Muscle Performance and Resistance Exercise PDF

*Muscle performance* refers to the capacity of a muscle to do work (force × distance).11 Despite the simplicity of the definition, muscle performance is a complex component of functional movement and is influenced by all of the body systems. Factors that affect muscle performance include the mor...

*Muscle performance* refers to the capacity of a muscle to do work (force × distance).11 Despite the simplicity of the definition, muscle performance is a complex component of functional movement and is influenced by all of the body systems. Factors that affect muscle performance include the morphological qualities of muscle; neurological, biochemical, and biomechanical influences; and metabolic, cardiovascular, respiratory, cognitive, and emotional function. For a person to anticipate, respond to, and control the forces applied to the body and carry out the physical demands of everyday life in a safe and efficient manner, the body's muscles must be able to produce, sustain, and regulate muscle tension to meet these demands. The key elements of muscle performance are *strength*, *power*, and *endurance*.11 If any one or more of these areas of muscle performance is impaired, activity limitations (functional limitations) and participation restriction (disability) or increased risk of dysfunction may ensue. Many factors, such as injury, disease, immobilization, disuse, and inactivity, may result in impaired muscle performance, leading to weakness and muscle atrophy. When deficits in muscle performance place a person at risk for injury or hinder function, the use of resistance exercise is an appropriate therapeutic intervention to improve the integrated use of strength, power, and muscular endurance during functional movements, to reduce the risk of injury or re-injury, and to enhance physical performance. *Resistance exercise* is any form of active exercise in which dynamic or static muscle contraction is resisted by an outside force applied manually or mechanically.103,248 Resistance exercise, also referred to as *resistance training*,8,9,167 is an essential element of rehabilitation programs for persons with impaired function and an integral component of conditioning programs for those who wish to promote or maintain health and physical well-being, potentially enhance performance of motor skills, and reduce the risk of injury and disease.8,9,242 A comprehensive examination and evaluation of a patient or client are the foundations on which a therapist determines whether a program of resistance exercise is warranted and can improve a person's current level of function or prevent potential dysfunction. Many factors influence how appropriate, effective, or safe resistance training is and how the exercises are designed, implemented, and progressed. Factors, such as the underlying pathology; the extent and severity of muscle performance impairments; the presence of other deficits; the stage of tissue healing after injury or surgery; and a patient's or client's age, overall level of fitness, and ability to cooperate and learn, all must be considered. Once a program of resistance exercise is developed and prescribed to meet specific functional goals and outcomes, direct intervention by a therapist initially to implement the exercise program or to begin to teach and supervise the prescribed exercises for a smooth transition to an independent, home-based program is imperative. This chapter provides a foundation of information on resistance exercise, identifies the determinants of resistance training programs, summarizes the principles and guidelines for application of manual and mechanical resistance exercise, and explores a variety of regimens for resistance training. It also addresses the scientific evidence, when available, of the relationship between improvements in muscle performance and enhanced functional abilities. The specific techniques described and illustrated in this chapter focus on manual resistance exercise for the extremities, primarily used during the early phase of rehabilitation. Additional exercises performed independently by a patient or client using resistance equipment are described and illustrated in Chapters 17 through 23. The use of resistance exercise for spinal conditions is presented in Chapter 16. **Muscle Performance** **and Resistance Exercise:** **Definitions and Guiding** **Principles** The three elements of muscle performance11---strength, power, and endurance---can be enhanced by some form of resistance exercise. To what extent each of these elements is altered by exercise depends on how the principles of resistance training are applied and how factors such as the intensity, frequency, and duration of exercise are manipulated. Because the physical demands of work, recreation, and everyday living usually involve all three aspects of muscle performance, most resistance training programs seek to achieve a balance of strength, power, and muscular endurance to suit an individual's needs and goals. In addition to having a positive impact on muscle performance, resistance training can produce many other benefits.8-10 These potential benefits are listed in Box 6.1. After a brief description of the three elements of muscle performance, guiding principles of exercise prescription and training are discussed in this section. **Strength, Power, and Endurance** **Strength** *Muscle strength* is a broad term that refers to the ability of contractile tissue to produce tension and a resultant force based on the demands placed on the muscle.182,192,210 More specifically, muscle strength is the greatest measurable force that can be exerted by a muscle or muscle group to overcome resistance during a *single* maximum effort.11 *Functional* *strength* relates to the ability of the neuromuscular system to produce, reduce, or control forces, contemplated or imposed, during functional activities, in a smooth, coordinated manner.42,219 Insufficient muscular strength can contribute to major functional losses of even the most basic activities of daily living. ***Strength training.*** The development of muscle strength is an integral component of most rehabilitation or conditioning **158** Muscle Performance and Resistance Exercise: Definitions and Guiding Principles programs for individuals of all ages and all ability levels. 8,10,90,168,221 *Strength training* (*strengthening exercise*) is defined as a systematic procedure of a muscle or muscle group lifting, lowering, or controlling heavy loads (resistance) for a relatively low number of repetitions or over a short period of time.9,31,103 The most common adaptation to heavy resistance exercise is an increase in the maximum force-producing capacity of muscle---that is, an increase in muscle strength, primarily as the result of neural adaptations and an increase in muscle fiber size.9,10,192 **Power** *Muscle power*, another aspect of muscle performance, is related to the strength and speed of movement and is defined as the work (force × distance) produced by a muscle per unit of time (force × distance/time).11,182,192,210 In other words, it is the *rate* of performing work. The rate at which a muscle contracts and produces a resultant force and the relationship of force and velocity are factors that affect muscle power.31,210 Because work can be produced over a very brief or an extended period of time, power can be expressed by either a single burst of high-intensity activity (such as lifting a heavy piece of luggage onto an overhead rack or performing a high jump) or by repeated bursts of less intense muscle activity (such as climbing a flight of stairs). The terms *anaerobic* *power* and *aerobic power*, respectively, are sometimes used to differentiate these two aspects of power.192 ***Power training.*** Many motor skills in our lives are composed of movements that are explosive and involve both strength and speed. Therefore, re-establishing muscle power may be an important priority in a rehabilitation program. Muscle strength is a necessary foundation for developing muscle power. Power can be enhanced by either increasing the work a muscle must perform during a specified period of time or reducing the amount of time required to produce a given force. The greater the intensity of the exercise and the shorter the time period taken to generate force, the greater is the muscle power. For power training regimens, such as *plyometric training* or *stretch-shortening drills*, the speed of movement is the variable that is most often manipulated293 (see Chapter 23). **Endurance** Endurance is a broad term that refers to the ability to perform low-intensity, repetitive, or sustained activities over a prolonged period of time. *Cardiopulmonary endurance* (*total* *body endurance*) is associated with repetitive, dynamic motor activities, such as walking, cycling, swimming, or upper extremity ergometry, which involve use of the large muscles of the body.8,9 This aspect of endurance is explored in Chapter 7. *Muscle endurance* (sometimes referred to as *local endurance*) is the ability of a muscle to contract repeatedly against a load (resistance), generate and sustain tension, and resist fatigue over an extended period of time.8,9,11,234 The term *aerobic power* sometimes is used interchangeably with muscle endurance. Maintenance of balance and proper alignment of the body segments requires sustained control (endurance) by the postural muscles. In fact, almost all daily living tasks require some degree of muscle and cardiopulmonary endurance. Although strength and muscle endurance, as elements of muscle performance, are associated, they do not always correlate well with each other. Just because a muscle group is strong, it does not preclude the possibility that muscular endurance is impaired. For example, a strong worker has no difficulty lifting a 10-pound object several times, but does the worker have sufficient muscle endurance in the upper extremities and the stabilizing muscles of the trunk and lower extremities to lift 10-pound objects several hundred times during the course of a day's work without excessive fatigue or potential injury? ***Endurance training.** Endurance training* (*endurance exercise*) is characterized by having a muscle contract and lift or lower a light load for many repetitions or sustain a muscle contraction for an extended period of time.9,10,192,210,260 The key parameters of endurance training are low-intensity muscle contractions, a large number of repetitions, and a prolonged time period. Unlike strength training, muscles adapt to endurance training by increases in their oxidative and metabolic capacities, which allows better delivery and use of oxygen. For many patients with impaired muscle performance, endurance training has a more positive impact on improving function than strength training. In addition, using low levels of resistance in an exercise program minimizes adverse forces on joints, produces less irritation to soft tissues, and is more comfortable than heavy resistance exercise. **CHAPTER 6** Resistance Exercise for Impaired Muscle Performance **159** **BOX 6.1 Potential Benefits of Resistance** **Exercise** Enhanced muscle performance: restoration, improvement or maintenance of muscle strength, power, and endurance Increased strength of connective tissues: tendons, ligaments, intramuscular connective tissue Greater bone mineral density or less bone demineralization Decreased stress on joints during physical activity Reduced risk of soft tissue injury during physical activity Possible improvement in capacity to repair and heal damaged soft tissues due to positive impact on tissue remodeling Possible improvement in balance Enhanced physical performance during daily living, occupational, and recreational activities Positive changes in body composition: ↑ lean muscle mass or ↓ body fat Enhanced feeling of physical well-being Possible improvement in perception of disability and quality of life **Overload Principle** **Description** The overload principle is a guiding principle of exercise prescription that has been one of the foundations on which the use of resistance exercise to improve muscle performance is based. Simply stated, if muscle performance is to improve, a load that exceeds the metabolic capacity of the muscle must be applied---that is, the muscle must be challenged to perform at a level greater than that to which it is accustomed. 