) and why men participate in strength training more frequently than women. McCabe and James (³⁰³) found that 27.8% of men, but only 4.6% of women, said their primary reason for exercise participation was to increase muscle size. Men also report using a greater number of strategies to increase muscle size, whereas women, whose primary reasons for exercise are to lose weight (36.4% of women) or “tone up” (19.1% of women), report using a greater number of strategies to lose weight (³⁰³). Gray (¹⁶⁹) reported that 58–90% of men want to increase their muscularity, with only 2–5% wanting to be smaller (¹⁶⁹). Men are also more likely than women to spend their exercise time trying to build muscle mass (²²⁵); they are more likely to take anabolic steroids (³⁵⁷); and they are more likely to flex their muscles around friends (⁶⁰). Therefore, as strength training increases muscle size and strength (³⁸⁷) and women are generally more attracted to men who are stronger, have bigger muscles, and have mesomorphic builds (^{117,118,120,152,169,283,407}), this helps to explain greater male than female participation in strength training. Men acknowledge physical attractiveness as a motivator for their exercise participation, albeit to a lesser degree than women. Mealey (³⁰⁷) found that 20% of men ranked “appearance to opposite sex” as their number 1 reason of 7 for their gym workouts and 22% ranked it as their number 2 reason. Strength exercise as an investment strategy for enhancing physical appearance is also reported more by men than women (²⁵¹). Finally, among male anabolic steroid users, desires to increase muscle mass and strength, to look good, and attract sexual partners are some of the more highly rated motivators for steroid use (⁸⁸). Nevertheless, it should be pointed out that men, particularly young adult men, often overestimate how much muscularity is desired or found attractive by women (^123,169,283).

Intrasexual competition and mate selection might also affect specific strength training practices. For example, Mealey (³⁰⁷) found that the higher men rated “appearance to the opposite sex” as a motivator for exercise, the more time they spent performing upper-body exercises. Mealey (³⁰⁷) concluded: “When given free choice in whether to use a new technology that has the potential to reduce biological sex differences, people, in fact, used the technology in ways that enhance those differences. Men tailored their workouts in ways that would enhance the musculature of the upper body, and women tailored their workouts in ways that would enhance leg and hip muscle.” In addition, both women and men find the obliques, abdominals, glutes, biceps, and shoulders the most attractive muscles on a man's body (¹²³). This aligns with the greater emphasis that men place on their trunk than lower-body muscles in their workouts (^140,225). Upper-body strength, size, and attractiveness contribute significantly more to male than female body esteem (^102,151). Women rate muscular strength, width of shoulders, and size of various muscle groups higher for male attractiveness than men rate those same qualities as part of female attractiveness (¹⁵¹). Moreover, men rate these same factors higher for importance to their own physical attractiveness than they do as part of a women's attractiveness (¹⁵¹).

Finally, whereas the drive for muscularity likely underlies greater male than female participation in strength training, an analogous drive for thinness likely underlies female exercise participation and preference. Men are more physically attracted to women with lower-body weights and lower waist-to-hip ratios (^118,119,423). Thus, weight management, muscle “toning,” and improved appearance and attractiveness are motivators for exercise that are more pronounced in men than women. Gray (¹⁶⁹) found that 55–70% of women wanted to decrease their body fat compared with 16–32% of men. Jonason (²²⁵) found that women spend more of their exercise time trying to lose weight than men do. In reporting on female physical attractiveness, men rated the female waist, buttocks, hips, and legs higher than the arms and biceps (¹⁵¹). The body areas rated by men as being of greatest importance in female attractiveness generally align with the body areas that women say are important for their own physical attractiveness (¹⁵¹).

