Therapeutic Exercise 6장 손상된 근육수행력 개선을 위한 저항운동

[문형철]

글쓰기에 대한 통찰을 얻기 위해

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 과부하의 원칙

Description

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 is the overload principle. 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.8,9,129,193,207 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.

P R E C A U T I O N : 

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.

골격근 장력생성에 영향을 주는 요인들

1)  근육의 단면적과 근육크기(근섬유의 숫자와 크기) 

2) 섬유배열과 섬유길이 - pinnation angle에 의해 장력이 결정됨. 

3) 근섬유 종류 -  type2섬유가 많으면 장력이 큼. 

4) 근육길이-장력관계 - 안정길이에서 최대 장력

5) 운동단위의 동원 - 신경발화와 동조화가 최대장력을 만듬. 

6) 근수축 방법과 근수축 속도 - 원심성수축, 등척성 수축, 구심성 수축의 순서로 장력형성. 

        구심성 수축에서 속도가 높아지면 장력이 약함. 원심성 수축에서 속도가 높아지면 장력이 커짐. 

Recovery from Exercise 운동후 회복

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.48,61,246 

- 저항운동후에 근피로의 적절한 회복시간이 고려되어야 함. 

-  강한 저항훈동 후 90-95% 근력이 회복되는데, 일반적으로 3-4분이 소요됨. 대부분 1분이내에 가장 많은 회복을 함. 

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).27,48,61,113,246 Faster recovery with light exercise is probably the result of neural as well as circulatory influences.48,61,246

나이에 따른 근육과 근수행력

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.

유년기-사춘기 이전

- 태어날때 근육은 체중의 25%, 성인은 체중의 40%

- 유년기에(그 이전에) 근섬유의 총 숫자는 정해짐

- type 1, 2 비율은 1세이전에 결정됨. 

- 근섬유 크기와 근육 질량은 청소년기까지 계속 직선적으로 증가함.

- 근력, 근지구력은 사춘기까지 연대기적으로 직선적으로 증가함

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.

사춘기

- 사춘기에 근섬유 크기와 질량이 빠르게 증가함. 근육질량은 매년 30%넘게 증가함.

- 소년의 경우 근력에 앞서 근육질량, 키, 체중이 최고에 다다름. 소녀의 경우 체중에 앞서 근력이 최고에 다다름.

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.

- 근육질량은 여성에서 16-20세에 피크, 남성에서 18-25세에 피크

- 근육질량은 25세부터 감소되기 시작함

- 근력은 50-60대까지 매 10년마다 8-10% 줄어듬. 

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.

- 60-70대까지 근력감소는 매 10년마다 15-20%가 줄어들고, 이후에는 매 10년마다 30%가 줄어듬.

- 80세가 되면 최대 근육질량에 비해 50%까지 감소됨.

- 근섬유 크기(단면적), type 1, 2 숫자, 그리고 알파운동뉴런은 모두 감소함. 우선적으로 type2 근섬유가 감소함.

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. 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.192,265,298 Muscle endurance also increases linearly during the childhood years.298 Muscle fiber number is essentially determined prior to or shortly after birth,242 although there is speculation that fiber number may continue to increase into early childhood.298 The rate of fiber growth (increase in cross-sectional area) is relatively consistent from birth to puberty. Change in fiber type distribut ion is relatively complete by the age of 1, shifting from a predominance of type II fibers to 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.283

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 sportrelated 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.26,92,93,148 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 pretraining level, as occurs in adults.90 This suggests that some maintenance level of training could be useful in children as with adults.91 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,6 other health-related benefits have been noted, including increased cardiopulmonary fitness, decreased blood lipids levels, and improved psychological well-being.26,90,148 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) and is closely supervised.6,271 

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 studies33,192 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.95 Overall, during the adolescent years, muscle  mass increases more than 5-fold in boys and approximately 3.5-fold in girls.33,192 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.292

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. 91 Benefits of strength training noted during puberty are similar to those noted in prepubescent children.90,93

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.186 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,298 or 8% per decade.109 This decline in strength appears to be minor until about age 50 283 and tends to occur at a later age or slower rate in active adults versus those who are sedentary.111,298 The potential for improving muscle performance with a resistance training program (Fig. 6.2 A&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 have been published by the American College of Sports Medicine (ACSM).8 

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. 111,186 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.119 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.35 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.35

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.35,109,141,240,276,283,304 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 speed of muscle contraction, muscle endurance, and the ability to recover from muscular fatigue occur with advanced age.141,276 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.109 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.109 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, more and more older adults are participating in fitness programs that include resistance exercises (Fig. 6.3). ACSM has also published guidelines for resistance training for healthy adults over 60 to 65 years of age.8

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 all35 investigations

demonstrating a positive impact.3,36,43,97,145,179,226,270 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.42,179 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 traininginduced

improvements are to be maintained, some degree

of resistance training must be continued.30 

저항운동에 대한 생리학적 적응

1. 골격근의 구조

1) 근력운동 - type 2 섬유의 비대, 근섬유의 hyperplasia(결합조직 비대), type2B가 type2A로 바뀜. 모세혈관 밀도 낮아짐. 미토콘드리아 밀도 변화 없음. 미토콘드리아 용량 작아짐. 