9,10,128,168,192,206 If the demands remain constant after the muscle has adapted, the level of muscle performance can be maintained but not increased. **Application of the Overload Principle** The overload principle focuses on the progressive loading of muscle by manipulating, for example, the intensity or volume of exercise. Intensity of resistance exercise refers to how much weight (resistance) is imposed on the muscle, whereas volume encompasses variables such as repetitions, sets, or frequency of exercise, any one or more of which can be gradually adjusted to increase the demands on the muscle. In a strength training program, the amount of resistance applied to the muscle is incrementally and progressively increased. For endurance training, more emphasis is placed on increasing the *time* a muscle contraction is sustained or the *number* *of repetitions* performed than on increasing resistance. **PRECAUTION:** To ensure safety, the extent and progression of overload must always be applied in the context of the underlying pathology, age of the patient, stage of tissue healing, fatigue, and the overall abilities and goals of the patient. The muscle and related body systems must be given time to *adapt* to the demands of an increased load or repetitions before the load or number of repetitions is again increased. **SAID Principle** The SAID principle (specific adaptation to imposed demands) 9,192 suggests that a framework of specificity is a necessary foundation on which exercise programs should be built. This principle applies to all body systems and is an extension of Wolff's law (body systems adapt over time to the stresses placed on them). The SAID principle helps therapists determine the exercise prescription and which parameters of exercise should be selected to create specific training effects that best meet specific functional needs and goals. **Specificity of Training** Specificity of training, also referred to as specificity of exercise, is a widely accepted concept suggesting that the adaptive effects of training, such as improvement of strength, power, and endurance, are highly specific to the training method employed. 9,183 Whenever possible, exercises incorporated in a program should mimic the anticipated function. For example, if the desired functional activity requires greater muscular endurance than strength, the intensity and duration of exercises should be geared to improve muscular endurance. Specificity of training also should be considered with respect to mode (type) and velocity of exercise24,80,205,225 as well as patient or limb position (joint angle)161,162,285 and the movement pattern during exercise. For example, if the desired functional outcome is the ability to ascend and descend stairs, exercise should be performed eccentrically and concentrically in a weight-bearing pattern and progressed to the desired speed. Regardless of the simplicity or complexity of the motor task to be learned, task-specific practice always must be emphasized. It has been suggested that the basis of specificity of training is related to morphological and metabolic changes in muscle as well as neural adaptations to the training stimulus associated with motor learning.218 **Transfer of Training** In contrast to the SAID principle, carryover of training effects from one variation of exercise or task to another also has been reported. This phenomenon is called transfer of training, overflow, or a cross-training effect. Transfer of training has been reported to occur on a very limited basis with respect to the velocity of training143,273 and the type or mode of exercise.80 Furthermore, it has been suggested that a cross-training effect can occur from an exercised limb to a nonexercised, contralateral limb in a resistance training program.283,284 A program of exercises designed to develop muscle strength also has been shown to improve muscular endurance at least moderately. In contrast, endurance training has little to no cross-training effect on strength.9,18,103 Strength training at one speed of exercise has been shown to provide some improvement in strength at higher or lower speeds of exercise. However, the overflow effects are substantially less than the training effects resulting from specificity of training. Despite the evidence that a small degree of transfer of training does occur in a resistance exercise program, most studies support the importance of designing an exercise program that most closely replicates the desired functional activities. As many variables as possible in the exercise program should match the requirements and demands placed on a patient during specific functional activities. **Reversibility Principle** Adaptive changes in the body's systems, such as increased strength or endurance, in response to a resistance exercise program are transient unless training-induced improvements are regularly used for functional activities or unless an individual participates in a maintenance program of resistance exercises.8,9,46,89,192 *Detraining,* reflected by a reduction in muscle performance, begins within a week or two after the cessation of resistance exercises and continues until training effects are lost.9,89,167,207 For this reason, it is imperative that gains in strength and endurance are incorporated into daily activities as early as possible in a rehabilitation program. It is also **160** Muscle Performance and Resistance Exercise: Definitions and Guiding Principles advisable for patients to participate in a maintenance program of resistance exercises as an integral component of a lifelong fitness program. **Skeletal Muscle Function** **and Adaptation to Resistance** **Exercise** Knowledge of the factors that influence the force-producing capacity of normal muscle during an active contraction is fundamental to understanding how the neuromuscular system adapts as the result of resistance training. This knowledge, in turn, provides a basis on which a therapist is able to make sound clinical decisions when designing resistance exercise programs for patients with weakness and functional limitations as the result of injury or disease or to enhance physical performance and prevent or reduce the risk of injury in healthy individuals. **Factors that Influence Tension** **Generation in Normal Skeletal Muscle** **NOTE:** For a brief review of the structure of skeletal muscle, refer to Chapter 4 of this textbook. For in-depth information on muscle structure and function, numerous resources are available.182,183,192,210 Diverse but interrelated factors affect the tension-generating capacity of *normal* skeletal muscle necessary to control the body and perform motor tasks. Determinants and correlates include morphological, biomechanical, neurological, metabolic, and biochemical factors. All contribute to the *magnitude, duration,* and *speed* of force production as well as how resistant or susceptible a muscle is to fatigue. Properties of muscle itself and as key neural factors and their impact on tension generation during an active muscle contraction are summarized in Table 6.1. 9,182,183,192,210 Additional factors---such as the energy stores available to muscle, the influence of fatigue and recovery from exercise, and a person's age, gender, and psychological/cognitive status, as well as many other factors---affect a muscle's ability to develop and sustain tension. A therapist must recognize that these factors affect a patient's performance during exercise as well as the potential outcomes of the exercise program. **Energy Stores and Blood Supply** Muscle needs adequate sources of energy (fuel) to contract, generate tension, and resist fatigue. The predominant fiber type found in the muscle and the adequacy of blood supply, which transports oxygen and nutrients to muscle and removes waste products, affect the tension-producing capacity of a **CHAPTER 6** Resistance Exercise for Impaired Muscle Performance **161** The larger the muscle diameter, the greater its tension-producing capacity. Short fibers with pinnate and multipinnate design in high force-producing muscles (ex. quadriceps, gastrocnemius, deltoid, biceps brachii). Long, parallel design in muscles with high rate of shortening but less force production (ex. sartorius, lumbricals). High percentage of type I fibers---low force production, slow rate of maximum force development, resistant to fatigue. High percentage of type IIA and IIB fibers---rapid high force production; rapid fatigue. Muscle produces greatest tension when it is near or at the physiological resting length at the time of contraction. The greater the number and synchronization of motor units firing, the greater the force production. The higher the frequency of firing, the greater the tension. Force output from greatest to least---eccentric, isometric, concentric muscle contraction. Concentric contraction: ↑ speed → ↓ tension. Eccentric contraction: ↑ speed → ↑ tension. **TABLE 6.1** Determinants and Correlates that Affect Tension Generation of Skeletal Muscle **Factor Influence** Cross-section and size of the muscle (includes muscle fiber number and size) Muscle architecture---fiber arrangement and fiber length (also relates to cross-sectional diameter of the muscle) Fiber-type distribution of muscle---type I (tonic, slow-twitch) and type IIA & IIB (phasic, fast-twitch) Length-tension relationship of muscle at time of contraction Recruitment of motor units Frequency of firing of motor units Type of muscle contraction Speed of muscle contraction (force-velocity relationship) muscle and its resistance to fatigue. The three main energy systems (ATP-PC system, anaerobic/glycolytic/lactic acid system, aerobic system) are reviewed in Chapter 7. **Fatigue** Fatigue is a complex phenomenon that affects muscle performance and must be considered in a resistance exercise program. Fatigue has a variety of definitions that are based on the type of fatigue being addressed. ***Muscle (local) fatigue.*** Most relevant to resistance exercise is the phenomenon of skeletal muscle fatigue. Muscle (local) fatigue---the diminished response of muscle to a repeated stimulus---is reflected in a progressive decrement in the amplitude of motor unit potentials.183,192 This occurs during exercise when a muscle repeatedly contracts statically or dynamically against an imposed load. This *acute* physiological response to exercise is *normal* and *reversible.* It is characterized by a gradual decline in the force-producing capacity of the neuromuscular system---that is, a *temporary* state of exhaustion (failure), leading to a decrease in muscle strength.23,56,183,253 The diminished response of the muscle is caused by a complex combination of factors, which include disturbances in the contractile mechanism of the muscle itself (associated with a decrease in energy stores, insufficient oxygen, reduced sensitivity and availability of intracellular calcium, and a build-up of H+) and perhaps reduced excitability at the neuromuscular junction or inhibitory (protective) influences from the central nervous system.183,192,210 The fiber-type distribution of a muscle, which can be divided into two broad categories (type I and type II), affects how resistant it is to fatigue.183,192,210 Type II (phasic, fast-twitch) muscle fibers are further divided into two additional classifications (types IIA and IIB) based on contractile and fatigue characteristics. Some resources subdivide type II fibers into three classifications.239 In general, type II fibers generate a great amount of tension within a short period of time, with type IIB being geared toward anaerobic metabolic activity and having a tendency to fatigue more quickly than type IIA fibers. Type I (tonic, slow-twitch) muscle fibers generate a low level of muscle tension but can sustain the contraction for a long time. These fibers are geared toward aerobic metabolism, as are type IIA fibers. However, type I fibers are more resistant to fatigue than type IIA. Table 6.2 compares the characteristics of muscle fiber types.9,182,183,210 Because different muscles are composed of varying proportions of tonic and phasic fibers, their function becomes specialized. For example, a heavy distribution of type I (slow twitch, tonic) fibers is found in postural muscles, which allows muscles, such as the soleus, to sustain a low level of tension for extended periods of time to hold the body erect against gravity or stabilize against repetitive loads. On the other end of the fatigue spectrum, muscles with a large distribution of type IIB (fast twitch, phasic) fibers, such as the gastrocnemius or biceps brachii, produce a great burst of tension to enable a person to lift the entire body weight or to lift, lower, push, or pull a heavy load but fatigue quickly. Clinical signs of muscular fatigue during exercise are summarized in Box 6.2.192,210 When these signs and symptoms develop during resistance exercise, it is time to decrease the load on the exercising muscle or stop the exercise and shift to another muscle group to allow time for the fatigued muscle to rest and recover. ***Cardiopulmonary (general) fatigue.