Strength training causes muscle hypertrophy in both men and women (³⁸⁷) (discussed below). As weight loss is a key motivator for female exercise participation, women report fear of becoming “big and bulky” as a reason they do not participate in strength training (^350,373). This fear of getting “big and bulky,” coupled with the simultaneous desire to lose or manage body mass, then leads to the ill-defined concept of “muscle toning,” which is a sort of negotiation or middle ground between weight loss and muscle hypertrophy. “Muscle toning” seems to mean a desire to maintain current muscle mass with a simultaneous decrease in fat. “Muscle toning” of a given body area would then be synonymous with the concept of “spot reduction,” which as mentioned earlier, was first examined in the 1960s by female investigators who studied female subjects (³³¹).

Barriers to Strength Training Participation

Factors that make participation in exercise less likely are often called “barriers.” Men and women report many of the same barriers to exercise, but some sex differences exist. Women participate in strength training at lower rates than men, so women have often been the focus of studies on barriers to strength training. Stankowski et al. (⁴³³) found that female university students were more likely than male university students to report they do not use the free weight area of the gym because they do not know how to use free weights safely (⁴³³). This finding corresponds with the finding that women are less comfortable than men with using free weights (³⁹⁸) and that women have less knowledge of free weight exercise than they do body weight exercise (²¹⁵).

In another study of female university students, lack of knowledge of how to lift weights was the seventh highest rated barrier of 12 to strength training participation (³⁵⁰). Barriers rated higher were feeling uncomfortable training alone (first), feeling uncomfortable in a crowded gym (second), too tired to exercise after work/school (third), uncomfortable around men in the gym (fourth), looking weak compared with others (fifth), and looking silly and uncoordinated (sixth). Other factors included not wanting to look big and bulky (eighth), training takes too much time (ninth), failed last time strength training was performed (10th), not wanting to feel sore (11th), and friends think I am weird (12th) (³⁵⁰). Ratamess et al. (³⁷³) found that the proportion of women who believed strength training would make their muscles “big and bulky” was greater in those who were not currently coached by a personal trainer (38%) compared with those who were currently coached by a personal trainer (16%). It is unclear if personal trainers directly affect this belief, if the belief is changed simply because of strength training, or if those who are less likely to hold this belief are more likely to seek out personal trainers. Fear of becoming “bulky” or “muscle bound,” along with concerns about safety, “lower class” associations, and becoming sweaty, have also been reported as barriers to strength training participation by women older than 60 years (²⁴³).

Physical Adaptations and Injuries From Strength Training

Familiarization on Muscle Strength

Improvements in performance on tests of muscle strength can occur within the first few strength training sessions (³³⁴). Thus, given that women participate in strength training less frequently than men and have less experience and comfort using free weights, one might think women would improve more quickly than men on tests of muscle strength within the first few training sessions. Ribeiro et al. (³⁸²) found that when men and women repeated the bench press, squat, and biceps curl 1RM tests in 4 consecutive sessions, women exhibited 2.5–4% greater improvements in performance from session 1 to session 4 than men (³⁸²). However, across a number of other studies on reliability of muscle strength measures, there seems to be no obvious sex differences in improved strength performance within the first few testing sessions (³³⁴).

Weeks of Training on Muscle Size and Strength

Both men and women were subjects in early strength training research (70% male; 30% female) (³³¹). In 1974, Wilmore (⁴⁸⁹) was the first to formally examine if adaptations in muscle size and strength from strength training are similar between men and women. In the study by Wilmore (⁴⁸⁹), 47 women and 26 men completed a total body strength training program through a university weight training class (10 weeks, 2 days per week). Women increased their leg press 1RMs, biceps curl 1RMs, bench press 1RMs, and grip strength 29.5, 10.6, 28.6, and 12.8%, respectively. Men increased their strength on these tests by 26.0, 18.9, 16.5, and 5.0%, respectively. Muscle hypertrophy occurred in both sexes but was more prominent in men. The results illustrated both men and women increase muscle strength after weeks of strength training. In 1978, Wilmore et al. (⁴⁹⁰) again found generally similar improvements in muscle strength in men and women after weeks of strength training.