2) 근지구력 운동 - 약간의 근비대 또는 변화없음. 모세혈관밀도 높아짐. 미토콘드리아 밀도와 용량 증가함. 

2. 신경요소

1) 근력운동 - 운동단위 동원 능력 좋아짐. 신경발화빈도 증가(근육반응시간 빨라짐), 신경발화 동조 능력 개선됨. 

3. 대사요소

1) 근력운동 - ATP 와 CP 저장능력 개선됨. 미오글로빈 저장능력 개선됨. 중성지방은 알려지지 않음. 

2) 근지구력 운동 - ATP와 CP 저장능력 개선됨. 미오글로빈 저장능력 개선, 중성지방 저장능력 개선됨

4. 효소

1) 근력운동 - 크레아틴 포스포키나아제 증가, myokinase증가 

2) 근지구력운동에서도 유사함

5. 신체조성

1) 근력운동 - lean body mass 증가, 지방 감소

2) 근지구력 운동 -  lean body mass변화없음, 지방감소

6. 결합조직

1) 근력운동 - 힘줄, 인대, 결합조직의 장력 증가, 뼈밀도 증가

2) 근지구력운동 - 힘줄, 인대, 결합조직장격 증가, 뼈밀도 증가

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 responses166 and then later adapt. That is, body systems accommodate over time to the newly imposed physical demands.8,121,172,193 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 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.9

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 largely attributed to neural responses, not adaptive changes in muscle itself.204,234,235 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.193

Neural adaptations are attributed to motor learning and improved coordination168,169,172,193 and include increased recruitment in the number of motor units firing as well as an increased rate and synchronization of firing.168,224,234,235 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.172,234,235

Skeletal Muscle Adaptations

Hypertrophy

As noted previously, the tension-producing capacity of muscle is directly related to the physiological crosssectional 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.207,276 After an extended period of moderate- to high-intensity resistance training, usually by 4 to 8 weeks1,172,295 but possibly as early as 2 to 3 weeks with very high-intensity resistance training,259 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.168,193,207,276 The greatest increases in protein synthesis and therefore hypertrophy are associated with high-volume, moderate-resistance exercise performed eccentrically.168 In addition, it is the type IIB muscle fibers that appear to increase in size most readily with resistance training.172,193

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.12,140,198 It has been postulated that fiber splitting occurs when individual muscle fibers increase in size to a point where they are inefficient, then subsequently split to form two distinct fibers.116 Critics of the concept of hyperplasia suggest that evidence of fiber splitting may actually 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.189 In a recent review article 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. 240 Transformation of type IIB to type IIA is common with endurance training,240 as well as during the early weeks of heavy resistance training,259 making the type II fibers more fatigue-resistant. There is some evidence that demonstrates type I to type II fiber type conversion in the denervated limbs of laboratory animals,216,311 in humans with spinal cord injury, and after an extended period of weightlessness associated with space flight.240 However, there is little to no evidence of type II to type I conversion under training conditions in rehabilitation or fitness programs. 193,240

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 high-intensity, low-volume training, capillary bed density actually decreases because of an increase in the number of myofilaments per fiber.172 Athletes who participate in heavy resistance training actually have fewer capillaries per muscle fiber than endurance athletes and even untrained individuals.154,274 Other changes associated with metabolism, such as a decrease in mitochondrial density, also occur with high-intensity resistance training.168,172 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.45,263,286,312

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.312 The connective tissue in muscle (around muscle fibers) also thickens, giving more support to the enlarged fibers.193 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.262,263

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.239,243 Consequently, physical activities and exercises, particularly those performed in weight-bearing positions, are typically recommended to minimize or prevent age-related bone loss.225 They are also prescribed to reduce the risk of fractures or improve bone density when osteopenia or osteoporosis is already present.52,112,239

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 bony area to be tested has been shown to increase or maintain bone mineral density.156,160,175,197,210 In contrast, a number of studies in young, healthy women230 and postmenopausal women227,247 have reported that there was no significant increase in bone mineral density with resistance  raining. 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.175,239 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.243 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.247

Research continues to determine the most effective forms of exercise to enhance bone density and prevent

age-related bone loss and fractures. For additional information on prevention and management of osteoporosis, refer to Chapter 11.