*** This type of fatigue is the diminished response of an individual (the entire body) as the result of prolonged physical activity, such as walking, jogging, cycling, or repetitive lifting or digging. It is related to the body's ability to use oxygen efficiently. Cardiopulmonary fatigue associated with endurance training is probably caused by a combination of the following factors.23,114 Decrease in blood sugar (glucose) levels. Decrease in glycogen stores in muscle and liver. Depletion of potassium, especially in the elderly patient. **162** Skeletal Muscle Function and Adaptation to Resistance Exercise **TABLE 6.2** Muscle Fiber Types and Resistance to Fatigue **Characteristics Type I Type IIA Type IIB** Resistance High Intermediate Low to fatigue Capillary density High High Low Energy system Aerobic Aerobic Anerobic Diameter Small Intermediate Large Twitch rate Slow Fast Fast Maximum Slow Fast Fast muscle-shortening velocity **BOX 6.2 Signs and Symptoms of Muscle** **Fatigue** An uncomfortable sensation in the muscle, even pain and cramping Tremulousness in the contracting muscle An unintentional slowing of movement with successive repetitions of an exercise Active movements: jerky, not smooth Inability to complete the movement pattern through the full range of available motion during dynamic exercise against the same level of resistance Use of substitute motions---that is, incorrect movement patterns---to complete the movement pattern Inability to continue low-intensity physical activity Decline in peak torque during isokinetic testing ***Threshold for fatigue.*** Threshold for fatigue is the level of exercise that cannot be sustained indefinitely.23 A patient's threshold for fatigue could be noted as the length of time a contraction is maintained or the number of repetitions of an exercise that initially can be performed. This sets a baseline from which adaptive changes in physical performance can be measured. ***Factors that influence fatigue.*** Factors that influence fatigue are diverse. A patient's health status, diet, or lifestyle (sedentary or active) all influence fatigue. In patients with neuromuscular, cardiopulmonary, inflammatory, cancer-related, or psychological disorders, the onset of fatigue is often abnormal.4,56,98 For instance, it may occur abruptly, more rapidly, or at predictable intervals. It is advisable for a therapist to become familiar with the patterns of fatigue associated with different diseases and medications. In multiple sclerosis, for example, the patient usually awakens rested and functions well during the early morning. By mid-afternoon, however, the patient reaches a peak of fatigue and becomes notably weak. Then, by early evening, the fatigue diminishes and strength returns. Patients with cardiac, peripheral vascular, and pulmonary diseases, as well as patients with cancer undergoing chemotherapy or radiation therapy, all have deficits that compromise the oxygen transport system. Therefore, these patients fatigue more readily and require a longer period of time for recovery from exercise.4,98 Environmental factors, such as outside or room temperature, air quality, and altitude, also influence how quickly the onset of fatigue occurs and how much time is required for recovery from exercise.169,192 **Recovery from Exercise** Adequate time for recovery from fatiguing exercise must be built into every resistance exercise program. This applies to both intrasession and intersession recovery. After vigorous exercise, the body must be given time to restore itself to a state that existed prior to the exhaustive exercise. Recovery from acute exercise, in which the force-producing capacity of muscle returns to 90% to 95% of the pre-exercise capacity, usually takes 3 to 4 minutes, with the greatest proportion of recovery occurring in the first minute.51,244 During recovery oxygen and energy stores are replenished quickly in muscles. Lactic acid is removed from skeletal muscle and blood within approximately 1 hour after exercise, and glycogen is replaced over several days. **FOCUS ON EVIDENCE** Studies over several decades have demonstrated that if light exercise is performed during the recovery period (*active* *recovery*)*,* recovery from exercise occurs more rapidly than with total rest (*passive recovery*).28,51,113,244 Faster recovery with light exercise is probably the result of neural as well as circulatory influences.51,244 **CLINICAL TIP** Only if a patient is allowed adequate time to recover from fatigue after each exercise session does muscle performance (strength, power, or endurance) improve over time.28,113 If a sufficient rest interval is not a recurring component of a resistance exercise program, a patient's performance plateaus or deteriorates. Evidence of overtraining or overwork weakness may become apparent (see additional discussion in a later section of this chapter). It has also been shown that fatigued muscles are more susceptible to acute strains.190 **Age** Muscle performance changes throughout the life span. Whether the goal of a resistance training program is to remediate impairments and activity limitations (functional limitations) or enhance fitness and performance of physical activities, an understanding of "typical" changes in muscle performance and response to exercise during each phase of life from early childhood through the advanced years of life is necessary to prescribe effective, safe resistance exercises for individuals of all ages. Key aspects of how muscle performance changes throughout life are discussed in this section and summarized in Box 6.3. **Early Childhood and Preadolescence** In absolute terms, muscle performance (specifically strength), which in part is related to the development of muscle mass, increases *linearly* with chronological age in both boys and girls from birth through early and middle childhood to puberty.191,261,290 Muscle endurance also increases linearly during the childhood years.290 Muscle fiber number is essentially determined prior to or shortly after birth,241 although there is speculation that fiber number may continue to increase into early childhood.290 The rate of fiber growth (increase in cross-sectional area) is relatively consistent from birth to puberty. Change in fiber type distribution is relatively complete by the age of 1, shifting from a predominance of type II fibers to a more balanced distribution of type I and type II fibers.290 Throughout childhood, boys have slightly greater absolute and relative muscle mass (kilograms of muscle per kilogram of body weight) than girls, with boys approximately 10% stronger than girls from early childhood to puberty.191 This difference may be associated with differences in relative muscle mass, although social expectations, especially by midchildhood, also may contribute to the observed difference in muscle strength. It is well established that an appropriately designed resistance exercise program can improve muscle strength in children above and beyond gains attributable to typical growth and development. Furthermore, training-induced strength gains in prepubescent children occur primarily as the result of neuromuscular adaptation---that is, without a significant increase in muscle mass.22,91 Reviews of the literature90,92 have cited many studies that support these findings. However, there **CHAPTER 6** Resistance Exercise for Impaired Muscle Performance **163** **164** Skeletal Muscle Function and Adaptation to Resistance Exercise **BOX 6.3 Summary of Age-Related Changes in Muscle and Muscle Performance Through the Life Span** **Infancy, Early Childhood, and Preadolescence** At birth, muscle accounts for about 25% of body weight. Total number of muscle fibers is established prior to birth or early in infancy. Postnatal changes in distribution of type I and type II fibers in muscle are relatively complete by the end of the first year of life. Muscle fiber size and muscle mass increase linearly from infancy to puberty. Muscle strength and muscle endurance increase linearly with chronological age in boys and girls throughout childhood until puberty. Muscle mass (absolute and relative) and muscle strength is just slightly greater (approximately 10%) in boys than girls from early childhood to puberty. Training-induced strength gains occur equally in both sexes during childhood *without* evidence of hypertrophy (increased muscle mass) until puberty. **Puberty** Rapid acceleration in muscle fiber size and muscle mass, especially in boys. During puberty, muscle mass increases more than 30% per year. Rapid increase in muscle strength in both sexes. Marked difference in strength levels develops in boys and girls. In boys, muscle mass and body height and weight peak before muscle strength; in girls, strength peaks before body weight. Relative strength gains as the result of resistance training are comparable between the sexes, with significantly greater muscle hypertrophy in boys. **Young and Middle Adulthood** Muscle mass peaks in women between 16 and 20 years of age; muscle mass in men peaks between 18 and 25 years of age. Decreases in muscle mass begin to occur as early as 25 years of age. Muscle mass constitutes approximately 40% of total body weight during early adulthood, with men having slightly more muscle mass than women. Muscle continues to develop into the second decade, especially in men. Muscle strength and endurance reach a peak during the second decade, earlier for women than men. By sometime in the third decade, strength declines between 8% and 10% per decade through the fifth or sixth decade. Strength and muscle endurance deteriorate less rapidly in physically active versus sedentary adults. Improvements in strength and endurance are possible with only a modest increase in physical activity. **Late Adulthood** Rate of decline of muscle strength accelerates to 15% to 20% per decade during the sixth and seventh decades and increases to 30% per decade thereafter. By the eighth decade, as loss of muscle mass continues; skeletal muscle mass has decreased by 50% compared to peak muscle mass during young adulthood. Muscle fiber size (cross-sectional area), type I and type II fiber numbers, and the number of alpha motoneurons all decrease. Preferential atrophy of type II muscle fibers occurs. Decreases in the speed of muscle contractions and peak power occur. Gradual but progressive decrease in endurance and maximum oxygen uptake. Loss of flexibility reduces the force-producing capacity of muscle. Minimal decline in performance of functional skills occurs during the sixth decade. Significant deterioration in functional abilities by the eighth decade is associated with a decline in muscular endurance. With a resistance training program, a significant improvement in muscle strength, power, and endurance is possible during late adulthood. Evidence of the impact of resistance training on the level of performance of functional motor skills is mixed but promising. is concern that children who participate in resistance training may be at risk for injuries, such as an epiphyseal fracture or avulsion fracture, because the