Only in the past 2 years have the results from all studies on the topic been subjected to meta-analysis (^230,387). Roberts et al. (³⁸⁷) summarized the results from 50 studies on sex differences in muscle size and strength after strength training in healthy adults aged 18–50 years. Effect sizes for relative improvements in lower-body muscle strength and upper-body and lower-body muscle hypertrophy were similar between men and women, although women exhibited larger relative improvements in upper-body muscle strength (³⁸⁷). Greater relative improvements in upper-body, but not lower-body, strength in women might be due to the greater baseline sex difference in muscle strength for upper-body muscles.

Jones et al. (²³⁰) summarized the results from 30 studies on sex differences in muscle size and strength from strength training in healthy adults who were over 50 years of age. Women demonstrated greater relative improvements in lower-body strength than men; however, no sex differences existed in relative improvements in upper-body strength or relative improvements in muscle size (²³⁰). Absolute increases in upper-body and lower-body muscle strength and size were all greater in men (²³⁰).

Straight et al. (⁴⁴⁰) reviewed the results from 35 studies on the impact of weeks of strength training on skeletal muscle fiber size in adults 55 years or older. They found men and women of this age experienced the same moderate to large increases in myosin heavy chain I and II fiber sizes after weeks of strength training (⁴⁴⁰).

Voluntary activation, as measured with the interpolated twitch technique, might also improve after weeks of strength training. However, evidence for this is mixed (^23,416), and further uncertainty exists as to whether a sex difference exists. Nevertheless, when individual data have been presented, both men and women have shown the capacity to improve voluntary activation after weeks of strength training (^183,332).

Importantly, in strength training intervention studies, male and female subjects complete identical strength training programs. However, as discussed earlier, when left to their own choices and preferences, men and women exercise differently. Thus, the results from meta-analyses of studies in which men and women complete identical strength training programs should not be interpreted as meaning that men and women experience the same outcomes from their everyday exercise programs.

Injury Count and Rate

Hospital records and survey results reveal some sex differences in injuries from strength training (^{132,170,186,203,228,241,242,361,366,372,431,443}) (Table 12). In the general population, the most obvious sex difference is that men experience a greater total number of injuries. This is perhaps unsurprising given more men than women participate in strength training. Of all resistance training–related injuries treated in hospitals in the USA and Australia, 80% occur in male subjects (^{170,228,242,366}). The rates of resistance training–related injuries (per 100,000 population) treated at hospital are also higher among male subjects than female subjects (^170,228).