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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 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 of exercise must also be considered. These elements are summarized in Box 6.4.

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 may also 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, effective stabilization is imperative.

Exercising on a stable surface, such as a firm treatment

table, helps hold the body steady. Body weight may also

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 the 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.

9,103,169 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, including 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.

Submaximal loading. Exercise at moderate to low intensities

is indicated:

At the beginning of an exercise program to eval‎uate the

patient’s response to resistance exercise, especially after

extended periods of inactivity

In the early stages of soft tissue healing when injured tissues

must be protected

After periods of 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

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

Near maximal or maximal loading. High-intensity exercise

is indicated:

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

P R E C A U T I O N : The intensity of exercise should

never be so great as to cause pain. As the intensity of

exercise increases and a patient exerts a maximum or nearmaximum

effort, cardiovascular risks substantially increase.

A patient needs to be continually reminded to incorporate

rhythmic breathing into each repetition of an exercise to

minimize these risks.

Initial Level of Resistance (Load)

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).63-65 A repetition

maximum (RM) is defined as the greatest amount of weight

(load) a muscle can move through the available range of

motion (ROM) a specific number of times.

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

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 lift through the available ROM just one time) as the

baseline measurement of a subject’s maximum effort but

used a 10 RM (the amount of weight that could be lifted

and lowered exactly 10 times) during training.64,65

In the clinical setting, a practical, time-saving way to

establish a baseline RM 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. Remember, a sign of fatigue is the inability to

complete the available ROM against the applied resistance.

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.

203,213,270,281

P R E C A U T I O N : 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.

Alternative Methods of Determining Baseline

Strength and a Beginning Exercise Load

Cable tensiometry193 and isokinetic or handheld dynamometry55

are alternatives to a repetition maximum for establishing

a baseline measurement of strength. A percentage

of body weight also has been proposed to estimate how

much resistance (load) should be used in a strengthening

program.236 Some examples for different exercises are listed

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.

Training Zone

After establishing the baseline RM, the amount of resistance

(exercise load) to be used at the initiation of resistance

training is often 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 traininginduced

adaptations in strength is low (30% to 40%) for

sedentary, untrained individuals or very high (80% to 95%)

for those already highly trained. For healthy but untrained

adults, a typical training zone usually falls between 60%

and 70% of an RM.9 The lower percentage of this range is

safer at the beginning of a program to enable an individual

to focus on learning exercise form and technique.

Exercising at a low to moderate percentage of the

established RM is also recommended for children and

the elderly.8,286 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 multiplied by

the resistance used.9,169 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 number

of repetitions performed and the intensity of the resistance.

The higher the intensity (load), the lower the number of

repetitions that are possible; and conversely, the lower the

load, the greater the number of repetitions possible. Therefore,

the exercise load directly dictates how many repetitions

and sets are possible.

Repetitions and Sets

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 strength, a percentage of the 1 RM used as the

exercise load influences the number of repetitions a patient

is able to perform. 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.16,193 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 zone193; 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 2 to 3 RM to 15 RM.16,170

Sets. A predetermined number of repetitions grouped

together is known as a set or bout. 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 a load 8 to 10 times, rest, and then lift the load 8 to 10

more 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. As few as one set and as many as six

sets have yielded positive training effects.9 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 singleset

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 is: Is the goal to improve

strength, muscular endurance, or both?

To Improve Muscle Strength

In DeLorme’s early studies63-65 three sets of a 10 RM performed

for 10 repetitions over the training period led to

gains in strength. Current recommendations are to use an

exercise load that causes fatigue after 6 to 12 repetitions

for two to three sets (6 to 12 RM).103 When fatigue no

longer occurs after the target number of repetitions has

been completed, the level of resistance is increased to once

again overload the muscle.

To Improve Muscle Endurance

Training to improve local endurance involves performing

many repetitions of an exercise against a submaximal

load.32,264 For example, as many as three to five sets of 40

to 50 or more 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 can also 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. Remember, when

injured muscles are immobilized, type I (slow twitch)

fibers atrophy at a faster rate than type II (fast twitch)

fibers.207,231 This underscores the need for early initiation

of endurance training.

