As shown in this symposium on "Resistance Training for Health and Disease," resistance training is well established as an effective method for developing musculoskeletal strength and is prescribed for fitness and the prevention and rehabilitation of orthopedic injuries (3,6-8,26,50). More recently, resistance training has been considered as a modality used for health purposes (55). Recognizing that various segments of the population may have special limitations (i.e., cardiac, frailty, and/or orthopedic complications), major health organizations including the American College of Sports Medicine (ACSM) (6,7), the American Heart Association (AHA) (27), the American Association for Cardiovascular and Pulmonary Rehabilitation (AACVPR) (3), and the Surgeon General (55) have developed resistance exercise guidelines appropriate for various groups. In recent years, resistance training program guidelines have been developed specifically for elderly persons (50) and patients with cardiovascular disease (3,7,27) as well as for healthy sedentary and physically active adults (6,7). Table 1 summarizes the guidelines, standards, and position statements for exercise prescription for resistance training established by various health organizations. The importance of a well-rounded program including aerobic endurance exercise and resistance training is well recognized, but the purpose of this paper will focus on the latter component. A brief history, the scientific basis for resistance exercise prescription as outlined in the recommended guidelines for the various segments of the population, and the implications for future research in the area of resistance training will be addressed.
HISTORICAL PERSPECTIVE: RESISTANCE TRAINING GUIDELINES
As outlined by Carpenter and Nelson (17) in this symposium series, resistance training was not recommended for rehabilitation or athletic performance until the 1950s and 1960s or for adult fitness programs until the 1970s. The evolution of formal guidelines can be traced to the post World War II era, when army physician DeLorme incorporated heavy progressive resistance exercises in rehabilitation programs designed for orthopedically disabled veterans (20). DeLorme and Watkins (21) emphasized the use of heavy resistance and a low number of repetitions to develop muscular strength, and light resistance and a high number of repetitions to develop muscular endurance. As a result of the improved recovery from injuries/accidents and increased strength and muscle mass, resistance training gained formal recognition in the medical community. Studies conducted during the 1950s and 1960s began manipulating and evaluating training volume variables (sets, repetitions, frequency of training, intensity, and rest periods) (10-12,16,40,45,58,59), the results from which formulate the basis for many of the current resistance exercise guidelines.
In the 1960s and 1970s, Cureton (19) and others (31) promoted the importance of a well-rounded program including aerobic endurance exercise and resistance training for adult fitness programs. During the same time period, a growing body of epidemiological research indicated a strong link between aerobic endurance activities and the prevention of cardiovascular disease (28,37,46). These research findings, in addition to the publication of Aerobics(18), Shorter winning the 1972 Olympic marathon, and the development of Masters Track and Field (first national meet in 1971), popularized aerobic endurance exercise. Unfortunately, health and fitness became synonymous with aerobic exercise and the perceived need for muscular strength and endurance training appeared to decline.
In 1978, ACSM issued its original position statement entitled "The Recommended Quantity and Quality of Exercise for Developing and Maintaining Fitness in Healthy Adults" (4). Reflecting the trends of the time, the focus of the statement was on establishing exercise guidelines for developing and maintaining cardiorespiratory fitness and body composition. The lack of research quantifying the amount of resistance training needed for the average adult was the main reason for its omission in the 1978 ACSM statement, not because it was felt unimportant (personal communication, M. L. Pollock, May, 1993). It was and remains the policy of the ACSM that position stands must be accompanied by research documentation. Unfortunately, the omission of resistance training guidelines was interpreted as a lack of importance. In 1980, the American Alliance for Health, Physical Education, Recreation, and Dance (AAHPERD) published its new Health Related Fitness Test Manual(1). The manual excluded upper body strength test items, implying their lack of importance.
In the early 1980s, health clubs and fitness/wellness centers recognized the impact of resistance training on athletic performance and general fitness. By the mid-1980s, the medical community had begun to recognize the potential health value of resistance training on functional capacity and other health-related factors (e.g., bone health, basal metabolism, weight control, and low back health). At this time, research concerning resistance training and its quantification for the average participant intensified. In 1989, AAHPERD added upper body strength training and testing as an integral component in their Physical Best Program (43). In 1990, ACSM (5) recognized the importance of the comprehensive fitness program and added a resistance training component to the 1978 Position Statement. As mentioned earlier, most major health organizations currently recommend resistance training as part of a comprehensive exercise program for health and fitness (Table 1). Thus, from 1965 to 1995, Cureton's (19) concept of a comprehensive fitness program had come full circle.
