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ADAPTATION TO EXERCISE: Progressive Resistance Exercise
Thomas D. Fahey
California State University, Chico
Chico, CA

Fahey, T.D. (1998). Adaptation to exercise: progressive resistance exercise. In: Encyclopedia of Sports Medicine and Science, T.D.Fahey (Editor). Internet Society for Sport Science: http://sportsci.org. 7 March 1998.

Individual Differences
Applying Basic Principles of Exercise Training

Exercise training is an adaptive process. The body will adapt to the stress of exercise with increased fitness if the stress is above a minimum threshold intensity. To achieve maximum effectiveness, we must consider factors involved in the adaptation of muscle to stress and deconditioning. These factors include overload, specificity, reversibility, and individual differences.

An interesting aspect of skeletal muscle is its adaptability. If a muscle is stressed (within tolerable limits), it adapts and improves its function. For example, weight lifters exercise their arms and shoulders, so their muscles hypertrophy and improve their strength. Larger muscles allow them to accommodate an increased load. Likewise, if a muscle receives less stress than it's used to, it atrophies. For example, the muscles of a casted leg atrophy in response to disuse.

The purpose of physical training is to stress systematically the body so it improves its capacity to exercise. Physical training is beneficial only as long as it forces the body to adapt to the stress of physical effort. If the stress is not sufficient to overload the body, then no adaptation occurs. If a stress cannot be tolerated, then injury or over-training results. Significant improvements in performance occur when the appropriate exercise stresses are introduced into the athlete's training program. Physical fitness is largely a reflection of the level of training. When an athlete is working hard, fitness is high. However, when heavy training ceases, fitness begins to deteriorate.

There are countless exercise devices and training programs available that are hailed as the best way to gain strength. In most instances, as long as a threshold tension is developed, increases in strength will occur. The type of strength developed is the important consideration in exercise and sports. Long distance running up steep hills, for example, will develop a certain amount of muscular strength. The muscular adaptations that result will differ from those produced from high resistance, low repetition squats (knee bends). The distance runner develops mainly sarcoplasmic protein (oxidative enzymes, mitochondrial mass, etc.), while the weight lifter develops mainly contractile protein. The nature of the adaptive response must always be considered when designing the training program. Factors that determine the rate and type of strength gains include overload, specificity, and reversibility.


Muscles increase their strength and size when they are forced to contract at tensions close to their maximum. Muscles must be overloaded to hypertrophy and improve strength. Experimental and empirical observations have allowed generalizations concerning the amount of overload necessary for strength gains.

Muscle protein accumulation occurs by increasing the rate of protein synthesis, decreasing the rate of protein degradation, or both. Fiber types differ in their response to overload. Slow fibers hypertrophy by decreasing the rate of protein degradation. Fast fibers hypertrophy by increasing the rate of protein synthesis. The rate of protein synthesis in a muscle is directly related to the rate of entry of amino acids into the cells. Amino acid transport into muscle is influenced directly by the intensity and duration of muscle tension. This was determined by experiments with isolated muscle. They were bathed in a solution containing labeled amino acids, such as 14C labeled a-amino isobutyric acid. It was found that amino acid uptake was highest when muscles were contracted. Uptake was greater as tension and the duration of tension increased.

Weight training studies and empirical observations of athletes have reinforced the importance of generating muscular tension. It must be developed at an adequate intensity and duration for the optimal development of strength. The majority of studies have found that the ideal number of repetitions are between four and eight (repetitions maximum, 4-8 RM). They should be practiced in multiple sets (3 or more). Strength gains are less when either fewer or greater numbers of repetitions are used. These findings are consistent with the progressive resistance training practices of athletes.

Athletes involved in speed-strength sports practice low repetition, high intensity exercise during or immediately preceding the competitive season. These athletes include shot-putters and discus throwers. Such training improves explosive strength, while allowing sufficient energy reserves for practicing motor skills. However, the effectiveness of this practice has not been established experimentally.

