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Resistance Training: Adaptations and Health Implications
By Len Kravitz, Ph.D.
The adaptational changes and health implications of resistance exercise are very dynamic and variable to each individual. For long-lasting change, there needs to be a systematic administration of a sufficient stimulus, followed by an adaptation of the individual, and then the introduction of a new, progressively greater stimulus. Whether training for sports performance or health enhancement, much of the success of the program will be attributable to the effectiveness of the exercise prescription in manipulating the progression of the resistance stimulus, the variation in the program design and the individualization of the program (Kraemer, 1994) . Most recently, the positive health benefits of physical activity have gained high recognition attributable to the Surgeon General's report on health and physical activity. The purpose of this article is to highlight many of the physiological adaptations and health benefits that occur with resistance training programs.
Note: For a recent article from the American Heart Association summarizing the documented benefits of resistance exercise for those with and without cardiovascular exercies: CLICK HERE!
Muscle fiber adaptations to resistance training
The increase in size of muscle is referred to as hypertrophy. The 'pump' one feels from a single exercise bout is referred to as transient hypertrophy. This short term effect is attributable to the fluid accumulation, from blood plasma, in the intracellular and interstitial spaces of the muscle. In contrast, chronic hypertrophy refers to the increase in muscle size associated with long-term resistance training. Increases in the cross-sectional area of muscle fibers range from 20% to 45% in most training studies (Staron et al., 1991) . Muscle fiber hypertrophy has been shown to require more than 16 workouts to produce significant effects (Staron et al., 1994) . In addition, fast-twitch (glycolytic) muscle fiber has the potential to show greater increases in size as compared to slow-twitch (oxidative) muscle fiber (Hather, Tesch, Buchanan, & Dudley, 1991).
It is generally believed that the number of muscle fibers you have is established by birth and remains fixed throughout the rest of your life. Therefore, the hypertrophy adaptations seen with resistance training are a net result of subcellular changes within the muscle which include: more and thicker actin and myosin protein filaments, more myofibrils (which embody the actin and myosin filaments), more sarcoplasm (the fluid in the muscle cell), and plausible increases in the connective tissue surrounding the muscle fibers (Wilmore & Costill, 1994) . To keep things in perspective, the largest muscle fiber in the body is no thicker than a human hair. Any evidence of muscle fiber splitting (referred to as hyperplasia), as has been described in animal studies, is presently inconclusive with human subject research, but conceivably possible.
Strength adaptations to resistance training
The increases in muscular strength during the initial periods of a resistance training program are not associated with changes in cross-sectional area of the muscle (Sale, 1988) . Changes in strength evidenced in the first few weeks of resistance training are more associated with neural adaptations (Moritani & deVries, 1979) , which encompass the development of more efficient neural pathways along the route to the muscle. The motor unit (motor nerve fiber and the muscle fibers it innervates) recruitment is central to the early (2 to 8 weeks) gains in strength. Collectively, the learned recruitment of additional motor units, which may respond in a synchronous (the coincident timing of impulses from 2 or more motor units) fashion (Wilmore & Costill, 1994) , the increased activation of synergistic muscles, and the inhibition of neural protective mechanisms (Kraemer, 1994) , all contribute to enhance the muscle's ability to generate more force. It is possible that two adjacent muscle fibers, with different motor nerves, could result in one fiber being activated to generate force while the other moves passively.
Long-term changes in strength are more likely attributable to hypertrophy of the muscle fibers or muscle group (Sale, 1988). The range of increase of strength is quite variable to the individual and may range from 7% to 45% (Kraemer, 1994) . It should be noted that strength results appear to be velocity specific. Velocity specificity best characterizes the probability that the greatest increases in strength occur at or near the velocity of the training exercise (Behm & Sale, 1993) . Therefore, slow-speed training will result in greater gains at slow movement speeds, while fast-speed training will realize the improvements in strength at faster movement speeds.