Table 12 - Injures associated with weight training among men and women.^*

Population	Year	Outcome assessed	Result
Reference			M	F
General population
Powell et al. (361)	1998	% Injured doing “weightlifting” in past 30 d	2.4	2.0
Jones et al. (228)	2000	WT-related injury rate per 100,000 population	32.4	7.5
		% Of all WT-related injuries treated at hospital ER	80.5	19.5
		% Of all WT-related fatalities	91	9
		% increase in WT-related injury rate from 1978 to 1998	28	64
Quatman et al. (366)	2009	% Of all WT injuries treated at hospital ER	83.5	16.5
		Odds† of any accident-related WT injury at ER		1.69
		Odds† of accident-related WT injury of feet at ER		2.44
		Odds† of accident-related WT injury of hands at ER	2.14
		Odds† of sprains and strains from WT at ER	1.34
		Odds† of trunk injury from WT at ER	1.55
		Odds† of leg injury from WT at ER	1.54
		Odds† of foot injury from WT at ER		2.63
		Head injuries as % of all injuries within sex	7.4	6.2
		Hand injuries as % of all injuries within sex	19.9	16.3
		Arm injuries as % of all injuries within sex	19.5	17.2
		Trunk injuries as % of all injuries within sex	36.9	27.4
		Leg injuries as % of all injuries within sex	3.8	5.7
		Foot injuries as % of all injuries within-sex	12.0	26.5
Kerr et al. (242)	2010	% WT-related injuries treated at hospital ER	82.3	17.7
		Upper-trunk injuries as % of all injuries within sex	26.8	18.4
		Lower-trunk injuries as % of all injuries within sex	20.8	14.7
		Leg injuries as % of all injuries within-sex	4.0	6.3
		Foot injuries as % of all injuries within sex	11.0	22.9
		Sprains/strains injuries as % of all injuries within sex	47.7	38.9
		Lacerations injuries as % of all injuries within sex	9.6	7.0
		Soft-tissue injuries as % of all injuries within sex	16.5	26.3
		Fracture/dislocate injuries as % of all injuries within sex	11.2	13.6
Gray and Finch (170)	2015	WT-related injury incidence per 100,000 subjects	929.6	366.0
		% Of all WT-related injuries treated at hospital	78	22
Military
Hauret et al. (186)	2015	WT-related injuries as % of all exercise/sport injuries	8.1	4.8
Competitive lifters
Raske and Norlin (372)	2002	WT-related injury incidence per 1,000 h of activity	2.4–2.7	2.2–3.2
Keogh et al. (241)	2006	WT-related injuries per 1,000 h training or competition	4.7	3.1
		% With shoulder injury in past 12 mo	41.5	47.4
		% With arm injury in past 12 mo	3.7	0
		% With elbow injury in past 12 mo	11.0	21.1
		% With chest injury in past 12 mo	4.9	0
		% With upper back injury in past 12 mo	2.4	0
		% With lower back injury in past 12 mo	29.3	21.0
		% With hip or buttocks injury in past 12 mo	1.2	NR
		% With thigh injury in past 12 mo	8.5	5.3
		% With knee injury in past 12 mo	12.2	0
Engebretsen et al. (132)	2013	% With any injury during 17 d of Olympic games	18.1	15.5
Stromback et al. (443)	2018	% With current neck injury	NR	20
		% With current shoulder injury	33.3	25.7
		% With current upper-back injury	NR	28.6
		% With current low back injury	41.7	22.9
		% With current hip injury	30.6	25.7
		% With current thigh injury	NR	17.1
		% With current knee injury	16.2	NR
Spence et al. (431)	2020	% Of lifters currently injured	17.9	30.4
Huebner et al. (203)	2020	% With shoulder injury in past 2 y that affected training	29.4	27.5
		% With back injury in past 2 y that affected training	16.4	16.7
		% With hip injury in past 2 y that affected training	10.8	13.1
		% With knee injury in past 2 y that affected training	23.0	17.6

^*ER = emergency room; F = female; M = male; NR = not reported; WT = weight training.

^†Odds ratio compared with the other sex.

Injury Type and Location

Some sex differences exist for resistance training injury type and location (Table 12). Approximately 90% of fatalities linked to use of strength training equipment occur in male subjects (²²⁸). Odds of an accident-related resistance training injury are higher in female subjects than male subjects (³⁶⁶). Odds of a sprain or strain injury from resistance training are higher in male subjects than female subjects (^242,366). Trunk injuries during resistance training are more common in male subjects than female subjects (^242,366). Foot injuries during resistance training are more common in female subjects than male subjects (^242,366).

Among competitive powerlifters and weightlifters, sex differences in injury rates, injury type, and injury location (Table 12) are less obvious than in recreational lifters. The shoulder is consistently reported as one of the most commonly injured body areas in both male and female competitive lifters (^203,241,443). Knee injuries are more common among male than female competitive lifters (^203,241,443). For other body areas, the results are mixed or show no sex differences.

Muscle Damage

Researchers have reviewed the effect of muscle-damaging exercise on physiological and functional outcomes in men and women (^83,84,201). The magnitude of strength loss and pain and soreness experienced after muscle-damaging exercise is similar between men and women (^83,84,201). These conclusions have been supported by a recent meta-analysis. Morawetz et al. (³²⁰) reported that mean losses in strength for men and women immediately after eccentric exercise of the elbow flexors are 54.5 and 61.0%, respectively, and mean losses in strength immediately after eccentric exercise of the knee extensors are 20.6 and 21.9%, respectively (³²⁰). The authors also confirmed no sex difference in the magnitude of muscle soreness experienced after eccentric exercise (³²⁰).

