Monograph 7


By R. C. Schafer, DC, PhD, FICC
Manuscript Prepublication Copyright 1997

Copied with permission from  ACAPress

    Physical Fitness
    Physical Performance
    Genetic Influences

Athletic Training and Practice
    Coordination, Balance, and Agility
    Mechanical Advantage and Bulk
    Overall Potential

References and Bibliography


The body can adapt to withstand a wide variety of threatening environmental forces. This results in systems designed to work in harmony during ever-changing internal and external stimuli. The nervous system provides the necessary intricate coordination, and physiologic measurements offer us data on the essential factors involved in the integrated aspects of organic function. While voluminous data can be gathered, our major concern in rehabilitation involves those aimed at restoring and maintaining homeostasis.

Physical Fitness

Fitness is an attribute of the human organism at its best. The development of optimal physical fitness produces changes increasing physical capacity and produces changes in altered metabolism at the cellular level. A reduced level of fitness predisposes to injury, its extent, and modifies its healing at both its macroscopic and microscopic aspects.

The degree of vascularity of the capillary network between skeletal muscle fibers and in associated tissues depends greatly on the type of habitual exercise. The quantity of interstitial fat, most marked in atrophied muscle, is also determined by the degree of exercise. Lymph vessels are not found within voluntary muscle.

Because every sport and every physical activity demands a different combination of physiologic capabilities, an individual's fitness must be specific for the task at hand. Even if the potential exists to develop certain capabilities, the decision to do so is a personal one.

Physical Performance

The common human performance parameters are those of strength, endurance, flexibility, speed, coordination, balance, and agility. Body type could also be included here. Intelligence, creativity, and motivation are other parameters. Clinicians must keep in mind that the quality of one's strength, power, flexibility, speed, coordination, balance, and agility are determined essentially by neuromuscular functions.

The range of ability to perform motor activity is determined by three essential aspects: (1) neuromuscular function (eg, strength and skill), (2) energy output (eg, aerobic and anaerobic processes), and (3) psychologic factors (eg, motivation, perseverance). These aspects are involved in almost all types of physical labor, though emphasis varies.

Performance is the result of central and peripheral neuromuscular mechanisms, the energy yield of split compounds, and profound psychologic effects. The methods of estimating physical performance capacity can be classified into two major groups: (1) physical fitness tests and (2) physiologic tests. Typical examples of physical fitness tests in sports are sprints, endurance runs, high jumps, long jumps, and throwing.

Determination of maximum oxygen intake and muscle strength comprise the common physiologic types of tests as in (1) determining the mechanical power developed on a bicycle ergometer or staircase; (2) determining the time element necessary to run a specified distance; (3) determining the duration of running on a treadmill at varying degrees of speed and grade; and (4) determining the distance run in a specified period.

Most studies used to evaluate physical performance have been in the field of athletics. We can use this specialized information by applying it to the nonathletic patient as it involves daily activities. While the principles remain the same, a concern is to remember that (1) the demands of the nonathlete are much less strenuous, and (2) the factors considered in physical fitness are not equal in all people who could be judged highly fit.

Genetic Influences

Little can be done to modify body type as much of the variables found in body build and its individual physiology, especially in regard to oxygen intake, are genetically determined. While skill has an influence on efficiency, body type places a finite limit on physical achievement goals. It is granted that a great deal of practice time is allotted to the development of skill, but many aspects of skill are inherited (eg, receptor organ sensitivity). The potential "superstar" probably starts life with a peculiar advantage.

Because body type and receptor-organ sensitivity are essentially governed genetically, training cannot turn an antelope into an ox or vice versa. Depending on one's genetic framework, training is an enhancement to potential expression. But many variables are not trainable. Skill is the result of practice, not training.

Practice tends to develop timing, accuracy, and conditioned responses so that conscious faculties can be concentrated on competitive strategies rather than on physical activities during competition. The term "pressure practice" is practice under highly competitive game conditions (eg, an intersquad scrimmage).


"Training" and "practice" have different goals in sports. Training refers to the improvement or maintenance of physical capacity such as systematic endurance or strength exercises. Normally, these are out of context with a particular sport. Practice means repeating specific skill-developing techniques used in a specific sport so that they may be executed at a higher level of performance. During practice, a swimmer might practice his push-off, a tennis player his forehand, a golfer his putting, etc.

To complicate terminology, what would be called practice in one sport might be called training in another. For example, weight lifting is practice for the weight lifter, training for the football tackle. Running is practice for the track athlete, training for the boxer.

As available time is also a concern, the relative degree of emphasis between training and practice varies from sport to sport. Training is minimal in tennis and golf, practice is secondary to training in most explosive-strength sports, and most highly skilled team sports require a careful blend of training and practice.

