Chapter 13 Strength Training Concepts in the Athlete

After studying this chapter, the reader will be able to do the following:

1. Describe the physiological adaptations within the muscle following strength exercises

2. Provide examples of the changes that occur with neural adaptations within muscle

3. List the major contributors to improving muscle strength

4. Explain the differences among strength, power, and endurance

5. Distinguish among how to train Type I, Type IIA, and Type IIB muscle fiber types

6. Identify the number of repetitions, sets, and amount of resistance necessary to increase muscle strength and hypertrophy

7. Describe the effects of aging on muscle

8. Describe the phases of periodization training

9. Describe the differences among eccentric, concentric, and isometric exercises

10. Provide examples of off-season, in-season, and maintenance programs for athletes

Strength training is an important part of any athlete’s rehabilitation or performance enhancement program, or both. Optimal resistance training programs require consideration of numerous variables. The muscle and muscle groups to be exercised, the type of exercise, frequency, intensity, and duration are all important variables that determine the success of any strength training program. What are the physiological and neural adaptations that occur within the muscle and how long does it take to make changes? Can muscle fiber types be converted with strength training? Should eccentric, concentric, or isometric exercises, or a combination of all three, be used? Periodization strength training programs can result in significant changes in muscle strength. What are the different phases of a periodization program, and how long should each phase last? How does the therapist incorporate neuromuscular exercises into a strength-training program? Answers to these questions are essential to professionals responsible for prescribing resistance exercise for improvement of sports performance or for returning the athlete to normal function following injury and treatment.

Strength is the ability of the muscle to exert a maximum force at a specified velocity.¹ Power is defined as the force exerted multiplied by the velocity of movement.¹ Muscular power is a function of both strength and speed of movement. For most large muscle groups, maximal mechanical power is achieved at 30% to 45% of one’s 1RM (one repetition at maximum effort). Endurance is the ability to sustain an activity for extended periods of time.¹ Local muscle endurance is best described as the ability to resist muscular fatigue.¹ The authors of this chapter believe that a good strength base is important to reestablishing function and improving performance. Often specific strengthening exercises have been labeled as nonfunctional because they are performed in the open kinetic chain (OKC). For the purposes of this chapter, a functional exercise is defined as an exercise specific to the muscle groups that are important to the activity the athlete wants to return to and that sufficient resistance, repetitions, and sets are used to stimulate the muscle to adapt by increasing strength. Several studies have demonstrated that when OKC exercises were used to strengthen the glenohumeral rotators, significant gains were made in the strength of the shoulder rotators and the velocity of the baseball in pitchers and the velocity of the tennis ball in a tennis serve.^2,³ Therefore strength training the rotators of the glenohumeral joint by moving the shoulder into internal and external rotation in an open kinetic chain position is a functional exercise (Figure 13-1).

Figure 13-1 A functional exercise: Strengthening the glenohumeral rotators in the plane of the scapula.

This chapter describes the neural and physiological adaptations in muscle as a result of strength training programs. Time frames for developing strength gains, in addition to the amount of resistance, sets, and repetitions necessary to make these changes, are discussed. The effects of aging on muscle and exercise adaptations in the elderly are also discussed. Finally is a review of the differences among eccentric, concentric, and isometric exercises.

Understanding the cellular and molecular adaptations of skeletal muscle in response to strength training is important to provide the framework to improve performance in the athlete and the health and quality of life of the general population with or without chronic diseases.

PHYSIOLOGICAL ADAPTATIONS OF MUSCLE

Neural Adaptations

The first signs of muscle adaptation to strengthening exercises are neural adaptations. Several studies have demonstrated that early strength gains induced by resistance training are primarily due to modifications of the nervous system. Moritani and DeVires,⁴ in a landmark study, found that “neural factors” accounted for the significant improvements observed during the first 4 weeks of an 8-week resistance-training program. Staron et al⁵ demonstrated that only after 6 weeks of training was significant muscle fiber hypertrophy detected. Furthermore, Staron et al⁶ demonstrated that with heavy resistance training the conversion of Type IIB fibers to Type IIA fibers occurred at 2 weeks in females and 4 weeks in males.

Views on the relative contribution of neural versus muscle adaptation with strength training lasting longer than 2 to 3 months are conflicting. Deschenes et al ¹ indicate that with prolonged resistance training, the degree of muscle hypertrophy is limited and that significant hypertrophic responses can occur within a finite period of time, lasting no more than 12 months. A secondary neural adaptation explains the continued strength gains with prolonged resistance training. The secondary phase of neural adaptations takes place between the sixth and twelfth months. In contrast, Shoepe et al⁷ demonstrated substantial muscle hypertrophy as a result of several years of resistance training, when compared with a group of sedentary individuals.

