Body Mass and Femur Length Are Inversely Related to Repetitions Performed in the Back Squat in Well-Trained Lifters

Abstract

Cooke, DM, Haischer, MH, Carzoli, JP, Bazyler, CD, Johnson, TK, Varieur, R, Zoeller, RF, Whitehurst, M, and Zourdos, MC. Body mass and femur length are inversely related to repetitions performed in the back squat in well-trained lifters. J Strength Cond Res 33(3): 890–895, 2019—The purpose of this research note was to examine whether relationships existed between anthropometrics (body mass, body fat percentage [BF%], and femur length) and descriptive characteristics (age and sex) with repetitions performed to failure at 70% of 1 repetition maximum (1RM) in the back squat. Fifty-eight subjects (males = 43, females = 15; age: 23 ± 3 years, training age: 5.5 ± 2.5 years, body mass: 80.65 ± 16.34 kg, BF%: 10.98 ± 3.53%, and femur length: 47.1 ± 2.6 cm) completed a 1RM squat followed by one set to failure at 70% of 1RM. Total repetitions performed at 70% of 1RM were 14 ± 4 (range: 6–26). Bivariate correlations showed significant inverse relationships between body mass (r = −0.352, p = 0.003), BF% (r = −0.278, p = 0.014), and femur length (r = −0.265, p = 0.019), with repetitions performed. No significant relationships existed between age and sex (p > 0.05), with repetitions performed. All these variables entered into a standard multivariate regression. The model R² was 0.200, and body mass had the largest influence (p = 0.057) because relative importance analysis demonstrated body mass to contribute to 43.87% of the variance (of the R²) in repetitions performed. No other variable was significant or approached significance (p > 0.05). Our results reveal that body mass, BF%, and femur length all are inversely related to repetitions performed at 70% of 1RM in the back squat.

Introduction

Percentage of 1 repetition maximum (1RM) or RM zones is commonly used to prescribe resistance training load (¹³). However, the previous literature has reported considerable between-individual differences for repetitions performed at similar relative intensities (¹²). For example, trained lifters have performed 29.9 repetitions to failure in the squat at 60% of 1RM, but with a SD of 7.4 (⁸). However, the reasons for these individual differences are largely unknown.

In theory, body mass and limb lengths should be inversely related to the ability to perform repetitions at submaximal intensities because body mass affects total load lifted and limb lengths affect load displacement. Furthermore, greater body mass is associated with greater body fat percentage (BF%) (⁵), and longer femurs and greater muscle lengths are associated with greater muscle damage (⁶) and impairment of muscle function during contraction (¹).

If anthropometric factors do affect repetitions performed, then load prescription can be further individualized through practical and easy assessments. However, to the best of our knowledge, there is no study that analyzes the relationship between anthropometric factors and repetitions performed in the back squat among trained males and females. Therefore, the purpose of this research note study was to examine whether relationships existed between body mass, BF%, and femur length with repetitions performed to failure at 70% of 1RM in the squat. We hypothesized that all anthropometric variables would have an inverse relationship with repetitions performed.

Methods

Experimental Approach to the Problem

Subjects reported to the laboratory for 1 day. Subjects then underwent anthropometric assessments (i.e., height, body mass, BF%, and femur length), according to the International Society for the Advancement of Kinanthropometry (ISAK) standards (⁹) followed by a 5-minute dynamic warm-up and squat 1RM testing. Squat 1RM testing was performed in accordance with the validated procedures and standards (¹³). Next, subjects had a 10-minute rest, and then they completed 2 single-repetition sets on the squat at 30, 40, 50, 60, 70, 80, and 90% of 1RM with 5-minute rest between the single-repetition sets, which was performed as part of a larger study. Subjects then rested for 10 minutes before completing 1 set on the squat to volitional failure with 70% of 1RM.

Subjects

Fifty-eight resistance-trained individuals were recruited (males = 43 and females = 15) (Table 1). For inclusion, subjects must have performed the back squat on an average of 1×·wk⁻¹ for ≥2 years and meet minimum strength requirements (males: 1RM ≥ 1.5× body mass and females: 1RM ≥ body mass). Subjects with contraindications to exercise (e.g., heart disease, serious musculoskeletal disorders, etc.) were excluded. Subjects were required to refrain from exercise for 48 hours before testing. Florida Atlantic University's institutional review board approved this investigation. Subjects gave written informed consent and completed health history and physical activity questionnaires (¹³).

Table 1.: Subject characteristics (N = 58).*†

Procedures

One Repetition Maximum Testing

Testing for 1RM was performed in accordance with previously validated procedures (¹³). To record the most accurate 1RM, the investigators used the average velocity (m·s⁻¹) by using a Tendo Weightlifting Analyzer (TENDO Sports Machines, Trencin, Slovak Republic), and subjects reported the rating of perceived exertion (RPE) according to the repetitions in reserve (RIR)-based RPE scale (¹³) to determine the load for each subsequent attempt. Each subject was given 5–7 minutes of rest between 1RM attempts. A 1RM was accepted as valid if one of 3 conditions was met: (a) Subject reporting a “10” on the RPE scale and the investigator determined an additional attempt with increased load would be unsuccessful, (b) subject reported a “9.5” RPE and then preceded to the subsequent attempt with a load increase of 2.5 kg or less, and (c) subject reported an RPE of ≤9 and failing the subsequent attempt with a load increase of 5 kg or less. The squat and bench press were performed under the rules and regulations of United States of America Powerlifting (¹⁰), which included a depth of the hip crease passing below the top of the knee.