Body builders usually do more sets and repetitions of exercises and more exercises per body part than weight lifters or strength athletes. Their goal is to build large, defined, symmetrical muscles. It is not known if typical body building training method is the most effective means of achieving their goals. Numerous studies have shown that high resistance, low repetition exercises are more effective than low resistance, high repetition exercises in promoting muscle hypertrophy.

Proper rest intervals are important for maximizing tension, both between exercises and training sessions. Insufficient rest results in inadequate recovery and a diminished capacity of the muscle to exert full force. Unfortunately, the ideal rest interval between exercises has not been determined. Most athletes strength train three to four days per week with large muscle exercises, such as the squat and bench press, rarely done more than twice a week. This practice has been empirically derived. It allows adequate recovery between training sessions.

The overload must be progressively increased for consistent gains in strength to occur. However, because of the high dangers of over-training in strength building exercises, constantly increasing the resistance is sometimes counter-productive. A relatively new practice among strength trained athletes is periodization of training. This practice varies the volume and intensity of exercises so the nature of the exercise stress frequently changes. Many athletes believe that this practice produces a faster rate of adaptation. Periodization of training will be discussed further in the section on the progressive resistance training programs of athletes.


Muscles adapt specifically to the nature of the exercise stress. The progressive resistance training program should stress the muscles how they are to perform. The most obvious example of specificity is that the muscle exercised is the muscle that adapts to training. Thus, if you exercise the leg muscles, they hypertrophy rather than the muscles of the shoulders.

There is specific recruitment of motor units within a muscle depending upon the requirements of the contraction . The different muscle fiber types have characteristic contractile properties. The slow twitch fibers are relatively fatigue-resistant, but have a lower tension capacity than the fast twitch fibers. The fast twitch fibers can contract more rapidly and forcefully, but they also fatigue rapidly.

The use of a motor unit is dependent on the threshold levels of its alpha motor neuron. The low threshold, slow twitch fibers are recruited for low intensity activities such as jogging (and for that matter, most tasks of human motion). However, for high speed or high intensity activities, such as weight lifting, the fast twitch motor units are recruited.

The amount of training that occurs in a muscle fiber is determined by the extent that it is recruited. High repetition, low intensity exercise, such as distance running, uses mainly slow twitch fibers. Endurance training improves the fibers' oxidative capacity. Low repetition, high intensity activity, such as weight training, causes hypertrophy of fast twitch fibers. There are some changes to the lower threshold slow twitch fibers. The training program should be structured to produce the desired training effect.

Increases in strength are very specific to the type of exercise, even when the same muscle groups are used (Figure 1). Specific motor units are recruited for specific tasks. If a person is weight training to improve strength for another activity, the exercises should be as close as possible to desired movements. Likewise, when attempting to increase strength after an injury or surgery, rehabilitation should include muscle movements as close as possible to normal activities.

Figure 1: The importance of specificity during strength training. Subjects performed squats for 8 weeks and made impressive strength gains. On different exercises that used the same muscles, strength gains were much less. (Adapted from Sale (4).)

Muscle fiber type appears to play an important role in determining success in some sports. Successful distance runners have a high proportion of slow twitch muscles (percent slow twitch fibers is highly related to maximum oxygen consumption). Sprinters have a predominance of fast twitch muscles. Several studies have shown that a high content of fast twitch fibers is a prerequisite for success in progressive resistance training. This is understandable, as the fast twitch fibers experience selective hypertrophy as a result of high resistance, low repetition exercise.

However, all sports do not require prerequisite fiber characteristics. For example, in world class shot-putters there is a surprisingly diverse muscle fiber composition. In those athletes, larger muscle fibers rather than percent fiber type, accounted for their performance. There are differences in the relative percentage of fast twitch fibers in explosive strength athletes. Having a high percentage of fast-twitch fibers is not necessary critical for success. Many strength athletes have a higher fast-to-slow twitch fiber area ratio than in sedentary subjects and endurance athletes. Individual differences in training intensity and technique can make up for deficiencies in the relative percentage of fast twitch fibers in these athletes. It would be interesting to speculate about the performance of a shot-putter with a high percentage of fast twitch fibers. What would performance be like in an athlete who developed good strength and technique? The high percentage of fast twitch fibers would probably be a decided advantage.