The neural adaptations elicited by resistance training include decreased co-contraction of antagonists and expansion in the dimensions of the neuromuscular junction, indicating greater content of presynaptic neurotransmitter and postsynaptic receptors.⁸^–¹⁰ Greater synchronicity of the discharge of motor units after strength training has also been reported.¹¹

Contractile Adaptations

The contractile protein in muscle includes the actin and myosin filaments. (See Chapter 2 for great detail.) Beyond the first few weeks of resistance exercises, an increased contractile capacity that exists within the muscle accounts for strength gains.^12,¹³

The turnover rate of muscle protein is one of the slowest in the body. Within skeletal muscle, synthesis and growth of contractile proteins lag behind that of other proteins, such as mitochondria and sarcoplasmic reticulum.^14,¹⁵ The synthesis and accretion of contractile proteins account for the hypertrophy that occurs with resistance training. This hypertrophy occurs mostly within the intracellular myofibrils (25% to 35%), in addition to hypertrophy within the whole muscle (5% to 8%).^1,¹⁶

Hypertrophy versus Hyperplasia

Resistance exercise is a potent stimulus to increase the size of muscle. For a muscle to become larger, it must either increase in cross-sectional area (hypertrophy) or increase the number of muscle fibers (hyperplasia). The number of muscle fibers is generally believed to be innate and does not change during life.¹⁷ In contrast, several researchers reported that muscle is capable of increasing its size as a result of an increase in fiber number.^18,¹⁹ The exact mechanism responsible for muscle hypertrophy is uncertain, but several theories have been expressed in the literature. Skeletal muscles are capable of remodeling under various conditions. The activation of myogenic stem cells within the muscle is one of the most important events that occurs during skeletal muscle remodeling.¹⁹ The muscle (myogenic) stem cells remain dormant under the basement of the myofibers, and on stimulation they differentiate into satellite cells to form myofibers.¹⁹ The muscle or myogenic stem cells start to generate, by a series of cell divisions, daughter cells that become satellite cells. Evidence suggests that strength training induces a significant increase in satellite cell content in skeletal muscle. Because the myonuclei in mature muscle fibers cannot divide, it is suggested that the incorporation of satellite cell nuclei into muscle fibers results in the maintenance of a constant nuclear/cytoplasmic ratio. Therefore new muscle fibers are formed following strength training. When resistance or endurance exercises promote satellite cell proliferation and differentiation can be detected in injured fibers and those with no discernible damage, muscle hyperplasia occurs in human skeletal muscle.¹⁸^–²¹

Resistance training has been shown to elicit a significant acute hormonal response, which is more critical to tissue growth and remodeling than chronic changes in resting hormonal concentrations. Anabolic steroids, such as testosterone and the growth hormones, have been shown to elevate during 15 to 30 minutes of exercise using high-volume, moderate-to-high-in-intensity, short rest periods and stressing a large muscle mass, when compared with low-volume, high-intensity protocols using long rest intervals.²²

Other anabolic hormones, such as insulin and insulin-like growth factor-1 (IGF-1), are critical to skeletal muscle growth. Blood glucose and amino acid levels regulate insulin. However, following resistance exercise, elevations in circulating IGF-1 have been reported, presumably in response to growth hormone–stimulated secretion.²²

Force developed by the myofilaments (actin and myosin) may stimulate the uptake of amino acids and thus result in muscle tissue growth.²³ Heavy forces encountered during resistance training lead to disruption in the Z lines. The disorganization after disruption of the Z disks may cause the myofibril to split and grow back full size.²⁴ Furthermore, the disruption and rebuilding of the muscle result in an increase in the connective tissue surrounding the muscle fibers. The ingestion use of amino acids before and after resistance training to promote hypertrophy is discussed at great length in Chapter 17.

In summary, as a result of strength training exercises, physiological adaptations of muscle result in an increase in strength. These adaptations include hypertrophy (within the first 6 to 8 weeks), hyperplasia, hormonal changes, increase in the connective tissue surrounding the muscle fibers, disruption of the myofilaments, and neuromuscular changes (within the first 2 weeks of training). In addition, metabolic adaptations occurring within the muscle fiber increase the ability of the muscle to generate adenosine triphosphate (ATP) for anaerobic metabolism. Anaerobic metabolism requires that the muscles increase phosphocreatine, glycogen stores, the enzyme creatine phosphokinase that breaks down PC, and the rate-limiting enzyme phosphofructokinase of glycolysis. Refer to Chapter 3 for a review of anaerobic metabolism.²⁵

Muscle Fiber Type: Specific Adaptations

The fact that a prolonged program of resistance training brings about fiber type conversion with the muscle is well documented. The most common finding is an increase in the percentage of Type IIA fibers with a decrease in the percentage of Type IIB fibers.^6,^26,²⁷

Apparently, as soon as a Type IIB muscle fiber is stimulated it starts a process of transformation toward the Type IIA, by changing the quality of proteins and expressing different types and amounts of myosin adenosine triphosphatase (mATPase).²⁸ Following a resistance-training program, few Type IIB fibers remain, which is reversed during detraining. However, when resistance training starts again, the conversion from Type IIB to Type IIA is quicker. Although resistance training promotes hypertrophy in all three major muscle fiber types in humans—I, IIA, IIB—the amount of hypertrophy differs from each fiber type. On the basis of the examination of pretraining to posttraining muscle samples, it has been established that muscle hypertrophy is greatest in Type IIA fibers, followed by Type IIB, with Type I fibers demonstrating the least amount of hypertrophy.^5,^6,^26,^27,²⁹ Sex differences are apparent in muscle cross-sectional examination before and after training; Type IIA fibers are the largest among men, whereas the Type I fiber is the greatest size among women.³⁰

Exercise Variables

In order to achieve the physiological adaptations described earlier, several variables must be considered. The variables that need to be carefully planned for in the development of an exercise program include the choice of exercise, order of exercises, number of sets, number of repetitions, intensity of exercise, duration of rest between sets and exercises, and frequency of training.