Conclusion

Sex differences in numerous strength training variables and outcomes have been reviewed. Key conclusions are summarized in Table 13 and explained below.

Table 13 - Summary of sex differences related to strength training.^*

Variable	Result	Sources
Muscle strength
Childhood—grip strength	M ≥ F
Adolescence—grip strength	M > F
Adult—upper-body	M > F	Figures 1, 3–5, , Supplemental Digital Content, Tables 3–6, http://links.lww.com/JSCR/A339
Adult—lower-body	M > F	Figures 6–9, , Supplemental Digital Content, Tables 7–12, http://links.lww.com/JSCR/A339
Adult—neck	M > F	Figure 10, Supplemental Digital Content, Table 13, http://links.lww.com/JSCR/A339
Adult—trunk	M > F	Figure 10, Supplemental Digital Content, Table 14, http://links.lww.com/JSCR/A339
Muscle endurance (repetitions to failure)
Adult—upper-body	M = F	Figure 11, Supplemental Digital Content, Table 15, http://links.lww.com/JSCR/A339
Adult—lower-body	M = F	Figure 11, Supplemental Digital Content, Table 15, http://links.lww.com/JSCR/A339
Muscle physiology, anatomy
Voluntary activation	M = F	Figure 12, Supplemental Digital Content, Table 16, http://links.lww.com/JSCR/A339
Muscle twitch forces	M > F	Figure 13, Supplemental Digital Content, Table 17, http://links.lww.com/JSCR/A339
Muscle mass—total body	M > F	Figure 14, Supplemental Digital Content, Table 18, http://links.lww.com/JSCR/A339
Muscle mass—upper-body	M > F	Figure 15, Supplemental Digital Content, Table 19, http://links.lww.com/JSCR/A339
Muscle mass—lower-body	M > F	Figure 15, Supplemental Digital Content, Table 20, http://links.lww.com/JSCR/A339
Muscle size—upper-body	M > F	Figure 16, Supplemental Digital Content, Table 21, http://links.lww.com/JSCR/A339
Muscle size—lower-body	M > F	Figure 17, Supplemental Digital Content, Table 22, http://links.lww.com/JSCR/A339
Muscle size—trunk	M > F	Figure 18, Supplemental Digital Content, Table 23, http://links.lww.com/JSCR/A339
Muscle fiber type—% muscle fibers that are type I	M = F	Figure 19, Supplemental Digital Content, Table 24, http://links.lww.com/JSCR/A339
Muscle fiber type—% muscle fibers that are type IIA	M = F	Figure 19, Supplemental Digital Content, Table 24, http://links.lww.com/JSCR/A339
Muscle fiber type—% muscle area that is type I	F > M	Figure 19, Supplemental Digital Content, Table 24, http://links.lww.com/JSCR/A339
Muscle fiber type—% muscle area that is type IIA	M > F	Figure 19, Supplemental Digital Content, Table 24, http://links.lww.com/JSCR/A339
Muscle fiber type area—type IIA/I	M > F	Figure 19, Supplemental Digital Content, Table 24, http://links.lww.com/JSCR/A339
Strength training participation rates	M > F
Program variables: Preferences and occurrences
Location	M = F
Interpersonal contact	M = F
Supervision—supervised exercise	M < F
Competition—include competition	M > F	Text
Equipment—use of free weights	M > F
Equipment—comfort with strength equipment	M > F
Equipment—knowledge of strength equipment	M > F
Muscles exercised at gym—upper-body	M > F	Text
Muscles exercised at gym—lower-body	M < F	Text
Muscles exercised at gym—trunk	M = F	Text
“Intensity” or load—high or heavy	M > F
Reasons for exercise participation
Affiliation	M = F
Appearance (physical), attractiveness	M < F
Challenge	M > F
Competition	M > F
Enjoyment, fun	M = F
Fitness	M = F
Health	M = F
Muscle	M > F
Mental health; stress, mood management; relax	M = F
Skill mastery, performance improvement	M ≥ F
Socializing with others	M = F
Social recognition, status	M > F
Strength, endurance	M > F
“Tone” muscle	M < F
Weight management	F > M
Muscle adaptations to strength training
Muscle strength—upper-body (% increase)	M ≤ F†	Text
Muscle strength—lower-body (% increase)	M ≤ F†	Text
Muscle hypertrophy—upper-body (% increase)	M = F	Text
Muscle hypertrophy—lower-body (% increase)	M = F	Text
Voluntary activation	M = F	Text
Injuries from weight training
Absolute number of injuries	M > F
Rate of injury—all body areas	M > F
Odds of injury—sprains, strains	M > F
Odds of injury—accident-related injury	M < F
Response to eccentric, muscle-damaging exercise
Force loss	M = F	Text
Muscle soreness	M = F	Text