Height, Body Mass, and Body Fat Percentage

Height (cm) was measured using a wall-mounted stadiometer (SECA, Hamburg, Germany), and body mass (kg) was assessed by a calibrated digital scale (Mettler-Toledo, Columbus, OH, USA). The average sum of 2 measurements of skinfold thickness acquired from 3 sites (males: abdomen, front thigh, and chest; and females: triceps, suprailiac, and thigh) on the right side of the body was used to estimate BF% (⁴); if any site was >2 mm different between measurements, then a third measurement would be taken.

Femur Length

Femur length (cm) was assessed to the nearest 0.1 cm with an anthropometric tape measure (Realmetbcn, Barcelona, Spain) and recognized as the distance between the marked trochanterion and tibiale laterale landmarks on the right leg. To measure femur length, subjects assumed a standing position with feet together and arms folded across the thorax. Reported femur length was determined as the average of 2 measurements; if measurement differences were >2 mm, a third measurement would be taken and the median of all 3 measurements would be used. Before data collection, technical error of measurement (TEM) was conducted (³) by one investigator recording femur lengths of 20 individuals on 2 visits separated by at least 24 hours. The intrainvestigator relative TEM was 1.37%, an acceptable reliability rating (⁷).

Repetitions to Failure

Subjects performed repetitions until volitional failure at 70% of 1RM. Volitional failure was determined as the subject either failing on a repetition or recording an RPE value of 10 after a repetition (¹³). The number of successful repetitions was recorded as the dependent variable.

Statistical Analyses

Total repetitions performed and the anthropometric characteristics are presented as mean ± SD. Pearson's correlations examined relationships between each independent variable with repetitions performed. A standard multivariate regression and relative importance analysis determined the relative contribution of each variable (except 1RM and relative strength due to violation of the multicollinearity principle) to repetitions performed. All regressions were calculated using SPSS version 24 (IBM Co., New York, NY, USA) with significance at p ≤ 0.05. The relative importance analysis was calculated using the “lmg” method in the R software package “relaimpo”. Finally, an independent t-test was used to compare repetitions performed between the 10 subjects with the longest femurs and the 10 subjects with the shortest femurs.

Results

Repetitions Performed and Bivariate Correlations

Total repetitions performed at 70% of 1RM were 14 ± 4 (range: 6–26). Body mass, BF%, and femur length all displayed a significant (p < 0.05) and inverse relationship to repetitions performed. Subjects' age, sex, 1RM, and 1RM/body mass ratio were not significantly related to repetitions performed. The r and p values for all variables are in Table 2.

Table 2.: Bivariate correlations between descriptive variables and repetitions performed at 70% of 1RM.*†

Multivariate Regression Analysis

Absolute and relative strength were not included in this analysis due to violation of the multicollinearity principle. The R² for the entire model was 0.200. Body mass approached significance (p = 0.057), and no other variables showed a significant relationship (p > 0.05). The associated coefficients, t values, p values, and tolerance statistics are in Table 3. The relative importance of body mass was the greatest of any variable (43.87% of the R²) (Table 3).

Table 3.: Results of the forced entry analysis for each variable.*†

Femur Length Comparison

Subjects with the 10 longest femurs (51.04 ± 1.30 cm) performed significantly fewer (p = 0.027) repetitions than the 10 subjects with the shortest femurs (43.32 ± 1.24 cm). The repetitions performed by the short femur group were 19 ± 6, whereas the long femur group performed 14 ± 3 repetitions (Figure 1).

Figure 1.: A, B) Plots of correlation between body mass and squat repetitions performed at 70% of 1RM (A) and correlation between femur length and squat repetitions performed at 70% of 1RM (B). 1RM = 1 repetition maximum.

Discussion

The aim of this research note was to assess whether relationships existed between anthropometric variables and repetitions performed at 70% of 1RM in the back squat. Our hypothesis was supported because all variables showed significant and inverse relationships with repetitions performed, with body mass having the greatest influence. However, the multivariate regression data showed no significant relationships and only body mass approaching significance, whereas all variables together explained 20% of the variance in repetitions performed.

The concept of repetitions performed at a given intensity being individualized is not new (^12,13); however, this is the first study to report a wide-ranging individual variance among a mixed-sex well-trained population in the back squat. Presently, 14 ± 4 repetitions were performed in the squat at 70% of 1RM with a range of 6–26 repetitions, indicating a large between-individual variance in repetitions at a given intensity. Our findings reveal 2 important outcomes: (a) More repetitions can be performed at a given intensity than previously reported (²) and (b) lighter individuals can perform more repetitions at a given percentage of 1RM.