SCIENTIFIC BASIS FOR RESISTANCE EXERCISE PRESCRIPTION
The effectiveness of a resistance training program is dependent upon several factors including frequency, volume of training (sets × repetitions × resistance), and mode of training (free weight vs variable resistance machines; dynamic (i.e., isotonic, isokinetic) vs isometric exercises; concentric vs eccentric contractions) (26,29). When prescribing an resistance exercise program, the clinician or fitness instructor must decide what constitutes an optimal balance of these factors to maximize benefits and needs to consider the individual's current age, health status, fitness level, rationale for strength development, and personal goals. Programs prescribed for competitive athletes that often include exercises designed specifically to improve the development of explosive power (i.e., clean and jerk, snatch, plyometrics) are usually not appropriate for sedentary middle-aged adults, elderly persons, or patients with chronic disease(s). The major health organizations recognized the need for developing resistance exercise guidelines for specific segments of the population (Table 1). Although these guidelines provide the basis for prescribing individualized exercise programs, the basic components common to all resistance exercise programs provide the framework for resistance exercise prescription, regardless of the intended population.
Repetition maximum (RM). One common factor in all effective strength training and rehabilitation programs is the inclusion of at least one set of the maximal or near-maximal number of repetitions possible for each exercise performed (26,29). The amount of weight to be used should be based on a percentage of the maximum amount of weight that can be lifted one time, generally referred to as a one-repetition maximum (1-RM). The maximum number of repetitions performed before fatigue prohibits the completion of an additional repetition, is a function of the weight load used, is referred to as a repetition maximum (RM), and generally reflects the intensity of the exercise. Consequently, a weight load that produces fatigue on the third repetition is termed a three-repetition maximum (3-RM) and corresponds to approximately 85% of the weight that could be lifted for a 1-RM. The number of repetitions performed to fatigue is an important consideration in designing a resistance training protocol, with the greatest strength gains appearing to result from resistances yielding 4- to 10-RM (26,42). Increasing the number of repetitions to 12- to 20-RM by decreasing the relative amount of resistance will favor increases in muscular endurance (26). In this regard it appears that the intensity of resistance training is the most important factor for developing muscular strength (8,42), while the total training volume (sets × repetitions × resistance) is more important for the development of muscular endurance and muscle mass (26,44,52,55). While the term "high-intensity" resistance training is usually reserved for 1-RM to 6-RM training loads, programs that emphasize exercising with a resistance that allow 8-12 repetitions are traditionally classified as "moderate intensity." Moderate intensity programs are usually recommended for most of the adult nonathletic populations including programs designed for adult fitness, health maintenance, and orthopedic rehabilitation. Moderate intensity programs recommended for cardiac patients and the elderly and more frail persons use 10/12 to 15 repetitions to fatigue.
Single versus multiple sets. The volume of training is a product of the number of sets performed for each exercise, the number of repetitions completed within each set, and the amount of weight (resistance) lifted. Although three sets of 6-12 repetitions performed 3 d·wk−1 is a typical exercise prescription for many resistance training programs, the optimal number of sets of an exercise to develop muscular strength remains controversial. It is surprising that there is a lack of well-controlled studies comparing single versus multiple set resistance training programs. Table 2 summarizes the results from resistance training studies comparing the volume of training using a variety of muscle groups. As depicted in Table 2, only one study has found a multiple set protocol to elicit greater strength gains than a single set (10), whereas the majority of studies indicate there is not a significant difference (33,44,49,52,53,56,57). The results of Berger's study (10), from which originated the basis for prescribing three sets of 6-10 repetitions, indicated that three sets of 6 to 10 repetitions was superior to one or two sets with similar repetitions following 12 wk of bench press exercise performed 3 d·wk−1. When comparing Berger's ANCOVA data, the groups started with a 1-RM bench press of 56.6 kg and finished with 69.3 kg (22.4% increase; 1 set group), 68.9 kg (21.8% increase; 2 set group), and 70.9 kg (25.3% increase; 3 set group). Although there was a statistically significant difference, the magnitude between the groups training with one set versus three sets was small (2.9%). Furthermore, no studies, including the Berger study, indicate that for the first 3-4 months of an resistance training program two sets are superior to one set. Recently, a study conducted by Starkey et al. (53) concluded that one set of moderate intensity resistance training (approximately 10 repetitions to volitional fatigue) was as effective as three sets (14 wk, 3 d·wk−1) for increasing knee extension and knee flexion dynamic strength and isometric torque and muscle thickness in previously untrained adults.