^*F = female; M = male.

^†Age-dependent result; see text for details.

Muscle strength is greater in men than women. The magnitude of this sex differences depends on age, muscle group, and muscle contraction type. Grip strength is perhaps slightly greater in boys than girls, but the difference is not statistically different and seems to be smaller in more recent studies than older ones. By age 15 years, female grip strength is ∼75% of male grip strength. This finding is fairly consistent throughout history. By the fourth decade of life, when grip strength has peaked for both sexes, female grip strength is ∼60% of male grip strength.

The magnitude of the sex difference between men and women is larger in upper-body muscles than lower-body muscles. Female upper-body muscle strength is usually 50–60% of male upper-body strength. Female lower-body muscle strength is usually 60–70% of male lower-body strength. Female trunk strength is roughly 60% of male trunk strength. These sex differences in muscle strength are observed both in the general population and in competitive powerlifters. Irrespective of the muscle group assessed, the magnitude of the sex difference in muscle strength depends on the type of muscle contraction. Sex differences in strength are greater in concentric than eccentric and isometric contractions. When measured isometrically, the sex difference in strength is greatest at long muscle lengths. A sex difference in muscle endurance as measured by the repetitions-to-failure test with equal relative loads is not obvious. This is due to both a dearth of data available for some exercises and mixed results for other exercises.

The sex difference in muscle strength is most likely explained by the sex difference in muscle mass and size because voluntary activation is equal between men and women. Men have more muscle mass than women in absolute terms and also as a proportion of total body mass. Men carry a greater proportion of their muscle mass in their upper bodies. Muscle twitch forces evoked from electrical stimulation delivered to peripheral nerves are greater in men than women. This sex difference is also more pronounced in upper-body than lower-body muscles and corresponds to muscle-specific sex differences in muscle mass, muscle size, and volitional muscle strength.

A sex difference in muscle fiber type might also contribute to the sex difference in muscle strength. Evidence is mixed as to whether there is a sex difference in the relative number of type I and II fibers in various muscles. However, a sex difference is evident in the relative areas of whole muscles occupied by type I and type II muscle fibers. Type I fibers occupy a greater area of female than male whole muscle area. Type II muscle fibers occupy a greater area of male than female whole muscle area. Thus, the ratio of the area of type IIA to type I fibers is consistently greater in men than women. Such a sex differences in muscle fiber type might help to explain greater resistance to fatigue in women than men for some muscle endurance tasks.

Men participate in strength training more frequently than women. Higher male than female participation has been documented in nearly every country. For men, strength training ranks higher on their hierarchy of desired physical activities. Men prefer strength training over stretching, yoga, dance, and group aerobics, whereas women prefer the latter activities over strength training. Thus, sex differences in exercise preferences contribute to sex differences in strength training participation.

