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 a resistance exercise program 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. 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.

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 and 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 muscle and its resistance to fatigue.

Fatigue. Fatigue is a complex phenomenon that affects muscle performance and must be considered in a resistance training 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. 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.

The diminished response of the muscle is caused by a combination of factors, which include:
• Disturbances in the contractile mechanism of the muscle itself because of a decrease in energy stores, insufficient oxygen, and a build-up of H+
• Inhibitory (protective) influences from the central nervous system
• Possibly a decrease in the conduction of impulses at the myoneural junction, particularly in fast-twitch fibers

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. 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. 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.

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 (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 (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.

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.

• Decrease in blood sugar (glucose) levels
• Decrease in glycogen stores in muscle and liver
• Depletion of potassium, especially in the elderly patient

Signs and Symptoms of Muscle Fatigue
• An uncomfortable sensation in the muscle, even pain and cramping
• Tremulousness in the contracting muscle
• 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. 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, oncologic, inflammatory, or psychological disorders, the onset of fatigue is often abnormal. 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.

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.

Recovery from Exercise. Adequate time for recovery from fatiguing exercise must be built into every resistance training 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, where 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.

Changes that occur in muscle during recovery are:
• Oxygen stores are replenished in muscles.
• Energy stores are replenished.
• Lactic acid is removed from skeletal muscle and blood within approximately 1 hour after exercise.
• Glycogen is replaced over several days.

Focus on Evidence. It has been known for some time that if light exercise is performed during the recovery period (active recovery), recovery from exercise occurs more rapidly than with total rest (passive recovery) Faster recovery with light exercise is probably the result of neural as well as circulatory influences.

PRECAUTIONS: Only if a patient is allowed adequate time to recover from fatigue after each exercise session does long-term muscle performance (strength, power, or endurance) improve.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.

Age. Muscle performance changes throughout the life span. Whether the goal of a resistance training program is to remediate impairments and 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.

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. Muscle endurance also increases linearly during the childhood years. Muscle fiber number is essentially determined prior to or shortly after birth, although there is speculation that fiber number may continue to increase into early childhood. 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.

Summary of Age-Related Changes in Muscle and Muscle Performance
Infancy, Early Childhood, and Preadolescence
• At birth, muscle accounts for about 25% of body weight.
• Total number of muscle fibers is established prior to or early during 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 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 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.
• Strength 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.
• Loss of muscle mass continues; by the eighth decade, 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.
• Decrease in the speed of muscle contractions and peak power.
• 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 during the sixth decade.
• Significant deterioration in functional abilities by the eighth decade 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.

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. This difference may be associated with differences in relative muscle mass, although social expectations, especially by mid-childhood, also may contribute to the observed difference in muscle strength.

There is no question that an appropriately designed resistance exercise program can improve muscle strength in children above and beyond gains attributable to typical growth and development. A review of the literature cited many studies that support this statement. However, there 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. The American Academy of Pediatrics and the American College of Sports Medicine support youth participation in resistance training programs if they are designed appropriately and carefully supervised. With this in mind, two important questions need to be addressed: (1) At what point during childhood is a resistance training program appropriate? (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.

However, there is lack of agreement on when and under what circumstances resistance training is an appropriate form of exercise. During the past two decades it has become popular for older (preadolescent) boys and girls to participate in resistance training programs, in theory, to enhance athletic performance and reduce the risk of sport-related injury. In addition, prepubescent children who sustain injuries during everyday activities may require rehabilitation that may include resistance exercises. Consequently, an understanding of the effects of exercise in this age group founded on current research 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. 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 pretraining level, as occurs in adults. This suggests that some maintenance level of training could be useful in children as with adults.

In addition, although evidence to suggest that a structured resistance training program for children (in addition to a general sports conditioning program) reduces injuries or enhances sports performance is inconclusive, other health-related benefits have been noted, including increased cardiopulmonary fitness, decreased blood lipids levels, and improved psychological well-being. 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) and is closely supervised.

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. Longitudinal studies ,of adolescent boys indicate that strength increases about 30% per year between ages 10 and 16, with muscle mass peaking before muscle strength. In adolescent girls, peak strength develops before peak weight. Overall, during the adolescent years, muscle mass increases more than 5-fold in boys and approximately 3.5-fold in girls. 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.

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. Benefits of strength training noted during puberty are similar to those noted in prepubescent children.

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. 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, or 8% per decade.This decline in strength appears to be minor until about age 50 and tends to occur at a later age or slower rate in active adults versus those who are sedentary.The potential for improving muscle performance with a resistance training program 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 have been published by the American College of Sports Medicine (ACSM).

Late Adulthood. The rate of decline in the tension-generating capacity of muscle in most cases accelerates to approximately 15% to 20% per decade in men and women in their sixties and seventies, and it increases to 30% per decade thereafter. 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.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.Loss of muscle strength during late adulthood, particularly during the eighties and beyond, is associated with a gradual increase in functional limitations as well as an increase in the frequency of falling.

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 age-related deterioration 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.The decline in the number of motor units appears to begin after age 60. All of these changes have an impact on strength and physical performance.

In addition to decreases in muscle strength, declines in speed of muscle contraction, muscle endurance, and the ability to recover from muscular fatigue occur with advanced age. 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. 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 adjusting one’s balance to prevent a fall.

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. 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.

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, more and more older adults are participating in fitness programs that include resistance exercises. ACSM has also published guidelines for resistance training for healthy adults over 60 to 65 years of age.

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 pretraining and post-training levels of functional abilities, such as balance, stair climbing, walking speed, and chair rise. The effect of strength and endurance training on functional abilities is promising but still inconclusive, with most but not all investigations demonstrating a positive impact.This 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 have also shown that if resistance training is discontinued, detraining gradually occurs; and subsequently strength and functional capabilities deteriorate close to pretraining levels.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.

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 one’s mental status. For example, fear of pain, injury, or reinjury, 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 also positively influence physical performance.

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.

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.

Buy the Book that holds this excerpt: Therapeutic Exercise: Foundations and Techniques (Therapeutic Exercise: Foundations & Techniques)

Related Articles