musculoskeletal system is immature.22,27,103,266 The American Academy of Pediatrics,6 the American College of Sports Medicine (ACSM),8 and the Centers for Disease Control and Prevention (CDC)36 support youth participation in resistance training programs if they are designed appropriately, initiated at a reasonable age, and carefully supervised (Fig. 6.1). With this in mind, two important questions need to be addressed: (1) At what point during childhood is a resistance training program appropriate? and \(2) What constitutes a safe training program? There is general consensus that during the toddler, preschool, and even the early elementary school years, free play and organized but age-appropriate physical activities are effective methods to promote fitness and improve muscle performance, rather than structured resistance training programs. The emphasis throughout most or all of the first decade of life should be on recreation and learning motor skills.278 There is lack of agreement, however, on when and under what circumstances resistance training is an appropriate form of exercise for prepubescent children. Although ageappropriate physical activity on a regular basis has been recommended for children for some time,6,8,36 during the past two or three decades it has become popular for older (preadolescent) boys and girls to participate in sport-specific training programs (including resistance exercises) before, during, and even after the season, in theory, to enhance athletic performance and reduce the risk of sport-related injury. Rehabilitation programs for prepubescent children who sustain injuries during everyday activities also may include resistance exercises. Consequently, an understanding of the effects of exercise in this age group must be the basis for establishing a safe program with realistic goals. **FOCUS ON EVIDENCE** In the preadolescent age group, many studies have shown that improvements in strength and muscular endurance occur on a relative basis similar to training-induced gains in young adults.27,92,93,148 It is also important to point out that, although only a few studies have looked at the effects of detraining in children, when training ceases, strength levels gradually return to a pre-training level, as occurs in adults.89 This suggests that some maintenance level of training could be useful in children as with adults.90 Although training-induced gains in strength and muscular endurance are well-documented, there is insufficient evidence to suggest that a structured resistance training program for children (coupled with a general sports conditioning program) reduces injuries or enhances sports performance.6 However, other health-related benefits of a balanced exercise program have been noted, including increased cardiopulmonary fitness, decreased blood lipids levels, and improved psychological well-being.22,27,89,148 These findings suggest that participation in a resistance training program during the later childhood (preadolescent) years may, indeed, be of value if the program is performed at an appropriate level (low loads and repetitions), incorporates sufficient rest periods, and is closely supervised.6,22,90,266 **Adolescence** At puberty, as hormonal levels change, there is rapid acceleration in the development of muscle strength, especially in boys. During this phase of development, typical strength levels become markedly different in boys and girls, which, in part, are caused by hormonal differences between the sexes. Strength in adolescent boys increases about 30% per year between ages 10 and 16, with muscle mass peaking before muscle strength.34,191 In adolescent girls, peak strength develops before peak weight.95 Overall, during the adolescent years, muscle mass increases more than 5-fold in boys and approximately 3.5-fold in girls.34,191 Although most longitudinal studies of growth stop at age 18, strength continues to develop, particularly in males, well into the second and even into the third decade of life.191 As with prepubescent children, resistance training during puberty also results in significant strength gains. During puberty, these gains average 30% to 40% above that which is expected as the result of normal growth and maturation.90 A balanced training program for the adolescent involved in a sport often includes off-season and preseason aerobic conditioning and low-intensity resistance training followed by more vigorous, sport-specific training during the season.22 The benefits of strength training noted during puberty are similar to those that occur in prepubescent children.89,93 **Young and Middle Adulthood** Although data on typical strength and endurance levels during the second through the fifth decades of life are more often from studies of men than women, a few generalizations can be made that seem to apply to both sexes.185 Strength reaches a maximal level earlier in women than men, with women reaching a peak during the second decade and in most men by age 30. Strength then declines approximately 1% per year,290 or 8% per decade.107 This decline in strength appears to be minor until about age 50278 and tends to occur at a later age or slower rate in active adults versus those who are sedentary.110,290 The potential for improving muscle performance with a resistance training program (Fig. 6.2 A and B) or by participation in even moderately demanding activities several times a week is high during this phase of life. Guidelines for young and middle-aged adults participating in resistance training as part of an overall fitness program have been published by the ACSM8 and the CDC.35 **Late Adulthood** The rate of decline in the tension-generating capacity of muscle in most cases accelerates to approximately 15% to 20% per **CHAPTER 6** Resistance Exercise for Impaired Muscle Performance **165** **FIGURE 6.1** Resistance training, if initiated during the preadolescent years, should be performed using body weight or light weights and carefully supervised. decade in men and women in their sixties and seventies, and it increases to 30% per decade thereafter.110,185 However, the rate of decline may be significantly less (only 0.3% decrease per year) in elderly men and women who maintain a high level of physical activity.118 These disparate findings and others suggest that loss of muscle strength during the advanced years may be due, in part, to progressively greater inactivity and disuse.39 Loss of muscle in the lower extremity and trunk strength and stability during late adulthood---most notably during the seventies, eighties, and beyond---is associated with a gradual deterioration of functional abilities and an increase in the frequency of falling.39,130 The decline in muscle strength and endurance in the elderly is associated with many factors in addition to progressive disuse and inactivity. It is difficult to determine if these factors are causes or effects of an age-related decrease in strength. Neuromuscular factors include a decrease in muscle mass (atrophy), decrease in the number of type I and II muscle fibers with a corresponding increase in connective tissue in muscle, a decrease in the cross-sectional size of muscle, selective atrophy of type II fibers, and change in the length-tension relationship of muscle associated with loss of flexibility, more so than deficits in motor unit activation and firing rate.35,107,141,239,271,278,295 The decline in the number of motor units appears to begin after age 60.141 All of these changes have an impact on strength and physical performance. In addition to decreases in muscle strength, declines in the speed of muscle contraction, muscle endurance, and the ability to recover from muscular fatigue occur with advanced age.141,271 The time needed to produce the same absolute and relative levels of torque output and the time necessary to achieve relaxation after a voluntary contraction are lengthened in the elderly compared to younger adults.107 Consequently, as velocity of movement declines, so does the ability to generate muscle power during activities that require quick responses, such as rising from a low chair or making balance adjustments to prevent a fall. Deterioration of muscle power with age has a stronger relationship to functional limitations and disability than does muscle strength.229 Information on changes in muscle endurance with aging is limited. There is some evidence to suggest that the ability to sustain low-intensity muscular effort also declines with age, in part because of reduced blood supply and capillary density in muscle, decreased mitochondrial density, changes in enzymatic activity level, and decreased glucose transport.107 As a result, muscle fatigue may tend to occur more readily in the elderly. In the healthy and active (community-dwelling) elderly population, the decline in muscle endurance appears to be minimal well into the seventies.141 During the past few decades, as the health care community and the public have become more aware of the benefits of resistance training during late adulthood, a greater number of older adults are participating in fitness programs that include resistance exercises (Fig. 6.3). ACSM and the CDC have published guidelines for resistance training for healthy adults over 60 to 65 years of age (see Box 6.18).8,37 **FOCUS ON EVIDENCE** A review of the literature indicates that when healthy or frail elderly individuals participate in a resistance training program of appropriate duration and intensity, muscle strength and endurance increase.\* Some of these studies have also measured pre-training and post-training levels of functional abilities, such as balance, stair climbing, walking speed, and rising from a chair. The effect of strength and endurance training on functional abilities is promising but still inconclusive, with most but not all39 investigations demonstrating a positive impact.3,40,48,97,145,177,227,265 The disparity of outcomes among investigations underscores the point that resistance training has a direct impact on muscle performance but only an indirect impact on functional performance, a more complex variable. Studies of elderly individuals also have shown that if resistance training is discontinued, detraining gradually occurs; subsequently, **166** Skeletal Muscle Function and Adaptation to Resistance Exercise **FIGURE 6.2** Conditioning and fitness programs for active young and middle-aged adults include resistance training with a balance of **(A)** upper extremity and **(B)** lower extremity strengthening exercises. **A B** \*39,40,47,120,145,177,202,246,263,265,298 strength and functional capabilities deteriorate close to pre-training levels.46,177 In summary, evidence indicates that the decline in muscle strength and functional abilities that occurs during late adulthood can be slowed or at least partially reversed with a resistance training program. However, as in other age groups, if these training-induced improvements are to be maintained, some degree of resistance training must be continued.295 **Psychological and Cognitive Factors** An array of psychological factors can positively or negatively influence muscle performance and how easily, vigorously, or cautiously a person moves. Just as injury and disease adversely affect muscle performance, so can a person's mental status. For example, fear of pain, injury, or re-injury, depression related to physical illness, or impaired attention or memory as the result of age, head injury, or the side effects of medication can adversely affect the ability to develop or sustain sufficient muscle tension for execution or acquisition of functional motor tasks. In contrast, psychological factors can positively influence physical performance. The principles and methods employed to maximize motor performance and learning as functions of effective patient education are discussed in Chapter 1. These principles and methods should be applied in a resistance training program to develop a requisite level of muscle strength, power, and endurance for functional activities. The following interrelated psychological factors as well as other aspects of motor learning may influence muscle performance and the effectiveness of a resistance training program. **Attention** A