Most fitness professionals can easily explain the beneficial effects of aerobic activity with respect to heart health, cardiorespiratory function, endurance fitness, and physical well-being. Certainly, increased aerobic capacity is a major objective of a comprehensive conditioning program. However, it is important to realize that muscles are the engines of our bodies. Muscles are where combustion occurs, where energy is released, where power is produced, and where movement originates. Muscles have long been recognized as key factors in physical performance and athletic achievement. Essentially all serious sports participants include regular resistance exercise in their training programs. However, recent research has shown that strength training has much greater application than improved sports performance or even enhanced physical fitness. Clearly, our muscular condition has a major influence on both our physical and mental health.
It is likely that age-related muscle loss is a starting point for a cascade of physiological problems, including bone loss, metabolic rate reduction, fat gain, diabetes, metabolic syndrome, heart disease, and all-cause mortality (57). Adults who do not perform resistance exercise lose almost 5 lbs of muscle every decade before age 50 years (19) and up to 10 lbs of muscle every decade after age 50 years (38). With this in mind, let’s examine the evidence-based benefits of resistance training for generally healthy individuals as well as for adults who have common risk factors.
Numerous studies have shown that muscle mass may be increased in adults of all ages (26), including individuals in their 90s (17,56). A representative research study with more than 1,600 participants (aged 21 through 80 years), revealed a 3-lb increase in lean (muscle) weight after 10 weeks of standard strength training (58), and a follow-up study showed approximately twice as much lean weight gain after 9 months of resistance exercise (59). Similar increases in lean weight were attained for strength training frequencies of 2 and 3 days per week and among all of the age groups.
Muscle loss (sarcopenia) is associated with bone loss (osteopenia) (27), which explains partly why approximately 45 million Americans suffer from insufficient bone mass (37). Women who do not engage in resistance exercise may experience a 1% to 3% per year reduction in bone mineral density (BMD) (55). Fortunately, several longitudinal studies have shown significant increases in BMD after participation in progressive strength training programs (57). Research reviews by Wolff et al. (62) and by Going and Laudermilk (20) revealed that resistance exercise increased BMD between 1% and 3% in both premenopausal and postmenopausal women. In one study, resistance-trained subjects increased BMD by 1% during a 12-month period (38) and, in another study, resistance-trained subjects who also consumed supplemental protein, calcium, and vitamin D increased BMD by 1% during a 9-month period (59). A 2-year study by Kerr et al. (31) showed more than 3% improvement in BMD compared with the control group. Although resistance exercise can increase BMD, the musculoskeletal effects are relatively site specific and termination of strength training results in reversal of the BMD gains (60).
At rest, every pound of untrained muscle uses between 5 and 6 calories per day for protein breakdown and synthesis (61). However, every pound of resistance-trained muscle uses approximately 9 calories per day for more extensive protein breakdown and repair processes (48). Resistance exercise produces tissue microtrauma that requires relatively large energy supplies for muscle remodeling. Research reveals that a single strength training session can increase resting energy expenditure by 5% to 9% for 3 days after the workout. In a study by Hackney et al. (21), a high-volume resistance training session (eight sets of eight exercises) raised resting energy expenditure an average of 9% in untrained subjects and an average of 8% in trained subjects during a 72-hour postexercise period. In a similar study by Heden et al. (22), a low-volume resistance training session (1 set of 10 exercises) raised resting energy expenditure an average of 5% in beginning participants during a 72-hour postexercise period. Many other studies have shown approximately a 7% increase in resting metabolic rate after several weeks of resistance training (26).
Most strength training studies that have shown significant increases in lean weight (∼1.0 lbs/month) also have demonstrated concurrent decreases in fat weight (almost 1.5 lbs/month) (26,56,58). Resistance exercise also has been effective for reducing intra-abdominal fat in older men and women (51,52).
Some people have questioned how three relatively brief (20-minute) circuit strength training sessions a week can result in a 1.5-lb/month fat loss. Assuming approximately 200 calories used during a 20-minute resistance training circuit, the total monthly energy expenditure directly caused by resistance exercise is only 2,400 calories (12 workouts × 200 calories per session). However, assuming the lowest reported increase in resting metabolism associated with strength training (100 calories/day) (22), the total monthly energy expenditure indirectly caused by resistance exercise is 3,000 calories (30 days × 100 calories/day), for a total calorie use of 5,400 calories (2,400 calories plus 3,000 calories), which is equivalent to 1.5 lbs of fat. In their comprehensive review article, “Evidence of Resistance Training as a Treatment Therapy in Obesity,” Strasser and Schobersberger (48) concluded that strength training is recommended in the management of obesity and related metabolic disorders. These include elevated resting blood pressure, elevated plasma cholesterol, elevated plasma glucose, and large waist girth — all of which are associated with the development of Type 2 diabetes and cardiovascular disease (48).
