|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., 1991) , or known risk factors. The extent of the increase in blood pressure is dependent on the time the contraction is held, the intensity of the contraction, and the amount of muscle mass involved in the contraction (Fleck, 1988) . More dynamic forms of resistance training, such as circuit training, that involve moderate resistance and high repetitions with short rests are associated with reductions in blood pressure. Studies have shown decreases in diastolic blood pressure (Harris & Holly, 1987) , no change in blood pressure (Blumenthal, Siegel, & Appelbaum, 1991) , and decreases in systolic blood pressure (Hagberg et al., 1984; Hurley, Hagberg, & Goldberg, 1988) . The effects of resistance training on blood pressure are varied due largely to differences in study design, which suggests that more research is necessary to clearly understand the role of resistance training in blood pressure management.
Heart size adaptations to resistance training
Studies of strength-trained athletes have shown that there is an increase in left ventricular wall thickness, absolute left ventricular wall mass, and septum (wall separating the left and right ventricles) wall thickness with resistance training (Stone et al., 1991) , as contrasted by increases in volume of the left ventricular chamber seen with aerobic-trained individuals. The extent to which the changes in the heart size from resistance training may affect cardiac output, stroke volume and heart efficiency are currently unknown.
Lipoprotein and lipid adaptations to resistance training
Epidemiological research has decisively demonstrated that low concentrations of total cholesterol and low-density lipoprotein cholesterol (LDL-C), and high levels of high-density lipoprotein cholesterol (HDL-C) are associated with a decrease in coronary heart disease (Kannel, 1983) . Lower concentrations of blood triglycerides and LDL-C, along with higher levels of HDL-C have been observed with endurance-trained individuals. Several investigators have reported favorable changes in blood lipids and lipoproteins following a strength training intervention (Kokkinos & Hurley, 1990) . However, Kokkinos and Hurley add that the lack of control in body composition, day-to-day variations in lipoproteins, dietary factors, and distinction of acute vs. chronic adaptations needs to be thoroughly addressed in future strength training research, to provide a more credible summary of the effect of resistance training on blood lipids and lipoproteins. In addition, more research is needed to determine if there is an optimal resistance training format that positively affects lipoprotein-lipid profiles.
Glucose metabolism adaptations to resistance training
An important risk factor for cardiovascular disease and diabetes is glucose tolerance. High blood glucose and high insulin levels can also have a deleterious effect on hypertension and blood lipids (Hurley, 1994) . Initially, improvements in glucose metabolism were associated with decreases in percent body fat and increases in aerobic capacity, thus suggesting that aerobic exercise would provide the better catalyst for improvements in glucose metabolism. However, improvements in glucose metabolism with strength training, independent of alterations in aerobic capacity or percent body fat, have been shown (Hurley et al., 1988; Smutok, Reece, & Kokkinos, 1993) . Interestingly, Smutok et al. (1993) concluded that strength training and aerobic training improved glucose tolerance and reduced insulin responses to oral glucose (in men) similarly. The strength training program consisted of two sets (90 second rests between sets) of exercise, using loads that could be lifted 12 - 15 times (per set) for 11 different exercises. Exercises included squats, leg extensions, leg curls, decline presses, pullovers, arm cross-overs, overhead presses, lateral raises, rows, hip and back exercises, and modified sit-ups. Additionally, it has been shown that body builders, who traditionally employ a high volume style of training, favorably alter glucose tolerance and insulin sensitivity (Stone et al., 1991) .
Practical Application: A Resistance Training Prescription for Health
It is evident from a number of the adaptations that occur with resistance training that there are several health-related benefits. Resistance training has been shown to reduce factors associated with coronary heart disease, diabetes and osteoporosis. Further research is needed to elucidate the effects of resistance training on blood lipids, lipoproteins and blood pressure (in hypertensives), and to ascertain what type of training programs may best alter these risk factors.
From this overview, there are some practical guidelines for the health fitness professional and personal trainer who wish to prescribe resistance training programs for health status improvement. They are as follows:
1. Develop programs that will utilize a greater amount of energy expenditure during the workouts. Programs that utilize the larger muscle groups provide a structural basis for the preferred loading that is recommended for improvements in bone mass and mineral density. This will also contribute to the caloric cost of the programs, helping to facilitate weight management goals.
2. Use moderate intensity programs, with multiple sets of 8 to 12 repetitions (Stone et al., 1991) . A frequency of 2 - 3 times a week of resistance training appears applicable and attainable. Programs designed to increase total workout volume (total repetitions x weight) are encouraged.
3. As with any effective exercise prescription, individualize the program, with a carefully planned, progressive overload.
4. Be guarded in the use of isometric contractions and high-intensity load training due to the marked increase observed in diastolic and systolic blood pressure.
5. Incorporate a variety of exercises. In order to avoid the effects of overtraining, muscle soreness, and injury, a prescription of resistance training using a variety of exercises is prudent.
6. With certain organic conditions, such as musculoskeletal conditions (i.e., arthritis), hypertension, and previous injuries, it may be advisable to seek the guidance of a qualified health practitioner for suggestions in designing a safe and effective resistance training program.
7. Take the time to teach the correct performance techniques of the resistance exercises. In the methodology sections in a number of the studies, the researchers emphasized the importance of teaching the subjects safe and correct resistance training mechanics.
8. Be aware that the training demands of resistance training may be greater for novice, low-fitness level, and elder individuals, due to the unique physiological challenges of the activity, and the level of fitness of the individuals. Often times, the use of longer rest periods between sets may be beneficial to help these populations adapt to the training demands.
9. Multiple-joint exercises are more demanding than single-joint exercises, and thus suggest that the training frequency (days per week) may need to be provide adequate recovery (up to 48 hrs) for the clients, especially when just beginning a resistance training program.
10. Develop an effective dialogue with your students. In an attempt to keep the training regimen satisfactory for the study, some researchers mentioned the importance of communication with the subjects in order to sustain the investigation. Effective communication is also consequential in developing and maintaining effective training programs for your students.
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