patient must be able to focus on a given task (exercise) to learn how to perform it correctly. Attention involves the ability to process relevant data while screening out irrelevant information from the environment and to respond to internal cues from the body. Both are necessary when first learning an exercise and later when carrying out an exercise program independently. Attention to form and technique during resistance training is necessary for patient safety and optimal long-term training effects. **Motivation and Feedback** If a resistance exercise program is to be effective, a patient must be willing to put forth and maintain sufficient effort and adhere to an exercise program over time to improve muscle performance for functional activities. Use of activities that are meaningful and are perceived as having potential usefulness or periodically modifying an exercise routine help maintain a patient's interest in resistance training. Charting or graphing a patient's strength gains, for example, also helps sustain motivation. Incorporating gains in muscle performance into functional activities as early as possible in a resistance exercise program puts improvements in strength to practical use, thereby making those improvements meaningful. The importance of feedback for learning an exercise or a motor skill is discussed in Chapter 1. In addition, feedback can have a positive impact on a patient's motivation and subsequent adherence to an exercise program. For example, some computerized equipment, such as isokinetic dynamometers, provide visual or auditory signals that let the patient know if each muscle contraction during a particular exercise is in a zone that causes a training effect. Documenting improvements over time, such as the amount of weight (exercise load) used during various exercises or changes in walking distance or speed, also provides positive feedback to sustain a patient's motivation in a resistance exercise program. **Physiological Adaptations** **to Resistance Exercise** The use of resistance exercise in rehabilitation and conditioning programs has a substantial impact on all systems of the body. Resistance training is equally important for patients with impaired muscle performance and individuals who wish to improve or maintain their level of fitness, enhance performance, or reduce the risk of injury. When body systems are exposed to a greater than usual but appropriate level of resistance in an exercise program, they initially react with a number of *acute* physiological responses and then later adapt---that is, body systems accommodate over time to the newly imposed physical demands.8,9,192 Training-induced adaptations to resistance exercise, known as *chronic* physiological responses, that affect muscle performance are summarized in Table 6.3 and discussed in this section. Key differences **CHAPTER 6** Resistance Exercise for Impaired Muscle Performance **167** **FIGURE 6.3** Incorporating resistance training into a fitness program has many benefits for older adults. in adaptations from strength training versus endurance training are noted. Adaptations to overload create changes in muscle performance and, in part, determine the effectiveness of a resistance training program. The time course for these adaptations to occur varies from one individual to another and is dependent on a person's health status and previous level of participation in a resistance exercise program.10 **Neural Adaptations** It is well accepted that in a resistance training program the initial, rapid gain in the tension-generating capacity of skeletal muscle is attributed largely to neural responses, not adaptive changes in muscle itself.109,183,203,235 This is reflected by an increase in electromyographic (EMG) activity during the first 4 to 8 weeks of training with little to no evidence of muscle fiber hypertrophy. It is also possible that increased neural activity is the source of additional gains in strength late in a resistance training program even after muscle hypertrophy has reached a plateau.168,192 Neural adaptations are attributed to motor learning and improved coordination109,167,169,192 and include *increased* *recruitment* in the number of motor units firing as well as an *increased rate and synchronization* of firing.109,167,225,235 It is speculated that these changes are caused by a decrease in the inhibitory function of the central nervous system (CNS), decreased sensitivity of the Golgi tendon organ (GTO), or changes at the myoneural junction of the motor unit.109,235 **Skeletal Muscle Adaptations** **Hypertrophy** As noted previously, the tension-producing capacity of muscle is directly related to the physiological cross-sectional area of the individual muscle fibers. *Hypertrophy* is an increase in the size (bulk) of an individual muscle fiber caused by an increase in myofibrillar volume.206,271 After an extended period of moderate- to high-intensity resistance training, usually by 4 to 8 weeks1,287 but possibly as early as 2 to 3 weeks with very high-intensity resistance training,255 hypertrophy becomes an increasingly important adaptation that accounts for strength gains in muscle. Although the mechanism of hypertrophy is complex and the stimulus for growth is not clearly understood, hypertrophy of skeletal muscle appears to be the result of an increase in protein (actin and myosin) synthesis and a decrease in protein degradation. Hypertrophy is also associated with biochemical changes that stimulate uptake of amino acids.167,192,206,271 The greatest increases in protein synthesis and, therefore, hypertrophy are associated with high-volume, moderateresistance exercise performed eccentrically.167,232 Furthermore, it is the type IIB muscle fibers that appear to increase in size most readily with resistance training.192,210 **168** Skeletal Muscle Function and Adaptation to Resistance Exercise **TABLE 6.3** Physiological Adaptations to Resistance Exercise **Variable Strength Training Adaptations Endurance Training Adaptations** Skeletal muscle structure Neural system Metabolic system and enzymatic activity Body composition Connective tissue Minimal or no muscle fiber hypertrophy ↑ in capillary bed density ↑ in mitochondrial density and volume (↑ number and size) No changes. ↑ ATP and PC storage: ↑ myoglobin storage ↑ of stored triglycerides ↑ creatine phosphokinase ↑ myokinase No change in lean body mass; ↓ % body fat ↑ tensile strength of tendons, ligaments, and connective tissue in muscle ↑ in bone mineralization with land-based, weight-bearing activities Muscle fibers hypertrophy: greatest in type IIB fibers. Possible hyperplasia of muscle fibers. Fiber type composition: remodeling of type IIB to type IIA; no change in type I to type II distribution (i.e., no conversion) ↓ or no change in capillary bed density: ↓ in mitochondrial density and volume Motor unit recruitment (↑ \# of motor units firing) ↑ rate of firing (↓ twitch contraction time) ↑ synchronization of firing ↑ ATP and PC storage ↑ myoglobin storage Triglycerides storage: change not known ↑ creatine phosphokinase ↑ myokinase ↑ lean (fat-free) body mass; ↓ % body fat ↑ tensile strength of tendons, ligaments, and connective tissue in muscle ↑ bone mineral density; no change or possible ↑ in bone mass **Hyperplasia** Although the topic has been debated for many years and evidence of the phenomenon is sparse, there is some thought that a portion of the increase in muscle size that occurs with heavy resistance training is caused by *hyperplasia*, an increase in the *number* of muscle fibers. It has been suggested that this increase in fiber number, observed in laboratory animals,116,117 is the result of longitudinal splitting of fibers.15,139,199 It has been postulated that fiber splitting occurs when individual muscle fibers increase in size to a point at which they are inefficient, then subsequently split to form two distinct fibers.116 Critics of the concept of hyperplasia suggest that evidence of fiber splitting actually may be caused by inappropriate tissue preparation in the laboratory.115 The general opinion in the literature is that hyperplasia either does not occur, or if it does occur to a slight degree, its impact is insignificant. 183,188 In a review article published in the late 1990s, it was the authors' opinion that if hyperplasia is a valid finding, it probably accounts for a very small proportion (less than 5%) of the increase in muscle size that occurs with resistance training.169 **Muscle Fiber Type Adaptation** As previously mentioned, type II (phasic) muscle fibers preferentially hypertrophy with heavy resistance training. In addition, a substantial degree of plasticity exists in muscle fibers with respect to contractile and metabolic properties.239 Transformation of type IIB to type IIA is common with endurance training,239 as well as during the early weeks of heavy resistance training,255 making the type II fibers more resistant to fatigue. There is some evidence that demonstrates type I to type II fiber type conversion in the denervated limbs of laboratory animals,216,302 in humans with spinal cord injury, and after an extended period of weightlessness associated with space flight.239 However, there is little to no evidence of type II to type I conversion under training conditions in rehabilitation or fitness programs.192,239 **Vascular and Metabolic Adaptations** Adaptations of the cardiovascular and respiratory systems as the result of low-intensity, high-volume resistance training are discussed in Chapter 7. Opposite to what occurs with endurance training, when muscles hypertrophy with highintensity, low-volume training, capillary bed density actually decreases because of an increase in the number of myofilaments per fiber.9 Athletes who participate in heavy resistance training actually have fewer capillaries per muscle fiber than endurance athletes and even untrained individuals.154,269 Other changes associated with metabolism, such as a decrease in mitochondrial density, also occur with high- intensity resistance training.9,167 This is associated with reduced oxidative capacity of muscle. **Adaptations of Connective Tissues** Although the evidence is limited, it appears that the tensile strength of tendons and ligaments as well as bone increases with resistance training designed to improve the strength or power of muscles.49,259,303 **Tendons, Ligaments, and Connective Tissue** **in Muscle** Strength improvement in tendons probably occurs at the musculotendinous junction, whereas increased ligament strength may occur at the ligament-bone interface. It is believed that tendon and ligament tensile strength increases in response to resistance training to support the adaptive strength and size changes in muscle.303 The connective tissue in muscle (around muscle fibers) also thickens, giving more support to the enlarged fibers.192 Consequently, strong ligaments and tendons may be less prone to injury. It is also thought that noncontractile soft tissue strength may develop more rapidly with eccentric resistance training than with other types of resistance exercises.258,259 **Bone** Numerous sources indicate there is a high correlation between muscle strength and the level of physical activity across the life span with bone mineral density.238 Consequently, physical activities and exercises, particularly those performed in weightbearing positions, are typically recommended to minimize or prevent age-related bone loss.226 They are also prescribed to reduce the risk of fractures or improve bone density when osteopenia or osteoporosis is already present.54,238 **FOCUS ON EVIDENCE** Although the evidence from prospective studies is limited and mixed, resistance exercises performed with adequate intensity and with site-specific loading through weight bearing of the boney area to be tested has been shown to increase or maintain bone mineral density.156,160,174,198,209 In contrast, a number of studies in young, healthy women231 and postmenopausal women228,245 have reported that there was no significant increase in bone mineral density with resistance training. However, the resistance exercises in these studies were not combined with site-specific weight bearing. In addition, the intensity of the weight training programs may not have been high enough to have an impact on bone density.174,238 The time course of the exercise program also may not have been long enough. It has been suggested that it may take as long as 9 months to a year of exercise for detectable and significant increases in bone mass to occur.8 In the spine, although studies to date have not shown that resistance training prevents spinal fractures, there is some evidence to suggest that the strength of the back extensors closely correlates with bone mineral density of the spine.245 Research continues to determine the most effective forms of exercise to enhance bone density and prevent agerelated bone loss and fractures. For additional information on prevention and management of osteoporosis, refer to Chapter 11. **CHAPTER 6** Resistance Exercise for Impaired Muscle Performance **169** **Determinants of Resistance** **Exercise** Many elements (variables) determine whether a resistance exercise program is appropriate, effective, and safe. This holds true when resistance training is a part of a rehabilitation program for individuals with known or potential impairments in muscle performance or when it is incorporated into a general conditioning program to improve the level of fitness of healthy individuals. Each of the interrelated elements noted in Box 6.4 and discussed in this section should be addressed to improve one or more aspects of muscle performance and achieve desired functional outcomes. Appropriate *alignment* and *stabilization* are always basic elements of any exercise designed to improve muscle performance. A suitable *dosage* of exercise must also be determined. In resistance training, dosage includes *intensity, volume, frequency,* and *duration* of exercise and *rest interval.