Coordination, Balance, and Agility

rdination may be defined as the ability to integrate separate abilities into a complex task. Well-coordinated movement, usually involving the large muscles, requires alert timing between the nervous and muscular systems, as seen in bowling, gymnastics, badminton, throwing, jumping hurdles, handball, tennis, ice hockey, baseball, golf, or soccer.

lance is a necessary attribute when one's base of support is reduced yet body position must be maintained. Ballet-like balance is required in such sports as tightrope walking, handstanding, surfing, karate, hockey, skiing, and to some varying degree in most ballplaying sports where movement is required in an "off-balance" position.

ility, the ability to change positions in space, involves speed with the addition of a sudden change in direction or height such as in a defensive maneuver or a change in attack. The number of positional changes available is obviously almost endless, thus total agility is difficult to evaluate. Agility is demanded in hockey players, running backs, gymnasts, infielders, divers, boxers, karate enthusiasts, and wrestlers.

Mechanical Advantage and Bulk

Mechanical advantage and disadvantage have distinct relationships with performance. Pace varies with limb length, for example, thus long limbs are an advantage in running, especially in long distance events. A higher center of gravity is a disadvantage in that it takes extra postural effort to maintain balance (eg, gymnastics, skating), but it has its advantages in sports (eg, basketball) where increased height places one closer to the goal.

Body bulk has both its advantages and disadvantages. Muscle bulk, especially in contact sports, provides both force inertia and protection for bones and joints. Body weight is of less consideration in swimming sports because much weight is supported, it offers some buoyancy advantages, or it provides necessary insulation due to subcutaneous fat (eg, open-water swimming). In contrast, due to gravitational pull, a heavy bulk is a disadvantage in running sports as the weight must be raised at each pace.

There are also disadvantages in that bulky hypertrophy increases viscous resistance to movement, produces problems from physical apposition, and increases the body mass to be moved. Thus, to avoid mass accumulation in an irrelevant part of the body, muscle training should be specific for the use desired, as indiscriminate muscle hypertrophy is likely to impair performance in endurance events.


Strength is nearly proportional to the quantity of body muscle. Muscle issue constitutes about 43% of body weight in the well-developed adult male and about 38% in the adult female. The average ratio between muscle strength and an individual's weight has been computed to be about 26:6. Thus, the aggregate strength of the muscles of a 150-lb male is about 4,000 lbs. Since only a portion of all muscle can be employed for a specific task, only a small part of one's total strength can be effectively used for a particular task.

As described previously, strength is developed in three manners: (1) isotonically by exercises against resistance in such a manner that body movements are allowed; (2) isometrically by exercises done against resistance in such a manner that body movements are restricted -sometimes offering a short cut to goals of equivalent repetitive drudgery; and (3) isokinetically, by exercises of a constant velocity against resistance that adapts to the angle of a joint. Isokinetic exercises are employed primarily to rehabilitate up to the point of normal strength, after which other forms of exercise are used.

Application.   Weight lifting is quite helpful in enhancing strength and endurance when judiciously applied, but it has little effect on developing flexibility or cardiorespiratory endurance. With weights, sets of about a dozen repetitions per bout are typical. A "bout" is one exercise series or program. Training with weights has both its zealot adherents and opponents, and both are well-armed with empiric evidence.

Most athletic movements are isotonic rather than isometric, and most all sports present strength demands above those required for normal living. Boxing, wrestling, judo, football, hockey, soccer, lacrosse, and nearly all other contact sports require above-average strength output, as do certain noncontact sports such as mountain climbing, gymnastics, and rowing. Strength development programs (eg, weight lifting, exercise equipment) are now popular in a wide number of sports in which they were rarely considered in times past (eg, tennis, swimming). Table 1.

Table 1.   Comparison of Various Exercises to Develop Strength
Goal Isometric Isotonic Isokinetic Variable Resistance
Eccentric contraction Zero Yes Zero Yes
Reciprocal contraction Zero Zero Yes Zero
Speed (velocity) Zero Variable Constant Variable
Specific to sport Low Medium Very high Medium
Motivation Low High Medium High
Safety Excellent Poor Excellent Poor

Certain exercises should be discouraged. For instance, deep-knee bends and the duck-waddle, both used for many years with several sports, exert severe stress on the cruciate ligaments of the knee, far outweighing any benefit to the quadriceps muscles.

Guided Progression.   Care must be taken in any exercise program for logical progression. Regardless of a patient's enthusiasm, the beginning level should be comfortably below that which may cause injury. Progression at any stage should be to a comfortable point requiring moderate effort. Stress should be felt, but pain should not be. The rationale is for the patient to work to full capacity against an orderly ever-increasing resistance.