With the exception of the Berger study (10), the literature supports the recommendation of prescribing single set programs performed to fatigue and indicates that quality (intensity) and not the quantity (volume) of resistance training may be the most important factor for developing muscular strength in sedentary persons (44,52,54). Despite the variety of muscle groups tested (pectorals, biceps, lumbar extensors, quadriceps, etc.), most studies have produced similar results (Table 2). It should be noted that most of the studies described were conducted over a 4- to 20-wk period, and longer duration studies may show greater strength gains with multiple set programs. However, the existing literature clearly indicates that for the first 3-4 months of resistance training, single set programs are equally effective as multiple set programs for improving muscular strength in previously untrained persons.
In addition, the amount of time required to complete a single set program is substantially less than one-half the time required to complete multiple set protocols. Messier and Dill (44) reported that the time required to complete a three-set free weight resistance training program averaged 50 min compared to the 20 min for a one-set group. This time efficiency should generally translate into improved exercise program compliance. Considering the similarities in strength gains for single and multiple set programs, single set programs are recommended because they are less time consuming, more cost efficient, and produce similar health and fitness benefits in the untrained person (43,46).
Frequency of training. The frequency of training for a muscle group is also an important component of a resistance training program design (15,22,26,30,32). The rest period must be sufficient to allow for muscular recuperation and development and to prevent overtraining. However, too much rest between training sessions can result in detraining. A 48-h rest period between concurrent training sessions is generally recommended (26), which corresponds with a 3 d·wk−1 frequency of training guideline for individual muscle groups. Although 3 d·wk−1 of resistance exercise is generally recommended for maximal strength gains, research indicates that isolated muscle groups are unique in their trainability and adaptability to resistance training (22,32,51). Table 3 summarizes the results from resistance training studies comparing frequency of training using a variety of muscle groups.
Two studies evaluating the effects of frequency of training have shown that four or more training sessions per week produced optimal strength gains in several muscle groups (30,36). Using the standard bench press exercise, Gillam (30) indicated that training 5 d·wk−1 over a 7-wk period was superior to 1, 2, 3, or 4 d·wk−1 training regimens. Interestingly, training 3 or 4 d·wk−1 produced similar results which were significantly greater than those obtained by the groups training 1 or 2 d·wk−1. Hunter (36) and Henderson (34) also found that increasing the frequency of bench press training to 4 and 3 d·wk−1, respectively, produced greater strength gains than lesser frequency protocols. In contrast, Berger (13) found that bench pressing either 2 or 3 d·wk−1 produced similar strength gains over the course of 12 wk. Similar findings have also been reported for studies evaluating strength gains in the lower limb muscles. Braith et al. (15) found 3 d·wk−1 to be superior to 2 d·wk−1 in increasing quadriceps (knee extension) strength and an earlier study by Barham (9) showed that performing the squat exercise 3 d·wk−1 was as effective as 5 d·wk−1, and that both training frequencies were superior to squatting 2 d·wk−1.
While the chest, arms, and legs may require a training frequency of 3 d·wk−1 or more to develop optimal strength gains, additional studies suggest that the muscles supporting the spine (i.e., lumbar extensors) and smaller muscles of the torso may respond as well with fewer training sessions per week. For example, Graves et al. (32) found no significant differences in dynamic and isometric strength generated by isolated lumbar extensor muscles among groups training 1, 2, or 3 d·wk−1 for 20 wk. When assessing cervical rotation strength, Leggett et al. (39) found that training frequencies of 2 and 3 d·wk−1 were superior to 1 d·wk−1 or 1 day every 2 wk over a 12-wk training period. Pollock et al. (49) indicated that training 2 d·wk−1 is superior to 1 d·wk−1 for increasing cervical extension strength, but because training 3 d·wk−1 was not evaluated, no inferences can be made in this regard. As for the muscles involved in torso rotation, DeMichele et al. (22) concluded that the 2 d·wk−1 training frequency obtained better adherence and equal strength gains compared with 3 d·wk−1; both of these groups elicited greater improvements than those that trained 1 d·wk−1.
Based on the findings of these studies, it is clear that there is no single optimal frequency of resistance training for all muscle groups. Whether the differences in the time course of strength gains occurring in isolated muscle groups result from variations in neural integration, muscle morphology, autoregulation, or other mechanisms warrants further investigation. Although clinicians and other health professionals must consider the specific needs of individual participants, particularly those who are frail or with orthopedic limitations, the conservative frequency of training of a minimum of 2 d·wk−1 guideline seems appropriate (Table 3). Participants who have time and want to achieve more benefits may choose to weight train 3 d·wk−1. In addition, when prescribing traditional resistance exercise programs (8-10 exercises; e.g., chest, back, shoulders, arms, abdomen, legs, hips), the minimum of 2 d·wk−1 training frequency guideline allows more time for recuperation, is less time consuming and thus may enhance adherence. Two d·wk−1 programs also appear to produce 80-90% of the strength benefits to more frequent programs in the untrained person.