* Each factor is a mechanism by which the muscle can be progressively overloaded to improve muscle performance. The *velocity* of exercise and the *mode(type)* of exercise must also be considered. ACSM denotes the key determinants of a resistance training program by the acronym, FITT, which represents frequency, intensity, time, and type of exercise.8 Consistent with the SAID principle discussed in the first section of this chapter, these elements of resistance exercise must be specific to the patient's desired functional goals. Other factors, such as the underlying cause or causes of the deficits in muscle performance, the extent of impairment, and the patient's age, medical history, mental status, and social situation also affect the design and implementation of a resistance exercise program. **Alignment and Stabilization** Just as correct alignment and effective stabilization are basic elements of manual muscle testing and dynamometry, they are also crucial in resistance exercise. To strengthen a specific muscle or muscle group effectively and avoid substitute motions, appropriate positioning of the body and alignment of a limb or body segment are essential. *Substitute motions* are compensatory movement patterns caused by muscle action of a stronger adjacent agonist or a muscle group that normally serves as a stabilizer (fixator).158 If the principles of alignment and stabilization for manual muscle testing137,158 are applied whenever possible during resistance exercise, substitute motions can usually be avoided. **Alignment** ***Alignment and muscle action.*** Proper alignment is determined by the direction of muscle fibers and the line of pull of the muscle to be strengthened. The patient or a body segment must be positioned so the direction of movement of a limb or segment of the body replicates the action of the muscle or muscle groups to be strengthened. For example, to strengthen the gluteus medius, the hip must remain slightly extended, not flexed, and the pelvis must be rotated slightly forward as the patient abducts the lower extremity against the applied resistance. If the hip is flexed as the leg abducts, the adjacent tensor fasciae latae becomes the prime mover and is strengthened. ***Alignment and gravity.*** The alignment or position of the patient or the limb with respect to gravity also may be important during some forms of resistance exercises, particularly if body weight or free weights (dumbbells, barbells, cuff weights) are the source of resistance. The patient or limb should be positioned so the muscle being strengthened acts against the resistance of gravity and the weight. This, of course, is contingent on the comfort and mobility of the patient. Staying with the example of strengthening the gluteus medius, if a cuff weight is placed around the lower leg, the patient must assume the side-lying position so abduction occurs through the full ROM against gravity and the additional resistance of the cuff weight. If the patient rolls toward the supine position, the resistance force is applied primarily to the hip flexors, not the abductors. **Stabilization** Stabilization refers to holding down a body segment or holding the body steady.158 To maintain appropriate alignment, ensure the correct muscle action and movement pattern, and avoid unwanted substitute motions during resistance exercise, **170** Determinants of Resistance Exercise **BOX 6.4 Determinants of a Resistance** **Exercise Program** *Alignment* of segments of the body during exercise *Stabilization* of proximal or distal joints to prevent substitution *Intensity:* the exercise load (level of resistance) *Volume:* the total number of repetitions and sets in an exercise session *Exercise order:* the sequence in which muscle groups are exercised during an exercise session *Frequency:* the number of exercise sessions per day or per week *Rest interval:* time allotted for recuperation between sets and sessions of exercise *Duration:* total time frame of a resistance training program *Mode* of exercise: type of muscle contraction, position of the patient, form (source) of resistance, arc of movement, or the primary energy system utilized *Velocity* of exercise *Periodization:* variation of intensity and volume during specific periods of resistance training *Integration of exercises into functional activities:* use of resistance exercises that approximate or replicate functional demands effective stabilization is imperative. Exercising on a stable surface, such as a firm treatment table, helps hold the body steady. Body weight also may provide a source of stability during exercise, particularly in the horizontal position. It is most common to stabilize the proximal attachment of the muscle being strengthened, but sometimes the distal attachment is stabilized as the muscle contracts. Stabilization can be achieved externally or internally. External stabilization can be applied manually by the therapist or sometimes by the patient with equipment, such as belts and straps, or by a firm support surface, such as the back of a chair or the surface of a treatment table. *Internal stabilization* is achieved by an isometric contraction of an adjacent muscle group that does not enter into the movement pattern but holds the body segment of the proximal attachment of the muscle being strengthened firmly in place. For example, when performing a bilateral straight leg raise, the abdominals contract to stabilize the pelvis and lumbar spine as the hip flexors raise the legs. This form of stabilization is effective only if the fixating muscle group is strong enough or not fatigued. **Intensity** The *intensity* of exercise in a resistance training program is the amount of resistance (weight) imposed on the contracting muscle during each repetition of an exercise. The amount of resistance is also referred to as the *exercise load* (training load)---that is, the extent to which the muscle is loaded or how much weight is lifted, lowered, or held. Remember, consistent with the overload principle, to improve muscle performance the muscle must be loaded to an extent greater than loads usually incurred. One way to overload a muscle progressively is to gradually increase the amount of resistance used in the exercise program.8,10,103,168,169 The intensity of exercise and the degree to which the muscle is overloaded is also dependent on the volume, frequency, and order of exercise or the length of rest intervals. **Submaximal Versus Maximal Exercise Loads** Many factors, such as the goals and expected functional outcomes of the exercise program; the cause of deficits in muscle performance; the extent of impairment; the stage of healing of injured tissues; the patient's age, general health, and fitness level; and other factors, determine whether the exercise is carried out against submaximal or maximal muscle loading. In general, the level of resistance is often lower in rehabilitation programs for persons with impairments than in conditioning programs for healthy individuals. Indications for submaximal loading for moderate to lowintensity exercise versus near-maximal or maximal loading for high-intensity exercise are summarized in Table 6.4. **PRECAUTION:** The intensity of exercise should never be so great as to cause pain. As the intensity of exercise increases and a patient exerts a maximal or near-maximal effort, cardiovascular risks increase substantially. A patient needs to be continually reminded to incorporate rhythmic breathing into each repetition of an exercise to minimize these risks. **CHAPTER 6** Resistance Exercise for Impaired Muscle Performance **171** **TABLE 6.4** Indications for Low-Intensity Versus High-Intensity Exercise **Submaximal Loading Near-Maximal or Maximal Loading** In the early stages of soft tissue healing when injured tissues must be protected. After prolonged immobilization when the articular cartilage is not able to withstand large compressive forces or when bone demineralization may have occurred, increasing the risk of pathological fracture to evaluate the patient's response to resistance exercise, especially after an extended period of inactivity. When initially learning an exercise to emphasize the correct form. For most children or older adults. When the goal of exercise is to improve muscle endurance. To warm-up and cool-down prior to and after a session of exercise. During slow-velocity isokinetic training to minimize compressive forces on joints. When the goal of exercise is to increase muscle strength and power and possibly increase muscle size. For otherwise healthy adults in the advanced phase of a rehabilitation program after a musculoskeletal injury in preparation for returning to high-demand occupational or recreational activities. In a conditioning program for individuals with no known pathology. For individuals training for competitive weight lifting or body building. **Initial Exercise Load (Amount of Resistance)** **and Documentation of Training Effects** It is always challenging to estimate how much resistance to apply manually or how much weight a patient should use during resistance exercises to improve muscle strength particularly at the beginning of a strengthening program. With manual resistance exercise, the decision is entirely subjective, based on the therapist's judgment during exercise. In an exercise program using mechanical resistance, the determination can be made quantitatively. **Repetition Maximum** One method of measuring the effectiveness of a resistance exercise program and calculating an appropriate exercise load for training is to determine a repetition maximum. This term was first reported decades ago by DeLorme in his investigations of an approach to resistance training called progressive resistive exercise (PRE).64,65 A *repetition maximum* (RM) is defined as the greatest amount of weight (load) a muscle can move through the full, available ROM with control a specific number of times before fatiguing. ***Use of a repetition maximum.