Haff and Triplett (²) have proposed that 11 repetitions can be performed at 70% of 1RM, yet our data report a range of 6–26 repetitions across 58 subjects, thus a percentage of 1RM tables are not applicable to everyone. If an entire team was prescribed 3 sets of 10 repetitions at 70% of 1RM, our results suggest a great variance in between-individual capabilities, thus a wide-range of recovery time due to some completing the sets at or close to failure, whereas others completing the sets far from failure. The unstandardized beta of −0.112 with a standard error of 0.058 for body mass suggests that for every 1-kg change in body mass, there would be a reciprocal change of 0.11 ± 0.06 repetitions performed. However, we propose caution when using that exact interpretation for programming purposes, as it is likely that there are between-individual differences in how much an equivalent change in body mass would alter repetition performance. In other words, applying the strict interpretation of a change of 0.11 repetitions for every 1-kg change in body mass is likely inappropriate and risky for some, thus we advise against using these data to predict repetition performance. Nonetheless, the general findings are unsurprising because greater body mass contributes to a greater load lifted between individuals despite relative barbell load being the same.

The inverse relationship between femur length and repetitions performed was predictable because longer muscle lengths during eccentric muscle actions are associated with greater damage (⁶), and such damage can result in impaired muscle function (¹) and reduced repetition performance. Furthermore, Vigotsky et al. (¹¹) demonstrated that a greater total height is associated with a lower 1RM, providing indirect support for our findings that greater total physiological work decreases the performance. Interestingly, in this study, an independent t-test was used to compare repetitions performed by the 10 subjects with the longest femurs (51.04 ± 1.30 cm) vs. the 10 subjects with the shortest femurs (43.32 ± 1.24 cm). This analysis showed significantly greater (p = 0.027) repetitions performed by the short femur group (19 ± 6) vs. the long femur group (14 ± 3). Furthermore, it does not seem that subjects' sex influenced this outcome because there were 4 females and 6 males in the short femur group, and 3 females and 7 males in the long femur group. Thus, longer femurs and greater resistance moment arms may decrease repetitions performed in the squat but may not affect repetitions performed to the same degree as body mass. However, because femur length and body mass are closely related, it is difficult to truly delineate influence between these factors.

Limitations of this study include only examining one exercise and intensity, and a few variables. Thus, it cannot be known whether the same relationships exist with other exercises (bench press and deadlift) and at other percentages of 1RM or other anthropometric measurements. Furthermore, there are likely other factors, which affect squat performance (i.e., stance width, bar position, etc.), that were not examined. In addition, it is possible that the 1RM protocol affected the number of repetitions performed; however, the average amount of repetitions performed (14 ± 4) was considerably larger than previously reported at 70% of 1RM (²), which suggests that the 1RM protocol did not harm repetitions performed. A possible critique could also be that individuals of varying relative strength levels fatigued differently and impacted the results; however, our results show no relationship between relative strength and repetitions performed, suggesting that this possible limitation did not impact the results.

In summary, our findings in a large well-trained (training age = 5.5 years) mixed-sex sample (n = 58) demonstrate that body mass is a primary variable affecting repetition performance in the back squat. However, it should be reiterated that despite the present model explaining 20% of the variance in repetitions performed, it left 80% of the variance unexplained. Thus, although 20% is a positive step toward explaining the between-individual repetition variance, more data are needed to fully elucidate this phenomenon. Future research is needed to directly test the relationship between change in body mass and change in repetitions performed. In addition, future investigations should examine the relationship between anthropometrics and repetitions performed at other intensities and in other exercises (i.e., bench press and deadlift).

Practical Applications

Practically, our findings can be used as a programming guide to help equate for effort between individuals. It is true that the RIR-based RPE scale could be used to equate for effort because this research note shows that repetitions allowed at a given intensity are individual. However, in a team setting, RIR-based RPE ratings may be inaccurate among some individuals (¹³). In general, our analysis suggests that for every 1-kg change in body mass, there would be an inverse change of approximately 0.10 in repetitions performed; thus, a body mass change of 10 kg would theoretically lead to a change of 1 repetition performed based on the present data. However, as previously stated, this strict interpretation should be used with caution because it is unlikely that those exact change values would apply to everyone; thus, inappropriate load prescription could be implemented for some. Furthermore, our model revealed that body mass only approached significance in the regression model, thus coaches and athletes should proceed with caution at this point if using these results to make training prescription decisions. Furthermore, these data are unique, in that, not only do they provide information to in part explain the between-individual difference in repetition performance, but also provide information on truly well-trained individuals because males had a relative squat 1RM of 1.88 times body mass and females of relative 1RM of 1.29 times body mass. Therefore, these findings are uniquely applicable to well-trained athletes. We acknowledge the limitations of the current correlational data; however, because the relevant measurements (i.e., body mass and femur length) are practical, these measurements can be easily implemented by coaches and athletes, which increase the applicability of the findings. Ultimately, we provide for the first time the evidence regarding why repetitions at a given intensity are individual in the back squat, which makes these findings applicable. However, because of between-individual variability, we believe that it is risky at this juncture for coaches and athletes to use these data to predict repetition performance. A large variance in repetition performance remains unexplained, thus more data are needed before practitioners can accurately predict squat repetition performance based on these factors.