RESISTANCE TRAINING GUIDELINES FOR HEALTHY ADULTS
The guidelines/statements shown in Table 1 reflect the science-based research conducted to determine minimal and optimal levels of exercise needed to induce health and fitness-related adaptations in the musculoskeletal systems. The addition of resistance training as a component of the comprehensive fitness program was an important inclusion by ACSM (5), and subsequent statements and guidelines have been published by ACSM (6,7), AHA (27), AACVPR (2,3), and the Surgeon General (55), recommending strength training for both healthy and diseased populations. At first glance, the ACSM recommendation for resistance training for healthy adults may appear minimal, but these minimal standards were based on the following premises: "First, the time it takes to complete a comprehensive, well-rounded program is important. Programs lasting more than 60 min per session are associated with higher dropout rates. (47) Second, although greater frequencies of training (15,26,30) and additional sets or combinations of sets and repetitions may elicit larger strength gains (10,20,26,35,59), the magnitude of difference is usually small" (5). Taking these assumptions into consideration, the minimal standard appears acceptable for most adults.
The guidelines recommended by the various health organizations are very similar in their application of exercise prescription variables. For safety and time allotment considerations, most resistance training programs should incorporate variable resistance equipment and traditional calisthenics and flexibility exercises. Intensity should start low and progress slowly, allowing time for adaptation. If a 1-RM test is administered for the purposes of assessing muscular strength at the beginning of an resistance training program, then 30-40% of the 1-RM for the upper body and 50-60% of 1-RM for the hips and legs should be used as the starting weight for the first exercise training session. When the participant can comfortably lift the weight 12 repetitions using good form and perceive it to be light to somewhat hard (12-13 on the Borg RPE scale) (14), 5% can be added to the next training session. Although completing one set of 8 to 12 repetitions at a comfortably hard level (RPE = 12-13) is the initial goal, the participant should strive to progress to a higher intensity (RPE = 15-16, hard). Since the level of fatigue (intensity) is an important factor for attaining a maximal benefit, exercising to a maximal effort gives the best results (RPE = 19-20; cannot complete another repetition using good form) (7,26). At this level of training progression to a higher weight should occur every 1-2 wk. If a subject cannot lift the weight a minimum of eight times then the weight should be reduced for the next training session.
Although the current minimal standard recommended by ACSM (1998) seems appropriate, most of the research used to formulate the resistance training guidelines were based on data from healthy adults under 50 yr of age. Recent recommendations for strength training in the elderly (50) and in cardiac patients (3,27) advocate lesser intensity resistance training for more fragile or diseased populations.
RESISTANCE TRAINING GUIDELINES FOR THE ELDERLY
As outlined by Evans (23) and Carpenter and Nelson (17), resistance training is essential for proper musculoskeletal development and maintenance and improves physical functional capacity and quality of life, especially in the elderly or more frail low fit individuals (25). The ACSM exercise prescription guidelines (6,7) for young adults and middle-aged adults are also appropriate for the elderly (Table 1), with slight but distinct differences in exercise prescription application (38,50). Because of the natural course of physiological degradation, elderly adults may be more fragile and thus more susceptible to fatigue, orthopedic injuries, and cardiovascular complications, and these factors need to be taken into consideration when prescribing resistance training programs (7,27,48,50). In addition, elderly adults are generally more sedentary as a group and by lowering the intensity of the activity and progressing more slowly than programs prescribed for younger adults may be more beneficial over the long term. The mode of exercise is also an important consideration when designing resistance exercise programs for the elderly. From a safety standpoint, variable resistance machines with selectorized weight stacks are generally recommended for several reasons: 1) the initial weight can be applied at a low level and increased in small increments (1 kg or less); 2) the equipment is usually designed to protect the lower back, thus reducing the risk of injury; 3) many machines are designed to avoid handgripping which reduces the risk of exercise-induced hypertension; 4) the machines are usually designed to allow the resistance to be applied evenly through the participants' full range of motion (ROM); 5) many types of equipment can be double pinned to allow the subject to exercise through their pain-free ROM; and 6) many resistance machines do not require the participant to balance or control the weight, as do dumbbells and barbells, which may reduce the likelihood of injury (50). In addition to the safety considerations, less time is generally consumed when using variable resistance machines when compared with free weight apparatus, allowing participants to complete their exercise sessions in a minimal amount of time, which often translates into improved program compliance. Recognizing the inherent risk of orthopedic injury in the elderly population, exercise sessions should begin at a lower intensity level (10-15 repetitions) and progress more slowly (every 2-4 wk) than programs designed for younger adults (every 1-2 wk), allowing time for adaptation. Considering the increasing numbers in the older population, further research to determine the benefits of resistance training as well as to establish exercise guidelines for the elderly is needed.