*** There are two main reasons for determining a repetition maximum: (1) to document a baseline measurement of the dynamic strength of a muscle or muscle group against which exercise-induced improvements in strength can be compared; and (2) to identify an initial exercise load (amount of weight) to be used during exercise for a specified number of repetitions. DeLorme reported use of a 1-RM (the greatest amount of weight a subject can move through the available ROM just one time) as the baseline measurement of a subject's maximum effort but used a multiple RM, specifically a 10-RM, (the amount of weight that could be lifted and lowered 10 times through the ROM) during training.65 Despite criticism that establishing a 1-RM involves some trial and error, it is a frequently used method for measuring muscle strength in research studies and has been shown to be a safe and reliable measurement tool with healthy young adults and athletes103,169 as well as active older adults prior to beginning conditioning programs.202,265,276 **PRECAUTION:** Use of a 1-RM as a baseline measurement of dynamic strength is inappropriate for some patient populations because it requires one maximum effort. It is not safe for patients, for example, with joint impairments, patients who are recovering from or who are at risk for soft tissue injury, or patients with known or at risk for osteoporosis or cardiovascular pathology. **CLINICAL TIP** To avoid the trial and error associated with establishing a 1-RM or to eliminate the need for an at-risk patient to exert a single, maximum effort, formulas have been developed and tables have been published18,140 that enable a therapist to calculate a 1-RM for each muscle group to be strengthened based on the patient performing a greater number of repetitions against a reduced load. Another practical, time-saving way to establish a baseline RM as an index for assessing dynamic strength for a particular muscle group is for a therapist to select a specific amount of resistance (weight) and document how many repetitions can be completed through the full range before the muscle begins to fatigue. If six repetitions, for example, were completed, the baseline resistance would be based on a 6-RM. Remember, a sign of fatigue is the inability to complete the full, available ROM against the applied resistance. **Alternative Methods of Determining Baseline** **Strength or an Initial Exercise Load** Cable tensiometry192 and isokinetic or handheld dynamometry57 are alternatives to a repetition maximum for establishing a baseline measurement of dynamic or static strength. A percentage of body weight also has been proposed to estimate how much resistance (load) should be used in a strength training program.236 Some examples for several exercises are noted in Box 6.5. The percentages indicated are meant as guidelines for the advanced stage of rehabilitation and are based on 10 repetitions of each exercise at the beginning of an exercise program. Percentages vary for different muscle groups. When a maximum effort is inappropriate, the level of perceived loading, as measured by the Borg CR 10 Scale, has been shown to be a useful tool in estimating an appropriate level of resistance and sufficient exercise intensity for muscle strengthening.13 **Training Zone** After establishing the baseline RM, the amount of resistance (exercise load) to be used at the initiation of resistance training often is calculated as a *percentage* of a 1-RM for a particular muscle group. At the beginning of an exercise program the percentage necessary to achieve training-induced adaptations in strength is low (30% to 40%) for sedentary, untrained individuals or very high (\>80%) for those already highly trained. For healthy but untrained adults, a typical training zone usually falls between 40% and 70% of the baseline RM.8,10,14 The lower percentage of this range is safer at the beginning of a program to enable an individual to focus on learning correct exercise form and technique before progressing the exercise load to 60% to 70%. **172** Determinants of Resistance Exercise **BOX 6.5 Percentage of Body Weight as an** **Initial Exercise Load for Selected** **Exercises** Universal bench press: 30% body weight Universal leg extension: 20% body weight Universal leg curl: 10% to 15% body weight Universal leg press: 50% body weight Exercising at a low to moderate percentage of the established RM is recommended for children and the elderly.8,10 For patients with significant deficits in muscle strength or to train for muscular endurance, using a low load---possibly at the 30% to 50% level---is safe yet challenging. **Volume** In resistance training the *volume* of exercise is the summation of the total number of repetitions and sets of a particular exercise during a single exercise session times the intensity of the exercise.8,10,168 The same combination of repetitions and sets is not and should not be used for all muscle groups. There is an inverse relationship between the sets and repetitions of an exercise and the intensity of the resistance. The higher the intensity (load), the lower the number of repetitions and sets possible. Conversely, the lower the load, the greater the number of repetitions and sets possible. Therefore, the exercise load directly dictates how many repetitions and sets are possible. ***Repetitions.*** The number of repetitions in a dynamic exercise program refers to the number of times a particular movement is repeated. More specifically, it is the number of muscle contractions performed to move the limb through a series of continuous and complete excursions against a specific exercise load. If the RM designation is used, the number of repetitions at a specific exercise load is reflected in the designation. For example, 10 repetitions at a particular exercise load is a 10-RM. If a 1-RM has been established as a baseline level of dynamic strength, a percentage of the 1-RM used as the exercise load influences the number of repetitions a patient is able to perform before fatiguing. The "average," untrained adult, when exercising with a load that is equivalent to 75% of the 1-RM, is able to complete approximately 10 repetitions before needing to rest.18,192 At 60% intensity about 15 repetitions are possible, and at 90% intensity only 4 or 5 repetitions are usually possible. For practical reasons, after a beginning exercise load is selected, the target number of repetitions performed for each exercise before a brief rest is often within a range rather than an exact number of repetitions. For example, a patient might be able to complete between 8 and 10 repetitions against a specified load before resting. This is sometimes referred to as a *RM zone,*192 it gives the patient a goal but builds in some flexibility. The number of repetitions selected depends on the patient's status and whether the goal of the exercise is to improve muscle strength or endurance. No optimal number for strength training or endurance training has been identified. Training effects (greater strength) have been reported employing a 2- to 3-RM to a 15-RM.18,170 ***Sets.*** A predetermined number of consecutive repetitions grouped together is known as a *set* or *bout* of exercise*.* After each set of a specified number of repetitions, there is a brief interval of rest. For example, during a single exercise session to strengthen a particular muscle group, a patient might be directed to lift an exercise load 8 to 10 times, rest, and then lift the load another 8 to 10 times. That would be two sets of an 8- to 10-RM. As with repetitions, there is no optimal number of sets per exercise session, but 2 to 4 sets is a common recommendation for adults.8 As few as one set and as many as six sets, however, have yielded positive training effects.10,168 Single-set exercises at low intensities are most common in the very early phases of a resistance exercise program or in a maintenance program. Multiple-set exercises are used to progress the program and have been shown to be superior to single-set regimens in advanced training.170 **Training to Improve Strength or Endurance:** **Impact of Exercise Load and Repetitions** Overall, because many variations of intensity and volume cause positive training-induced adaptations in muscle performance, there is a substantial amount of latitude for selecting an exercise load/repetition and set scheme for each exercise. The question becomes: Is the goal to improve strength, power, or muscular endurance? **To Improve Muscle Strength** In DeLorme's early studies,64,65 three sets of a 10-RM performed for 10 repetitions over the training period led to gains in strength. Current recommendations for strength training vary somewhat. One resource14 suggests that a threshold of 40% to 60% of maximum effort is necessary for adaptive strength gains to occur in a healthy but untrained individual. However, other resources recommend using a moderate exercise load (60% to 80% of a 1-RM) that causes fatigue after 8 to 12 repetitions for 2 or 3 sets.8,168 When fatigue no longer occurs after the target number of repetitions has been completed, the level of resistance is increased to overload the muscle once again. **To Improve Muscle Endurance** Training to improve muscle (local) endurance involves performing many repetitions of an exercise against a submaximal load.9,168,260 For example, as many as three to five sets of 40 to 50 repetitions against a low amount of weight or a light grade of elastic resistance might be used. When increasing the number of repetitions or sets becomes inefficient, the load can be increased slightly. Endurance training also can be accomplished by maintaining an isometric muscle contraction for incrementally longer periods of time. Because endurance training is performed against very low levels of resistance, it can and should be initiated very early in a rehabilitation program without risk of injury to healing tissues. **CLINICAL TIP** When injured muscles are immobilized, type I (slow twitch) muscle fibers atrophy at a faster rate than type II (fast twitch) fibers.206 There is also a slow to fast muscle fiber type **CHAPTER 6** Resistance Exercise for Impaired Muscle Performance **173** conversion with disuse. These changes give rise to a much faster rate of atrophy of antigravity muscles compared with their antagonists,183 underscoring the need for early initiation of endurance training following injury or surgery. **Exercise Order** The sequence in which exercises are performed during an exercise session has an impact on muscle fatigue and adaptive training effects. When several muscle groups are exercised in a single session, as is the case in most rehabilitation or conditioning programs, large muscle groups should be exercised before small muscle groups, and multi-joint exercises should be performed before single-joint exercises.10,103,167,168 In addition, after an appropriate warm-up, higher intensity exercises should be performed before lower intensity exercises.10 **Frequency** *Frequency* in a resistance exercise program refers to the number of exercise sessions per day or per week.8,10 Frequency also may refer to the number of times per week specific muscle groups are exercised or certain exercises are performed.8,168 As with other aspects of dosage, frequency is dependent on other determinants, such as intensity and volume as well as the patient's goals, general health status, previous participation in a resistance exercise program, and response to training. The greater the intensity and volume of exercise, the more time is needed between exercise sessions to recover from the temporarily fatiguing effects of exercise. A common cause of a decline in performance from overtraining (see discussion later in the chapter) is excessive frequency, inadequate rest intervals, and progressive fatigue. Some forms of exercise should be performed less frequently than others because they require greater recovery time. It has been known for some time that high-intensity *eccentric* exercise, for example, is associated with greater microtrauma to soft tissues and a higher incidence of delayed-onset muscle soreness than concentric exercise.16,106,212 Therefore, rest intervals between exercise sessions are longer and the frequency of exercise is less than with other forms of exercise. Although an optimal frequency per week has not been determined, a few generalizations can be made. Initially in an exercise program, short sessions of exercises sometimes can be performed on a daily basis several times per day as long as the intensity of exercise and number of repetitions are low. This frequency often is indicated for early postsurgical patients when the operated limb is immobilized and the extent of exercise is limited to nonresisted isometric (setting) exercises to minimize the risk of muscle atrophy. As the intensity and volume of exercise increases, a frequency of 2 to 3 times per week, every other day, or up to five exercise sessions per week is common.8,10,103,167 A rest interval of 48 hours for training major muscle groups can be achieved by exercising the upper extremities one day and the lower extremities during the next exercise session. Frequency is again reduced for a maintenance program, usually to two times per week. With prepubescent children and the very elderly, frequency typically is limited to no more than 2 to 3 sessions per week.8,10,36,37 Highly trained athletes involved in body building, power lifting, and weight lifting, who know their own response to exercise, often train at a high-intensity and volume up to 6 days per week.10,168,170 **Duration** Exercise *duration* is the total number of weeks or months during which a resistance exercise program is carried out. Depending on the cause of impaired muscle performance, some patients require only a month or two of training to return to the desired level of function or activity, whereas others need to continue the exercise program for a lifetime to maintain optimal function. As noted earlier in the chapter, strength gains observed early in a resistance training program (after 2 to 3 weeks) primarily are the result of neural adaptation. For significant changes to occur in muscle, such as hypertrophy or increased vascularization, at least 6 to 12 weeks of resistance training is required.1,8,192 **Rest Interval (Recovery Period)** ***Purpose of rest intervals.