The type of exercise should be specific to the specific muscle deficits revealed in the initial evaluation. Furthermore, the type of exercises should be specific to the muscle groups that are important to improving the performance of the athlete. For example, in the overhead-throwing athlete, the external rotators, infraspinatus and teres minor, provide a breaking action in the deceleration of the shoulder. Eccentric loading to the external rotators is a specific exercise to strengthen the external rotators, and eccentric activity of the external rotators is specific to the movement pattern and exercise performed by the athlete in competition. Furthermore, high-speed eccentric loading is damaging to the muscle. By increasing the eccentric strength of the external rotators, there is greater protection of the muscle from damage. This concept is discussed in greater detail later.

The order of the exercises performed by the athlete typically involves performance of large muscle group exercises before smaller muscle group exercises. Because the metabolic demand is greater for large muscle group exercises, exercises that recruit more than one muscle group, such as closed kinetic chain exercises, should be performed before isolation exercises.²⁸

Once again, debate exists in the literature regarding the number of sets and frequency of strength training. For the athlete the number of sets within a workout is directly related to individual training goals. Multiple-set programs optimize the development of strength and local muscular endurance.³¹ Gains in strength occur more rapidly with multiple-set programs compared with single-set protocols.³² Single-set exercise programs may be effective for individuals who are untrained or those just beginning a resistance training program. One-set workouts are also useful for maintenance programs. Furthermore, strength changes over a short-term training period and nonperiodized multiple-set program may not be different among one, two, or three sets of 10 to 12RM.³³ However, when single-set protocol is compared with multiple-set periodized programs, significant superior results are observed with the multiset periodized programs that last longer than 1 month.³⁴ Gotshalk et al³⁵ demonstrated that higher volumes of total work produced significantly greater increases in circulating anabolic hormones during the recovery phase following multiset heavy-resistance exercise protocols.

McLester et al ³⁶ demonstrated that training 1 day per week was an effective means of increasing strength, even in experienced recreational weight lifters. However, the previous study reported superior results with training 3 days per week when compared with 1 day per week when the total volume of the exercise was held constant.

Advanced training frequency varies considerably. Hoffman et al ³⁷ demonstrated that football players training 4 to 5 days per week achieved better results that those who trained either 3 or 6 days per week. Frequencies as high as 18 sessions per week have been reported in Olympic weight lifters.³⁸

The intensity of the exercise or the amount of resistance used for a specific exercise is the most important variable in resistance training. The most common method of determining the amount of resistance used in a strength-training program is the maximal load that can be lifted a given number of repetitions within one set. The greatest effects on strength measures or maximal power outputs are achieved when the strength training repetitions range between 6 and 12.²⁸ In other words, the maximum weight that can be lifted six times and six times only is the amount of resistance to start with. Addition of sets and repetitions occurs at subsequent workouts until 3 sets of 12 repetitions (reps) are reached. After reaching this reps and sets goal, the reps are reduced down to eight and weight is added, allowing only eight repetitions. Once 15 reps are achieved with a specific weight, the muscle will no longer continue to improve in strength. However, lighter loads allowing 15 to 20 reps are effective for increasing absolute local muscle endurance.^39,⁴⁰

Maximizing power requires a good strength base. Given that both force and time components are relevant to maximizing power, training to increase muscle power requires two general loading strategies. First, heavy resistance training recruits high-threshold fast-twitch muscle fibers that are necessary for strength. The second strategy is to incorporate lighter loads. Depending on the exercise, this may encompass 30% to 60% of 1RM.^41,⁴² Weight training for power has been referred to as “explosive strength training.” Paavolainen et al⁴³ demonstrated that explosive strength training could improve 5-km running time by improving running economy and muscle power, although a large volume of endurance training was performed concomitantly. The maximum amount of resistance used in the explosive strength training exercises was 40% of 1RM. When performing explosive weight-training exercises, the athlete moves as fast as possible throughout the range of motion, resulting in losing contact with the ground in an explosive squat or losing contact with the bar in a bench press. During a traditional bench press and squat weight-training exercises performed at an explosive velocity, one study has shown that 40% to 60% 1RM and 50% to 70% 1RM, respectively, may be most beneficial in the development of power.⁴⁴

The final variable that is important to muscle adaptation from strength training is the time intervals between sets. The rest interval depends on the intensity of the training. For example, it has been shown that acute force and power production may be compromised with short rest periods of 60 seconds or less.⁴⁵ Longitudinal studies have shown greater strength increases resulting from long rest periods between sets, 2 to 3 minutes versus 30 to 40 seconds.^46,⁴⁷

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