Progression means to permit a small but increasing overload to tissues to spur adaptation of bones, tendons, muscles, ligaments, and capsules. Failure to increase demands will stymie further development. Imposing an increase in loading too quickly produces reinjury. The objective is to slightly overload, not overwhelm, the musculature. Correct procedure, state Wallis/Logan, is to balance these two extremes by applying the Specific Adaptation to Imposed Demands (SAID) principle. Wilmore says this implies that the body responds to a given orderly demand with a specific and predictable adaptation. More simply, function improves with use.

Harrelson points out that it is not a prerequisite that full pain-free active ranges of motion are attained before beginning active resistive exercise. The patient may begin isometric manual resistance or active resistive exercise in a limited range of motion as long as the healing process is not jeopardized. This is usually achieved by light-load high repetitions, not a few heavy-load repetitions.

An ideal program should be balanced and conducted on a daily basis with varying intensity on alternate days, depending on how often and when the athlete competes. An intense workout the day before competition or hard labor is usually not wise.

Types of Strength

For discussion, strength can be divided into dynamic (isotonic), explosive, and static (isometric) strength:

1.   Dynamic strength is the ability to lift, move, and support body weight, calling on endurance when functions are strenuously repeated; ie, explosive movements repeated in rapid succession (eg, pull-ups, sprinting). Limits are imposed by speed resisting factors and the quantity and quality of energy-exchange factors. If extreme pain, breathlessness, or weakness comes at the end of an effort, a gain in active muscle strength will do much to improve performance.

2.   Explosive strength is the ability to release maximum power in the fastest possible time (eg, standing long jump). Single violent efforts are commonly seen in sprinting, jumping, and throwing where speed factors are combined with force/velocity features of active muscle. Response is determined by mechanical leverage (influenced by body type), immediate energy resources from tissue chemical coupling (influenced by glycogen and mineral ion levels), the quantity of actin and myosin filaments per fiber (influenced by training hypertrophy), and the number of fibers activated (influenced by learning experiences). The performance result of these forces is determined largely by the degree of dynamic viscosity.

3.   Static strength, a separate factor of physical fitness, is exertion of a maximum force for a brief period of time against a difficult to move object (eg, handgrip or arm-pull dynamometer). The importance of static strength is brought out in such activities as weight lifting. Such strength depends on the total number of active muscle fibers involved in a specific activity; ie, the functional (gross muscle less fat and connective tissue) cross-section of the muscle exerting the force, under control of the nervous system, with some assistance from the type of contraction and mechanical advantage in play.

Lessening of central inhibition and greater relaxation of antagonists also play a part in the performance effect. Limits are imposed by exhaustion, motivation, Valsalva effects, pain, and quickly diminishing endurance. Muscles required to contract against increasing resistance become progressively stronger and usually, but not inevitably, hypertrophied; ie, women need not fear that weight lifting with good style will bring gross overdevelopment.

Effects of Strength on Circulation.   Either progressing central or local fatigue adversely affects skill: diminished skill is commonly associated with approaching exhaustion. As muscle perfusion is greater in a strong muscle as compared with that of a weak muscle, fatigue, to a great extent, is due to inadequate perfusion. However, the overt signs of pallor and the energy-wasting poor coordination, confusion, and staggering gait are to be blamed on inadequate blood flow to the posture-regulating center. Strength also has an effect on recovery in that strength tends to minimize the microtrauma secondary to oxygen lack and local weakness.

General Health.   There is a close correlation to one's degree of strength and endurance and one's degree of health because of the relationship to a greater capacity for physical work and the lessened functional response to the challenges of stress. In this regard, the absence of disease would not indicate health.


Power, the rate of doing work, is determined by the rate at which energy can be released within muscle. While the type of contraction (ie, isometric, eccentric, or concentric), resistance, duration, quantity of repetitions, and number of exercise bouts are important in any exercise program, the most important factor appears to be that the contraction force developed by a muscle must be close to maximum if improved change is to be expected.

It has been well established that low-repetition high-resistance exercises develop power; high-repetition low-resistance exercises develop endurance. Many authorities believe that strength is the only training variable in enhancing the speed of muscle contraction and that tissue viscosity is relatively constant. This is only true, however, when strengthening actions mimic movements used in customary activities.


Endurance is the capacity to maintain strenuous activity of a large number of muscle groups for a duration sufficient to prolonged resistance to progressing fatigue and oxygen demands. It is essentially oxygen dependent and a manifestation of cardiovascular and respiratory function (both aerobic and anaerobic capacity).

The word "endurance" is generally used to denote the ability of skeletal muscle to continue contractions relative to contraction length (time), contraction quantity (per unit of time), and contraction quality (force). During vigorous muscle effort, the endurance factor is determined by the initial glycogen content of muscle fibers (influenced by diet and training).