Men and women do not necessarily have the same preferences for the features of their exercise programs. Sex differences are not apparent for preferred exercise location or interpersonal contact during exercise. However, men are more likely to prefer exercise that involves competition, high-intensity, and strengthening or developing upper-body muscles. Men are also more likely than women to be comfortable with, and prefer the use of, free weights. Women are more likely to prefer lower-body exercise, exercise that is noncompetitive, of lower intensity, and supervised. The female preference for supervision likely stems from greater female inexperience, discomfort, and disinterest in using free weights.

Sex differences exist in motives or reasons for exercise. Men are more likely than women to be motivated by challenge, competition, social recognition and status, and improving muscle size and strength. Women are more likely than men to be motivated by improving physical appearance and attractiveness, muscle “toning,” and weight loss and management. Both sexes are equally motivated by enjoyment/fun, improved fitness, improved health, stress and mood management, and socializing with others.

Sex differences in exercise preferences and motives are likely driven by intrasexual competition and mate selection. Women generally find strong, muscular men most physically attractive, and men generally find women with lower body weights and lower waist-to-hip ratios most physically attractive. These mate preferences likely give rise to the drive for muscularity in men, which leads to greater male than female participation in activities or strategies designed to cause muscle hypertrophy (e.g., strength training and anabolic steroid use). Fear of getting “big and bulky,” coupled with a more pronounced desire to use cardiovascular exercise to lose or manage body mass, helps to explain less interest in strength training among women.

Men and women increase muscle size and strength after weeks of strength training. Relative improvements in upper-body, but not lower-body, strength are greater in young women than men and might be attributed to the more pronounced sex difference in upper-body than lower-body strength at baseline. Older adult women experience greater relative improvements in lower-body strength than older adult men. No sex differences in relative increases in muscle size are evident in younger or older adults. A sex difference in voluntary activation after weeks of strength training is unlikely based on the existing, albeit limited, literature.

Men account for most resistance training injuries that receive medical attention at hospitals. Men have higher injury rates, more sprains or strains, more fatalities, and more trunk injuries from resistance training than women. Women have higher rates of accident-related injuries and foot injuries. Among competitive lifters, few sex differences in injury rate, type and location exist. No sex difference exists in strength loss or soreness experienced after muscle-damaging exercise.

Practical Applications

Strength training programs should address individual goals. The guidelines for strength training prescriptions are sex-neutral (¹⁸), and the results from the current review do not constitute endorsement that the guidelines be sex-specific. Nevertheless, there may be instances when individual exercise prescriptions are not possible, and thus, knowledge of average sex differences might be helpful. One example is an exercise class. Irrespective of whether a class is single or mixed sex, knowledge of sex differences in strength training variables and outcomes can inform what class features are most appropriate or might be most enjoyable. Women will generally require lighter absolute loads, prefer exercises for the lower body and trunk, and prefer more supervision and instruction than men. Men will generally prefer upper-body exercises, higher exercise intensities, and challenge and competition (e.g., rankings or points systems that are publicly displayed for social recognition of achievement). Such sex-specific considerations might optimize enjoyment and adherence to exercise, which might then maximize improvements in health, physical fitness, and athletic performance. The results from the current review might also have implications for current discussions on sex and gender within exercise and sports science. First, there are clear differences in muscle anatomy and strength of the average man and woman. Thus, for most sports, allowing biological male subjects to compete against biological female subjects disadvantages female subjects unnecessarily and, in contact sports, might place female subjects at increased risk of injury. Second, tasks in the military, law enforcement, firefighting, and other occupations require maximal or repeated submaximal strength efforts against absolute loads. Men will generally perform these tasks more successfully and with lower effort because of their greater strength levels, although women can be expected to perform more similar to men on lower-body strength tasks. Both men and women can improve their performances on such tasks by increasing their strength through strength training. Third, as detailed elsewhere (³²⁷), some researchers have claimed that differences in physical activity rates between men and women are due primarily to social or environmental factors. The current review presents evidence that biological and intrapersonal factors contribute to the practice of certain types of physical exercise. Thus, aims of achieving sex “equity” or parity in strength training participation through public or organizational policy are shortsighted, given that men and women, on average, have different preferences and biological propensities.