A prevalent issue in analyzing the diverse results of strength adaptations in training studies depends upon the subjects' preparation for the investigation. Although several researchers often select untrained subjects, the failure to plan and control for a learning effect (subject improves because they learn the correct performance of the muscle action) may result in erroneous conclusions from the study.
Bone tissue adaptations to resistance training
In response to loading of the bone, created by muscular contractions or other methods of mechanical forces, the bone begins a process of bone modeling which involves the manufacture of protein molecules that are deposited in the spaces between bone cells. This leads to the creation of a bone matrix which ultimately becomes mineralized as calcium phosphate crystals, resulting in the bone acquiring its rigid structure. This new bone formation occurs chiefly on the outer surface of the bone, or periosteum.
Activities that stimulate bone growth need to include progressive overload, variation of load, and specificity of loading (Conroy, Kraemer, Maresh, & Dalsky, 1992) . Specificity of loading refers to exercises that directly place a load on a certain region of the skeleton. With osteoporosis, the sites of fractures that are most devastating are in the axial skeleton (the spine and hip). Conroy et al. recommended that more intense loading of the spine and hip be done during early adulthood when the body is more capable of taking on an increased load and developing its peak bone mass. Progressive overload is necessary so the bone and associated connective tissue are not asked to exceed the critical level that would place them at risk. Programs to increase bone growth should be structural in nature, including exercises such as squats and lunges which direct the forces through the axial skeleton and allow greater loads to be utilized (Conroy & Earle, 1994) . The magnitude required to produce an effective stimulus for bone remodeling appears to be a 1 repetition maximum (RM) to 10 RM load (Kraemer, 1994) . For example, if a person can do 10 repetitions, but not 11 repetitions, of a particular exercise at 120 lbs, he/she is said to have a 10 RM of 120 lbs. Although multiple sets are recommended for bone modeling stimulation, the intensity of the exercise, mechanical strain on the bone, and specificity of the bone loading exercise are considered more important factors.
Body composition adaptations to resistance training
Resistance training programs can increase fat-free mass and decrease the percentage of body fat. One of the outstanding benefits of resistance exercise, as it relates to weight loss, is the positive impact of increasing energy expenditure during the exercise session and somewhat during recovery, and on maintaining or increasing fat-free body mass while encouraging the loss of fat body weight (Young & Steinhard, 1995) . It is more likely that body composition is affected and controlled by resistance training programs using the larger muscle groups and greater total volume (Stone, Fleck, Triplett, & Kramer, 1991) . Volume in resistance training is equal to the total workload, which is directly proportional to the energy expenditure of the workbout. Total volume is determined by the total number of repetitions (repetitions x sets) performed times the weight of the load (total repetitions x weight). Often you will see total volume calculated multiplying the number of sets x repetitions x load. For example, three sets of 12 repetitions with 50 lbs would be expressed, 3 x 12 x 50 = 1,800 lbs of volume. An impressive finding to highlight with resistance training is that the energy expenditure following the higher total volume workouts appears to be elevated, compared to other forms of exercise, and thus, further contributes to weight loss objectives.
Heart rate adaptations of resistance training
Heart rate is acutely elevated immediately following a workbout and affected by the amount of resistance, the number of repetitions and the muscle mass involved in the contraction (small vs. large mass exercises) (Fleck, 1988) . Interestingly, in terms of chronic adaptations, there appears to be a reduction in heart rate from resistance training, which is considered beneficial (Stone et al., 1991) . Long term adaptations observed in the research, from no change up to a 11% decrease in heart rate, may be explained by the differences in intensity, volume, rest between sets, use of small vs. large muscle mass, duration of study and fitness level of the subjects.