Simultaneous participation in a training program designed to stimulate both strength and endurance has been found to interfere with gains in strength. Strength athletes may inhibit their ability to gain strength by participating in vigorous endurance activities. Muscles may be unable to adapt optimally to both forms of exercise.


Muscles will atrophy as a result of disuse, immobilization, and starvation. Muscles adapt to increasing levels of stress by increasing their function. Disuse leads to decreasing strength and muscle mass. Atrophy results in a decrease in both contractile and sarcoplasmic protein.

The muscle fiber types do not atrophy at the same rate. Joint immobilization results in a faster rate of atrophy for the slow twitch muscle. This has important implications for rehabilitation. Often, increasing strength is a major goal following immobilization. Endurance should also be stressed because of the relatively greater loss of slow twitch muscle capacity.

Immobilization affects muscle length. If a muscle is fixed in a lengthened position, sarcomeres are added, while they are lost if the muscle is immobilized in a shortened position. Immobilization also leads to a variety of biochemical changes including decreased glycogen, adenosine diphosphate (ADP), creatine phosphate (CP),and creatine. All of these factors can affect muscular performance after immobilization has ended.

Individual Differences

As with other forms of exercise, people vary in the rate they gain strength. Some of these differences can be attributed to the relative predominance of fast— and slow-twitch motor units in muscles. Usually, endurance athletes will have a more slow twitch fibers (Type I motor units) in their active muscles. Strength athletes will have more fast twitch fibers. Intense progressive resistance training mainly enlarges fast-twitch fibers. People who have more fast-twitch fibers will tend to gain strength faster than those who do not.

Muscle strength is related to the cross-sectional area of the muscle. However, this strong relationship diminishes when "explosive athletes" and endurance athletes are compared. What most studies suggest is that strength is highly related to muscle size. However, people who have a disproportionate amount of fast-twitch fibers will gain strength faster than those who do not. Fast-twitch fibers tend to be stronger than other fiber types, so people who have more of them will tend to be stronger and have greater potential for strength gains.

Several studies have shown that fiber composition is genetically determined. Genetic researchers often investigate the influence of heritability on a trait by studying monozygotic and dizygotic twins (identical and nonidentical twins). Fiber distribution and muscle enzyme activity in monozygotic twins is almost identical in most of these studies.

Genetics is not the sole determinant of individual differences in strength. Numerous studies have shown that many successful strength-speed athletes do not have a predominance of fast-twitch motor units in critical muscles. Further, in athletic subjects, fiber composition is only marginally related to the time subjects can maintain isometric force and perform explosive squat jumps. Genetics exert a strong influence on the ability to gain strength. A good training program can make up for "genetic deficiencies."

Applying Basic Principles of Exercise Training

Adaptation is the whole purpose of exercise training. Adaptation requires a systematic application of exercise stress. The stress should be sufficient to stimulate an adaptation, but not so severe that breakdown and injury occur.


1. Edgerton, V.R. Mammalian muscle fiber types and their adaptability. Amer. Zool. Vol?: 113-125, 1978.

2. Fahey, T.D. and G. Hutchinson. Weight Training for Women. Mt. View, CA: Mayfield Publishing, 1992.

3. Fahey, T.D. Basic Weight Training for Men and Women. Mt. View, CA: Mayfield Publishing, 1994.

4. Komi, P.V. (ed.) Strength and Power in Sport. London: Blackwell Scientific Publications, 1992.

5. Komi, P.V. Physiological and biomechanical correlates of muscle function: effects of muscle structure and stretch-shortening cycle on force and speed. Exercise Sci. Sports Rev. 12: 81-121, 1984.

6. Mastropaolo, J.A. A test of the maximum-power theory for strength. Eur. J. Appl. Physiol. 65: 415-420, 1992.

7. Sale, D.G. Neural adaptation to resistance training. Med. Sci. Sports Exerc. 20 (suppl.): S135-S145, 1988.

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