Muscle loss and fat gain increase the risk of Type 2 diabetes, a disease that is predicted to affect one of three adults by the middle of this century (8). Numerous studies have demonstrated significant improvements in insulin sensitivity and glycemic control as a result of resistance exercise (24). As previously presented, strength training also has been shown to reduce abdominal fat, which seems to be associated with insulin resistance in aging adults (32). An extensive research review by Flack and associates (18) concluded that resistance exercise may be an effective intervention for counteracting age-associated changes in insulin sensitivity and for preventing Type 2 diabetes in middle-aged and older adults. A meta-analysis conducted by Strasser and colleagues (47) likewise concluded that strength training should be recommended for both the prevention and management of Type 2 diabetes.
There is evidence that resistance exercise may be preferable to aerobic activity for improving insulin sensitivity (11) and for lowering HbA1c (11), and that higher-volume/higher-intensity strength training protocols may be more effective than lower-volume/lower-intensity workouts (18). The American Diabetes Association recommends resistance exercise for all of the major muscle groups, 3 days/week, progressing to 3 sets of 8 to 10 repetitions each, performed at a higher level of training intensity (46).
Several studies have shown significant reductions in resting blood pressure (systolic and diastolic) after two or more months of regular resistance exercise (circuit and standard strength training) (28). Combined resistance and endurance exercise also has demonstrated favorable blood pressure responses (58). In a large 10-week study, participants who performed 20 minutes of strength training and 20 minutes of aerobic activity 3 days/week experienced a significant 4.6-mmHg decrease in resting systolic blood pressure, and those who performed the same exercise program 2 days/week experienced a significant 2.3-mmHg decrease in resting systolic blood pressure (58). Kelley and Kelley (29) conducted a meta-analysis of relevant research and concluded that resistance exercise is effective for reducing resting blood pressure. Another meta-analysis of randomized controlled trials determined that resistance exercise resulted in similar blood pressure reductions as aerobic activity, averaging 6.0 mmHg lower systolic pressure and 4.7 mmHg lower diastolic pressure for the studies reviewed (14). Likewise, a 2010 review published in ACSM’s Current Sports Medicine Reports concluded that the effects of resistance training on resting blood pressure were comparable to those of aerobic training (41). However, recent guidelines from the American Heart Association (3) stated that “Although resistance training could yield modest reductions in blood pressure, data are insufficient to support a specific resistance training program” (page 916). Nonetheless, considering that about 35% of Americans have hypertension, the blood pressure benefits associated with strength training should not be overlooked (9, 40).
An even larger number of Americans (∼45%) have undesirable blood lipid profiles (35). Some studies have not found an association between resistance exercise and blood lipid levels (33), and the American Heart Association does not believe that the data are sufficient to support a specific resistance training program (3). However, other research in this area has revealed improved blood lipid profiles after participation in strength training programs (30). Fahlman and colleagues (16) found that resistance training was effective for improving triglyceride levels, low-density lipoprotein (LDL) cholesterol, and high-density lipoprotein (HDL) cholesterol in older women. The American College of Sports Medicine Position Stand on Exercise and Physical Activity for Older Adults (1) reports that resistance training has been shown to reduce triglyceride levels by 11% to 18%, to decrease LDL cholesterol by 13% to 23%, and to increase HDL cholesterol by 8% to 21% (page 1,519). Research indicates that combined strength training and aerobic activity may have a more favorable impact on blood lipid profiles than either exercise performed independently (42).
Research clearly demonstrates that resistance exercise can reverse some of the degenerative processes associated with inactive aging, including muscle loss (50), movement control (5), functional abilities (25), physical performance (23), and walking speed (44). The functional benefits of strength training extend to nonagenarians (17) and the frail elderly (56). In a study of nursing home residents (mean age, 89 years), 28 relatively brief sessions of resistance exercise (twice weekly for 14 weeks) resulted in a nearly 4-lb increase in lean weight, a 3-lb decrease in fat weight, a 60% increase in overall muscle strength, and a 14% improvement in functional independence (56).
Most of the research on exercise and cognition has featured aerobic activity alone or combined endurance and strength training (39). A meta-analysis by Colcombe and Kramer (13) demonstrated that programs of resistance training and aerobic exercise were more effective than aerobic exercise alone for improving cognitive function in inactive older adults. However, studies restricted to only resistance training interventions also have shown significant improvements in various cognitive abilities (12).
Studies on strength training and psychological measures have shown enhanced self-esteem among older adults (54), younger adults (53), women (10), cancer patients (15), and cardiac rehabilitation participants (6). Annesi and Westcott (4) found that 10 weeks of combined strength and endurance training significantly improved physical self-concept, total mood disturbance, depression, fatigue, positive engagement, revitalization, tranquility, and tension in adults and older adults. In a classic study by Singh and colleagues (45), more than 80% of the previously depressed elderly individuals who performed standard resistance exercise 3 days/week were no longer clinically depressed after completing 10 weeks of the strength training program. Based on the studies presented, it would seem that resistance training may have a positive influence on cognitive and psychological components of mental health.
O’Connor and colleagues (39) completed a comprehensive review of the research on resistance exercise and mental health, including the effects of strength training on low-back pain, arthritic discomfort, and fibromyalgia because these debilitating physical conditions also may have a negative influence on various psychological factors. Research supports the role of resistance exercise for reducing low-back pain (43), for decreasing arthritic discomfort (34), and for easing the symptoms of fibromyalgia (7).