Blood pressure adaptations to resistance training
Conservative estimates postulate that 50 million Americans, approximately 1 in 4 adults, have high blood pressure. More than 90% of these cases are identified as primary hypertension, which increases the risk of heart failure, kidney disease, stroke, and myocardial infarction (Tipton, 1984) . During a resistance exercise bout, systolic and diastolic blood pressures may show dramatic increases, which suggest that caution should be observed in persons with cardiovascular disease (Stone et al., 1
Article PageHomeResistance Training: Adaptations and Health ImplicationsBy Len Kravitz, Ph.D.The adaptational changes and health implications of resistance exercise are very dynamic and variable to each individual. For long-lasting change, there needs to be a systematic administration of a sufficient stimulus, followed by an adaptation of the individual, and then the introduction of a new, progressively greater stimulus. Whether training for sports performance or health enhancement, much of the success of the program will be attributable to the effectiveness of the exercise prescription in manipulating the progression of the resistance stimulus, the variation in the program design and the individualization of the program (Kraemer, 1994) . Most recently, the positive health benefits of physical activity have gained high recognition attributable to the Surgeon General's report on health and physical activity. The purpose of this article is to highlight many of the physiological adaptations and health benefits that occur with resistance training programs.Note: For a recent article from the American Heart Association summarizing the documented benefits of resistance exercise for those with and without cardiovascular exercies: CLICK HERE!Muscle fiber adaptations to resistance trainingThe increase in size of muscle is referred to as hypertrophy. The 'pump' one feels from a single exercise bout is referred to as transient hypertrophy. This short term effect is attributable to the fluid accumulation, from blood plasma, in the intracellular and interstitial spaces of the muscle. In contrast, chronic hypertrophy refers to the increase in muscle size associated with long-term resistance training. Increases in the cross-sectional area of muscle fibers range from 20% to 45% in most training studies (Staron et al., 1991) . Muscle fiber hypertrophy has been shown to require more than 16 workouts to produce significant effects (Staron et al., 1994) . In addition, fast-twitch (glycolytic) muscle fiber has the potential to show greater increases in size as compared to slow-twitch (oxidative) muscle fiber (Hather, Tesch, Buchanan, & Dudley, 1991).It is generally believed that the number of muscle fibers you have is established by birth and remains fixed throughout the rest of your life. Therefore, the hypertrophy adaptations seen with resistance training are a net result of subcellular changes within the muscle which include: more and thicker actin and myosin protein filaments, more myofibrils (which embody the actin and myosin filaments), more sarcoplasm (the fluid in the muscle cell), and plausible increases in the connective tissue surrounding the muscle fibers (Wilmore & Costill, 1994) . To keep things in perspective, the largest muscle fiber in the body is no thicker than a human hair. Any evidence of muscle fiber splitting (referred to as hyperplasia), as has been described in animal studies, is presently inconclusive with human subject research, but conceivably possible.Strength adaptations to resistance trainingThe increases in muscular strength during the initial periods of a resistance training program are not associated with changes in cross-sectional area of the muscle (Sale, 1988) . Changes in strength evidenced in the first few weeks of resistance training are more associated with neural adaptations (Moritani & deVries, 1979) , which encompass the development of more efficient neural pathways along the route to the muscle. The motor unit (motor nerve fiber and the muscle fibers it innervates) recruitment is central to the early (2 to 8 weeks) gains in strength. Collectively, the learned recruitment of additional motor units, which may respond in a synchronous (the coincident timing of impulses from 2 or more motor units) fashion (Wilmore & Costill, 1994) , the increased activation of synergistic muscles, and the inhibition of neural protective mechanisms (Kraemer, 1994) , all contribute to enhance the muscle's ability to generate more force. It is possible that two adjacent muscle fibers, with different motor nerves, could result in one fiber being activated to generate force while the other moves passively.Long-term changes in strength are more likely attributable to hypertrophy of the muscle fibers or muscle group (Sale, 1988). The range of increase of strength is quite variable to the individual and may range from 7% to 45% (Kraemer, 1994) . It should be noted that strength results appear to be velocity specific. Velocity specificity best characterizes the probability that the greatest increases in strength occur at or near the velocity of the training exercise (Behm & Sale, 1993) . Therefore, slow-speed training will result in greater gains at slow movement speeds, while fast-speed training will realize the improvements in strength at faster movement speeds.A prevalent issue in analyzing the diverse results of strength adaptations in training studies depends upon the subjects' preparation for the investigation. Although several researchers often select untrained subjects, the failure to plan and control for a learning effect (subject improves because they learn the correct performance of the muscle action) may result in erroneous conclusions from the study.Bone tissue adaptations to resistance trainingIn response to loading of the bone, created by muscular contractions or other methods of mechanical forces, the bone begins a process of bone modeling which involves the manufacture of protein molecules that are deposited in the spaces between bone cells. This leads to the creation of a bone matrix which ultimately becomes mineralized as calcium phosphate crystals, resulting in the bone acquiring its rigid structure. This new bone formation occurs chiefly on the outer surface of the bone, or periosteum.