RESISTANCE TRAINING GUIDELINES FOR THE CARDIAC PATIENT
Like healthy adults, cardiac patients require a minimum level of muscular exertion to accomplish activities associated with daily living but often lack the physical strength and/or confidence to perform these tasks. Cardiac rehabilitation programs, however, have traditionally emphasized dynamic aerobic exercise for the maintenance and improvement of cardiovascular fitness as well as for the known health benefits. The AHA, ACSM, and AACVPR (3,7,27) described the importance of muscular fitness in preparing the cardiac patient to return to work and to participate in household and leisure time activities. Many of these vocational and recreational activities place demands on the body that more closely resemble resistance exercise than aerobic exercise. Therefore, it seems prudent to incorporate resistance training into the patient's exercise program, especially since recent research indicates that complementary resistance training has many favorable health and fitness benefits.
Updated resistance training guidelines for cardiac patients developed by the AHA (27) and AACVPR (3)(Table 1) are similar to the guidelines established for healthy adults (6,7). The primary differences involve reduced exercise intensity, slower progression of the training volume variables, and increased patient monitoring and program supervision. Guidelines for cardiac patients include the use of a lighter weight performed with 10-15 (27) or 12-15 repetitions (3). Although resistance training has been shown to be a safe procedure in regard to precipitating a cardiovascular event (24,41), much variance exists among recommendations as to the level of fatigue (moderate to maximum) required in such programs (3,7,27). Thus, it appears that low risk patients who have a MET capacity greater than 7 can be cleared for heavy resistance training (10/12-15 repetitions to fatigue) while other more high risk patients should keep their fatigue to a moderate level (RPE ≤ 15). An additional point to consider when prescribing exercise programs for cardiac patients is that pharmacotherapy (i.e., beta blockers) may alter the normal hemodynamic responses to exercise. Recognizing this, the RPE scale developed by Borg (14) is gaining increasing acceptance as an effective method for monitoring the cardiac patient's level of exertion during resistance exercise testing and training sessions (27).
Most of the current research is based on maximal efforts to volitional fatigue; thus, the results of varying levels of fatigue/intensity (e.g., RPE of 12-13 or 15-16) have not been carefully studied. As research becomes available, a clearer recommendation concerning the quality and quantity of strength training for various segments of the population needs to be formulated.
The amount of emphasis to be placed on performing resistance training compared with aerobic training in daily regimens has received considerable attention. In addition, whether this emphasis should be different among the young, middle-aged, elderly, or diseased populations remains to be determined. As the evidence mounts concerning the importance of resistance training for both health and fitness, a more balanced program related to comprehensive fitness will be incorporated for all age groups. The total time available for training will continue to be a major consideration because of adherence and injury problems related to programs of greater frequency and duration (47,48). Health and fitness professionals, as well as the participants, will have to make value judgments concerning the time available to accomplish their goals. Whether there are enough data available now to warrant a shift to 50-50 balance or at a minimum 60% aerobic and 40% strength and flexibility recommendation remains questionable.
While most of the quantification studies have addressed strength gains, new studies manipulating exercise program variables (frequency, intensity, volume, etc.) need to be designed to address the role of resistance exercise on health factors. Most of the research to date relates to short-term studies and there is a lack of randomized clinical trials evaluating the effect of resistance training on health outcomes. Although the issue of importance and emphasis of resistance training is debatable, it seems prudent to increase the emphasis of strength training in the middle-aged and elderly populations. Resistance training should be emphasized considering its impact on the attenuation of osteoporosis and muscle atrophy and related risks associated with falling and reduced functional capacity (17,23). In addition, patients with cardiovascular disease also need a well-rounded program, but the emphasis should continue toward greater amounts of aerobic training.
When prescribed appropriately, resistance training is effective for developing fitness, health, and for the prevention and rehabilitation of orthopedic injuries. The results of data provided by this symposium on Resistance Training for Health shows that there is enough existing evidence to conclude that resistance training, particularly when incorporated into a comprehensive fitness program, can offer substantial health benefits which can be obtained by persons of all ages. These benefits, including improvements in functional capacity, translate into an improved quality of life.
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