Endurance increases as the result of enhanced muscle hypertrophy, glycogen reserves, myoglobin, and increased vasculature. Local endurance is an effect of muscle strength. We witness this in the power of serves in a long tennis match and the final paces of the runner. In such instances, repetitions of isotonic overload exercises enhance local endurance. Both quantity of movements and the time duration involved are reflections of endurance; eg, the number of push-ups or pull-ups, time to run a mile. Endurance exhibits in a variety of ways in sports, especially gymnastics, crew, cross-country skiing, mountain climbing, wrestling, marathon runs, and swims.

Circuit Training.   Circuit training is frequently used to develop endurance and strength both locally and generally in athletes. A series of exercises (eg, 8-10) is done in sequence. A battery of stretching curls, push-ups, sit-ups, pull-ups, and shuttle runs may be completed. These exercises are added to those primarily designed for a specific activity. During the first circuit program, each participant is judged on each exercise for the maximum number of repetitions. This maximum may be determined either by time or by the point of exhaustion.

The score obtained is divided by three to arrive at the individual's training rate for each exercise. At subsequent sessions, the participant completes three circuits of the prescribed exercises as fast as possible at the training rate. As training progresses, the time necessary to complete the three circuits will gradually decrease. Then adjustments can be made by increasing the number of repetitions. An alternative method to continually retesting the maximum level is to set three progressively difficult grades of performance, but this largely ignores individual differences.

Cardiovascular-Respiratory Training.   Circuit training can be modified to be appropriate for even youth and the elderly showing signs of dysfunctional cardiovascular-respiratory efficiency. Careful monitoring and supervision are mandatory to avoid overstress. With progression keyed to adaptive changes, such training can lead to increased respiratory volume, increased blood volume and cardiac output, improved heart rate and stroke volume, and an increase in total hemoglobin. Shands also reports benefits in improved blood supply to active musculature have also been noted.


Speed is highly correlated with muscle power and difficult to isolate. Speed (fastness) can refer to running time or reaction time. It can refer to the legs of a racer, the fists of a boxer, the arms of a goalie, the eyes of a skeet shooter. As inertia is proportional to mass, total speed considers three aspects: time/distance in initiating body movement (explosive force of active muscles); time/distance rate of acceleration to maximum; and time/distance loss of acceleration as the event is prolonged. During the last stage of a run, the reducing metabolic energy must cope with continuing resistances in air flow, tissue viscosity, surface friction, and other energy losses.

Great speed is obviously essential in runners, tennis players, and football backs and ends. Like a heavy truck, it is difficult for a bulky body frame to accelerate quickly to maximum speed. The ideal physique of a sprinter would be one with powerful legs and little weight elsewhere. In sprinting, performance is enhanced by a preliminary warm-up that raises intramuscular temperature. A slight increase in temperature enhances muscle-tissue viscosity and use of energy resources influencing muscle contraction.

Reaction Speed.   Reaction speed is determined by the interval between when stimuli are received by receptor organs (eg, eyes, ears, hands, soles) and when the muscles react. A fast reaction time is necessary for high efficiency in such events as table tennis, dashes, and boxing.


Plyometrics are exercises that link speed of motion and strength to cause an explosive reactive-type motion. They are helpful in developing speed, power, and locomotor skill. This is achieved by maximizing the myotactic (stretch) reflex. Muscles are fully stretched by eccentric contractions immediately before a concentric contraction. The greater the stretch on the muscle from its resting length, the greater the load the concentric contraction can overcome.

Application is commonly done with large rubber bands (eg, Thera-Bands) or surgical tubing. The subject muscles are stretched against the resistance of the band or tube, then the patient moves the part or limb as fast as possible through all normal ranges of motion.

Such drills, states Harrelson, were once confined to off-season strength training programs but have now become part of therapeutic rehabilitation in the late stages. Application two or three times weekly is sufficient when combined with other regimens. Judicious use of plyometrics is important because overuse injuries easily occur when supervision is not alert.

Overall Potential

While performance assessment is often studied in an isolated compartmentalized fashion (eg, strength, endurance, aerobic-anaerobic capacity, etc) by the exercise physiologist, care must be taken to avoid oversimplified specific-muscle training that does not enhance overall performance. For example, the development of isometric strength does not assure an improvement in dynamic performance. Multiple intrinsic and extrinsic factors and adaptations affect performance on any one day. A "star" is more than the sum of his parts.


Physical performance during youth is greatly determined by body size, body type, the degree of maturation of the nervous system, and the degree of sexual maturation (more important in males than females). Added to this under conditions of good nutrition, the body of both the immature and mature thrives on use, and use encourages adaptation to maintain a functional reserve of about 25% greater than demand.


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