Activities that stimulate bone growth need to include progressive overload, variation of load, and specificity of loading (Conroy, Kraemer, Maresh, & Dalsky, 1992) . Specificity of loading refers to exercises that directly place a load on a certain region of the skeleton. With osteoporosis, the sites of fractures that are most devastating are in the axial skeleton (the spine and hip). Conroy et al. recommended that more intense loading of the spine and hip be done during early adulthood when the body is more capable of taking on an increased load and developing its peak bone mass. Progressive overload is necessary so the bone and associated connective tissue are not asked to exceed the critical level that would place them at risk. Programs to increase bone growth should be structural in nature, including exercises such as squats and lunges which direct the forces through the axial skeleton and allow greater loads to be utilized (Conroy & Earle, 1994) . The magnitude required to produce an effective stimulus for bone remodeling appears to be a 1 repetition maximum (RM) to 10 RM load (Kraemer, 1994) . For example, if a person can do 10 repetitions, but not 11 repetitions, of a particular exercise at 120 lbs, he/she is said to have a 10 RM of 120 lbs. Although multiple sets are recommended for bone modeling stimulation, the intensity of the exercise, mechanical strain on the bone, and specificity of the bone loading exercise are considered more important factors.
Body composition adaptations to resistance training
Resistance training programs can increase fat-free mass and decrease the percentage of body fat. One of the outstanding benefits of resistance exercise, as it relates to weight loss, is the positive impact of increasing energy expenditure during the exercise session and somewhat during recovery, and on maintaining or increasing fat-free body mass while encouraging the loss of fat body weight (Young & Steinhard, 1995) . It is more likely that body composition is affected and controlled by resistance training programs using the larger muscle groups and greater total volume (Stone, Fleck, Triplett, & Kramer, 1991) . Volume in resistance training is equal to the total workload, which is directly proportional to the energy expenditure of the workbout. Total volume is determined by the total number of repetitions (repetitions x sets) performed times the weight of the load (total repetitions x weight). Often you will see total volume calculated multiplying the number of sets x repetitions x load. For example, three sets of 12 repetitions with 50 lbs would be expressed, 3 x 12 x 50 = 1,800 lbs of volume. An impressive finding to highlight with resistance training is that the energy expenditure following the higher total volume workouts appears to be elevated, compared to other forms of exercise, and thus, further contributes to weight loss objectives.
Heart rate adaptations of resistance training
Heart rate is acutely elevated immediately following a workbout and affected by the amount of resistance, the number of repetitions and the muscle mass involved in the contraction (small vs. large mass exercises) (Fleck, 1988) . Interestingly, in terms of chronic adaptations, there appears to be a reduction in heart rate from resistance training, which is considered beneficial (Stone et al., 1991) . Long term adaptations observed in the research, from no change up to a 11% decrease in heart rate, may be explained by the differences in intensity, volume, rest between sets, use of small vs. large muscle mass, duration of study and fitness level of the subjects.
Blood pressure adaptations to resistance training
Conservative estimates postulate that 50 million Americans, approximately 1 in 4 adults, have high blood pressure. More than 90% of these cases are identified as primary hypertension, which increases the risk of heart failure, kidney disease, stroke, and myocardial infarction (Tipton, 1984) . During a resistance exercise bout, systolic and diastolic blood pressures may show dramatic increases, which suggest that caution should be observed in persons with cardiovascular disease (Stone et al., 1
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