*** Rest is a critical element of a resistance training program and is necessary to allow time for the body to recuperate from the acute effects of exercise associated with muscle fatigue or to offset adverse responses, such as exercise-induced, delayed-onset muscle soreness. Only with an appropriate balance of progressive loading and adequate rest intervals can muscle performance improve. Therefore, rest between sets of exercise and between exercise sessions must be addressed. ***Integration of rest into exercise.*** Rest intervals for each exercising muscle group are dependent on the intensity and volume of exercise. In general, the higher the intensity of exercise the longer the rest interval. For moderate-intensity resistance training, a 2- to 3-minute rest period after each set is recommended. A shorter rest interval is adequate after low-intensity exercise. Longer rest intervals (\>3 minutes) are appropriate with high-intensity resistance training, particularly when exercising large, multi-joint muscles, such as the hamstrings, which tend to fatigue rapidly.8,10 While the muscle group that was just exercised is resting, resistance exercises can be performed by another muscle group in the same extremity or by the same muscle group in the opposite extremity. Patients with pathological conditions that make them more susceptible to fatigue, as well as children and the elderly, should rest at least 3 minutes between sets by performing a nonresisted exercise, such as low-intensity cycling, or performing the same exercise with the opposite extremity. Remember, active recovery is more efficient than passive recovery for neutralizing the effects of muscle fatigue. **174** Determinants of Resistance Exercise Rest between exercise sessions must also be considered. When strength training is initiated at moderate intensities (typically in the intermediate phase of a rehabilitation program after soft tissue injury), a 48-hour rest interval between exercise sessions (that is, training every other day) allows the patient adequate time for recovery. **Mode of Exercise** The *mode* of exercise in a resistance exercise program refers to the form of exercise, the type of muscle contraction that occurs, and the manner in which the exercise is carried out. For example, a patient may perform an exercise dynamically or statically or in a weight-bearing or nonweight-bearing position. Mode of exercise also encompasses the form of resistance---that is, how the exercise load is applied. Resistance can be applied manually or mechanically. As with other determinants of resistance training, the modes of exercise selected are based on a multitude of factors already highlighted throughout this section. A brief overview of the various modes of exercise is presented in this section. An in-depth explanation and analysis of each of these types of exercise can be found in the next section of this chapter and in Chapter 7. **Type of Muscle Contraction** Figure 6.4 depicts the types of muscle contraction that may be performed in a resistance exercise program and their relationships to each other and to muscle performance.182,210,248 Isometric (static) or dynamic muscle contractions are two broad categories of exercise. Dynamic resistance exercises can be performed using *concentric* (shortening) or *eccentric* (lengthening) contractions, or both. When the velocity of limb movement is held consistent by a rate-controlling device, the term *isokinetic* contraction is sometimes used.248 An alternative perspective is that this is simply a dynamic (shortening or lengthening) contraction that occurs under controlled conditions.182 **Position for Exercise: Weight-Bearing** **or Nonweight-Bearing** The patient's body position or the position of a limb in nonweight-bearing or weight-bearing positions also alters the mode of exercise. When a nonweight-bearing position is assumed and the distal segment (foot or hand) moves freely during exercise, the term open-chain exercise (or a variation of this term) is often used. When a weight-bearing position is assumed and the body moves over a fixed distal segment, the term closed-chain exercise is commonly used.182,210,248 Concepts and issues associated with the use of this terminology are addressed later in this chapter. **Forms of Resistance** *Manual* resistance and *mechanical* resistance are the two broad methods by which resistance can be applied. A *constant* or *variable* load can be imposed using mechanical resistance (e.g., free weights or weight machines). *Accommodating* resistance138 can be implemented by use of an isokinetic dynamometer that controls the velocity of active movement during exercise. *Body weight* or partial body weight is also a source of resistance if the exercise occurs in an antigravity position. Although an exercise performed against only the resistance of the weight of a body segment (and no additional external resistance) is defined as an active rather than an activeresistive exercise, a substantial amount of resistance from the weight of the body can be imposed on contracting muscles by altering a patient's position. For example, progressive loads can be placed on upper extremity musculature during push-ups by starting with wall push-ups while standing, progressing to push-ups while leaning against a countertop, push-ups in a horizontal position (Fig. 6.5), and finally push-ups from a head-down position over a large exercise ball. **CHAPTER 6** Resistance Exercise for Impaired Muscle Performance **175** **FIGURE 6.4** Types of muscle contractions: their relationships to muscle performance and their tension-generating capacities. **FIGURE 6.5** Body weight serves as the source of resistance during a push-up. **Energy Systems** Modes of exercise also can be classified by the energy systems used during the exercise. Anaerobic exercise involves highintensity (near-maximal) exercise carried out for a very few number of repetitions because muscles rapidly fatigue. Strengthening exercises fall into this category. Aerobic exercise is associated with low-intensity, repetitive exercise of large muscle groups performed over an extended period of time. This mode of exercise primarily increases muscular and cardiopulmonary endurance (refer to Chapter 7 for an in-depth explanation). **Range of Movement: Short-Arc or Full-Arc** **Exercise** Resistance through the full, available range of movement (full-arc exercise) is necessary to develop strength through the ROM. Sometimes resistance exercises are executed through only a portion of the available range. This is known as shortarc exercise. This form of exercise is used to avoid a painful arc of motion or a portion of the range in which the joint is unstable or to protect healing tissues after injury or surgery. **Mode of Exercise and Application to Function** Mode-specific training is essential if a resistance training program is to have a positive impact on function. When tissue healing allows, the type of muscle contractions performed or the position in which an exercise is carried out should mimic the desired functional activity as closely as possible.205 **Velocity of Exercise** The velocity at which a muscle contracts significantly affects the tension that the muscle produces and subsequently affects muscular strength and power.217 The velocity of exercise is frequently manipulated in a resistance training program to prepare the patient for a variety of functional activities that occur across a wide spectrum of slow to fast velocities. **Force-Velocity Relationship** The force-velocity relationship is different during concentric and eccentric muscle contractions, as depicted in Figure 6.6. **Concentric Muscle Contraction** During a maximum effort concentric muscle contraction, as the velocity of muscle shortening increases, the force the muscle can generate *decreases.* EMG activity and torque decrease as a muscle shortens at faster contractile velocities, possibly because the muscle may not have sufficient time to develop peak tension.53,182,210,248,294 **Eccentric Muscle Contraction** Findings are less consistent for eccentric than concentric muscle actions. During a maximum effort eccentric contraction, as the velocity of active muscle lengthening increases, force production in the muscle initially *increases* to a point but then *quickly levels off.*38,63,182,210,248 The initial increase in force production may be a protective response of the muscle when it is first overloaded. It is thought that this increase may be important for shock absorption or rapid deceleration of a limb during quick changes of direction.78,248 The rise in tension also may be caused by stretch of the noncontractile tissue in muscle.63 In contrast, other research indicates that eccentric force production is essentially unaffected by velocity and remains constant at slow and fast velocities.53,121 **Application to Resistance Training** A range of slow to fast exercise velocities has a place in an exercise program. Resistance training with free weights is safe and effective only at slow to medium velocities of limb movement so the patient can maintain control of the moving weight. Because many functional activities involve reasonably fast velocities of limb movement, training at only slow velocities is inadequate. The development of the isokinetic dynamometer during the late 1960s138,200 gave clinicians a tool to implement resistance training at fast as well as slow velocities. In recent years, some variable resistance exercise units (pneumatic and hydraulic) and elastic resistance products also have afforded additional options for safely training at fast velocities. *Velocity-specific training* is fundamental to a successful rehabilitation program. Results of numerous studies since the 1970s have shown that training-induced strength gains in a resistance exercise program primarily occur at the training velocities,24,80,149 with limited transfer of training (physiological overflow) above and below the training velocities.143,273 Accordingly, training velocities for resistance exercises should be geared to match or approach the demands of the desired functional activities.57,149 Isokinetic training, using *velocity spectrum rehabilitation* regimens, and *plyometric training,* also known as *stretchshortening* *drills*, often emphasize high-speed training. These approaches to exercise are discussed later in this chapter and in Chapter 23, respectively. **FIGURE 6.6** Force