One aspect of aging is a reduction in muscle mitochondrial content and function. Circuit strength training has been shown to increase both muscle tissue mitochondrial content and oxidative capacity (49). A most interesting study by Melov and associates (36) examined genetic changes in the muscle mitochondria of older adults after 6 months of standard resistance exercise. The research participants (mean age, 68 years) experienced favorable changes in 179 genes associated with age and exercise performance. After the strength training program, the older adults’ mitochondrial gene expression was similar to that of moderately active young adults (mean age, 24 years). The researchers concluded that specific aging factors in skeletal muscle may be reversed through progressive resistance exercise.
RECOMMENDATIONS FOR BEGINNING STRENGTH TRAINERS
Regardless of age, sex, or apparent physical condition, people who wish to begin a resistance training program should complete a health-screening questionnaire, such as the one recommended on page 25 of ACSM’s Guidelines for Exercise Testing and Prescription, 9th edition (2). Everyone who reveals two or more of the following risk factors should be required to obtain medical clearance (and preferably physician’s guidelines) for participation in the strength training program (see Table 2.2 on page 27 of ACSM’s Guidelines for Exercise Testing and Prescription, 9th edition) (2) for detailed defining criteria of these cardiovascular disease risk factors).
* Age (men, ≥45 years; women, ≥55 years)
* Family history of heart disease
* Cigarette smoking
* Sedentary lifestyle
* Obesity (body mass index, ≥30)
* Hypertension (systolic, ≥140 mmHg; diastolic, ≥90 mmHg)
* Dislipidemia (LDL cholesterol, ≥130; HDL cholesterol, <40)
It should be noted that an HDL cholesterol reading of 60 or higher cancels out one of the risk factors. It also is suggested that individuals with uncontrolled hypertension obtain physician recommendations for performing resistance exercise.
The most recent ACSM resistance training recommendations call for relatively brief workouts that provide two to four sets of 8 to 12 repetitions each for the major muscle groups by performing 8 to 10 multijoint exercises two or three nonconsecutive days per week (see Table 7.6 on page 185 of ACSM’s Guidelines for Exercise Testing and Prescription, 9th edition) (2). For example, beginners who perform one set of the nine machine exercises in the Table would experience two sets of resistance training for most of their major muscle groups (quadriceps, hamstrings, pectoralis major, anterior deltoids, latissimus dorsi, teres major, rhomboids, middle trapezius, posterior deltoids, triceps, biceps, erector spinae, rectus abdominis).
The key to successful strength training experiences is sensible progression. A double progressive training protocol begins with a resistance that can be performed for at least eight repetitions (∼80% of maximum resistance for most exercises). The client continues to train with this resistance until 12 good (full range, controlled speed) repetitions can be completed (∼70% of maximum resistance for most exercises). At this point, progression in repetitions should be changed to progression in resistance (∼5% heavier weight load) that will reduce the number of repetitions and permit a new progression in repetitions within the 8 to 12 range.
Various modes of resistance are effective for increasing muscle mass and strength. For example, free-weight training offers many multiple joint exercises performed from a standing position that concurrently activate muscles of the legs, core, upper body, and arms while incorporating balance, coordination, and movement control. Medicine ball strength training enables safe and effective power production with minimal joint stress because the resistance can be released at the end of each explosive exercise action. Resistance bands are easy to use and are especially well suited for pressing exercises as the increasing resistance roughly matches the increasing force output during band extension in squats, chest presses, and shoulder presses. Well-designed resistance machines provide supportive structure, proper movement patterns, and resistance curves that parallel strength curves in most exercises. Older individuals, people with orthopedic limitations, and those who have issues with balance, coordination, or flexibility may be advised to begin strength training with resistance machines. It also is recommended that older adults do not hold their breath or hold the resistance in a static position for more than a few seconds because both of these procedures may cause excessive blood pressure elevation.
Muscle loss may initiate a cascade of health issues including bone loss, metabolic rate reduction, fat gain, diabetes, metabolic syndrome, heart disease, and all-cause mortality. Resistance training has been shown to add muscle mass, increase bone mineral density, raise resting metabolic rate, decrease body fat, improve glycemic control, reduce resting blood pressure, improve blood lipid profiles, enhance physical function, improve mental health, and reverse specific aging factors in skeletal muscle. Beginning participants should be assessed for cardiovascular disease risk factors and should be trained in accordance with the ACSM recommendations for performing safe and effective resistance exercise.
BRIDGING THE GAP
Age-related muscle loss (3% to 8% per decade) initiates a cascade of undesirable physiological responses, including bone loss, metabolic slow down, and fat gain that are associated with many prevalent diseases and disabilities. Resistance exercise has been shown to be effective for reversing muscle loss, increasing bone density, recharging resting metabolism, decreasing body fat, improving glycemic control, reducing resting blood pressure, improving blood lipid profiles, facilitating physical function, enhancing mental health, reversing aging factors, and attenuating low-back pain and arthritic discomfort. Resistance training health benefits may be attained by performing 2 or 3 weekly workouts of 8 to 10 multijoint exercises using loads that permit 8 to 12 controlled repetitions.
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