Exercise Order

The sequence in which exercises are performed during an

exercise session has an impact on muscle fatigue and the

adaptive training effects. When multiple muscle groups

are exercised in a single session, as is often the case in

rehabilitation or conditioning programs, large muscle

groups should be exercised before small muscle groups

and multijoint muscles before single-joint muscles.9,103,168

In addition, after an appropriate warm-up, higher intensity exercises should be performed before lower intensity

exercises.9

Frequency

Frequency in a resistance exercise program refers to the

number of exercise sessions per day or per week.236 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, and progressive fatigue. Some

forms of exercise should be performed less frequently than

others because they require greater recovery time. Highintensity

eccentric exercise, for example, is associated with

greater microtrauma to soft tissues and a higher incidence

of delayed-onset muscle soreness than other modes of

exercise.13,108,212 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, so long as the intensity and number

of repetitions are low, short sessions of exercises sometimes

can be performed on a daily basis several times per

day. This frequency is often indicated for early postsurgical

patients when the operated limb is immobilized and the

extent of exercise is limited to low-intensity isometric (setting)

exercises to prevent or minimize atrophy. As the

intensity and volume of exercise increases, every other day

or up to five exercise sessions per week is common.8,103,168

Frequency is again reduced for a maintenance program,

usually to two times per week. With prepubescent children314

and the very elderly,8 frequency is usually limited

to two to three sessions per week. 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.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 an impairment in 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) 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.1,8,193

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 exerciseinduced,

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 (4 to 5 minutes) are appropriate with

high-intensity resistance training, particularly when exercising

large, multijoint muscles, such as the hamstrings,

which tend to fatigue rapidly.9,61 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

an unresisted 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.61

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

non-weight-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  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 the 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.184,250 

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.250 An alternative perspective is

that this is simply a dynamic (shortening or lengthening)

contraction that occurs under controlled conditions.184

Position for Exercise: Weight-Bearing

or Non-Weight-Bearing

The patient’s body position or the position of a limb in

non-weight-bearing or weight-bearing positions also alters

the mode of exercise. When a non-weight-bearing position

is assumed and the distal segment (foot or hand) moves

freely during exercise, the term open-chain exercise 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.184,250 Concepts

associated with this approach to classifying exercises and

definitions of this terminology are addressed later in the

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 active-resistive 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 on an incline board.

Energy Systems

Modes of exercise can also be classified by the energy systems

used during the exercise. Anaerobic exercise involves

high-intensity (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 short-arc exercise. This form of exercise is used to avoid a painful arc of motion or a portion of the range

where 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.206

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 range 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

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.50,184,250,303

Eccentric Muscle Contraction

Findings are less consistent for eccentric than concentric

muscle actions. During an eccentric contraction, as the

velocity of active muscle lengthening increases, force production

in the muscle initially also increases but then quickly levels off.34,62,184,250 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.78,250 The

rise in tension may also be caused by stretch of the noncontractile

tissue in muscle.62 In contrast, other research

indicates that eccentric force production is essentially unaffected

by velocity and remains constant at slow and fast

velocities.50,122

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 1960s138,199 gave

clinicians a tool to implement resistance training at fast as

well as slow velocities. In recent years, some variableresistance

exercise units (pneumatic and hydraulic) as well

as elastic resistance products also have afforded additional

options for safely training at fast velocities.

Speed-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 speeds,23,80,149,200 with limited transfer of training

(physiological overflow) to other speeds of movement.

143,278 Accordingly, training velocities for resistance

exercises should be geared to match or approach the

demands of the desired functional activities.55,149

Isokinetic training, using velocity spectrum rehabilitation

regimens, and plyometric training, also known as

stretch-shortening drills, often emphasize high-speed training.

These approaches to exercise are discussed later in the

chapter.

Periodization

Periodization, also known as periodized training, is an

approach to resistance training that builds systematic variation

in exercise intensity and repetitions, sets, or frequency

at regular intervals over a specified period of time.102 This

approach to training was developed for highly trained

athletes preparing for competitive weight-lifting or powerlifting

events. The concept was designed to prevent overtraining

and psychological staleness prior to competition

and to optimize performance during competition.102

In preparation for competition, the training calendar is

broken down into cycles, or phases, that sometimes extend

over an entire year. The idea is to prepare for a “peak” performance

at the time of competition. Different types of

exercises at varying intensities, volume, frequency, and rest

intervals are performed over a specific time period. Table

6.4 summarizes the characteristics of each cycle.

Although periodized training is commonly implemented

prior to a competitive event, evidence to support the

efficacy of periodization is limited.102,170,193 Despite this,

periodized training has also been used on a limited basis in

the clinical setting for injured athletes during the advanced

stage of rehabilitation.96