Muscular strength can be defined as the force that a muscle group can exert against a resistance in a maximal effort. In 1948, Delorme (36) adopted the name "progressive resistance exercise" for his method of developing muscular strength through the utilization of counter balances and weight of the extremity with a cable and pulley arrangement. This technique gave load-assisting exercises to muscle groups that did not perform antigravity motions. McQueen (37) distinguished between exercise regimens for producing muscle hypertrophy and those for producing muscle power. He concluded that the number of repetitions for each set of exercise determines the different characteristics of the various training procedures.

When muscles contract, the limbs may appear to move in different ways. One type of motion is a static contraction, known as an isometric type of contraction. Another type of contraction is a shortening or dynamic contraction which is called an isotonic contraction. Dynamic contractions are accompanied by muscle shortening and by limb movement. Dynamic contractions can exhibit two types of motion. One activity is a concentric contraction in which the joint angle between the two bones become smaller as the muscular tension is developed. The other action is an eccentric contraction in which, as the muscles contract, the joint angle between the bones increases.

Owing to ambiguity in the literature concerning certain physiologic terms and differences in laboratory procedures, the following terms are defined:

• (1) Muscular strength: the contractile power of muscles as a result of a single maximum effort.
• (2) Muscular endurance: ability of the muscles to perform work by holding a maximum contraction for a given length of time or by continuing to move submaximal load to a certain level of fatigue.
• (3) Isometric: a muscular contraction of total effort but with no visible limb movement (sometimes referred to as "static" or "anaerobic").
• (4) Isotonic: raising and lowering a submaximal load, such as a weight, a given number of times (sometimes called "dynamic" or "aerobic").
• (5) Isokinetic training (accommodating resistance): muscular contraction at a constant velocity. As the muscle length changes, the resistance alters in a manner that is directly proportional to the force exerted by the muscle.
• (6) Concentric contraction: an isotonic contraction in which the muscle length decreases (that is, the muscle primarily responsible for movement becomes shorter).
• (7) Eccentric contraction: an isotonic contraction in which the muscle length of the primary mover is believed to increase. That is, the muscle primarily responsible for movement becomes longer.
• (8) Muscle overload: the workload for a muscle or muscle group that is greater than that to which the muscle is accustomed.
• (9) Variable resistance exercise: as the muscle contracts, the resistance changes in a predetermined manner (linear, exponentially, or as defined by the user).
• (10) Variable velocity exercise: as the muscle contracts with maximal or submaximal tension, the speed of movement changes in a predetermined manner (linear, exponentially, or as defined by the user).
• (11) Repetitions: the number of consecutive times a particular movement or exercise is performed.
• (12) Repetition maximum (1 RM): the maximum resistance a muscle or muscle group can overcome in a maximal effort.
• (13) Sets: the number of groups of repetitions of a particular movement or exercise.

Based on evidence presented in these early studies, hundreds of investigations have been published relative to techniques for muscular development, including isotonic exercises, isometric exercises, eccentric contractions, and many others. The effectiveness of each exercise type has been supported and refuted by numerous investigations but no definitive, irrefutable conclusions have been established.

Hellebrandt and Houtz (38) shed some light on the mechanism of muscle training in an experimental demonstration of the overload principle. They found that the repetition of contractions which place minimal stress on the neuromuscular system had little effect on the functional capacity of the skeletal muscles. They also found that the amount of work done per unit of time is the critical variable upon which extension of the limits of performance depends. The speed with which functional capacity increases suggests that the central nervous system, as well as the contractile tissue, is an important contributing component of training.

Results from the work of Hellebrandt and Houtz (38) suggest that an important consideration in both the design of equipment for resistive exercise and the performance of an athlete or a busy executive is that the human body relies on pre-programmed activity by the central nervous system. Since most human movements are ballistic and since the neural control of these patterns differs from slow controlled movements, it is essential that training routines employ programmable motions to suit specific movements. This control necessitates exact precision in the timing and coordination of both the system of muscle contraction and the segmental sequence of muscular activity. Research has shown that a characteristic pattern of motion is present during any intentional movement of body segments against resistance. This pattern consists of reciprocally organized activity between the agonist and antagonist. These reciprocal activities occur in consistent temporal relationships with the variables of motion, such as velocity, acceleration, and forces.

In addition to the control by the nervous system, the human body is composed of linked segments, and rotation of these segments about their anatomic axes is caused by force. Both muscle and gravitational forces are important in producing these turning effects, which are fundamental in body movements in all sports and daily living. Pushing, pulling, lifting, kicking, running, walking, and all human activities result from the rotational motion of the links which, in humans, are the bones. Since force has been considered the most important component of athletic performance, many exercise equipment manufacturers have developed various types of devices employing isometrics and isokinetics. When considered as a separate entity, force is only one factor influencing successful athletic performance. Unfortunately, these isometric and isokinetic devices inhibit the natural movement patterns of acceleration and deceleration.

The three factors underlying all athletic performances and the majority of routine human motions are force, displacement, and the duration of movement. In all motor skills, muscular forces interact to move the body parts through the activity. The displacement of the body parts and their speed of motion are important in the coordination of the activity and are also directly related to the forces produced. However, it is only because of the control provided by the brain that the muscular forces follow any particular displacement pattern and, without these brain center controls, there would be no skilled athletic performances. In every planned human motion, the intricate timing of the varying forces is a critical factor in successful performances.

In any human movement, the accurate coordination of the body parts and their velocities is essential for maximizing performances. This means that the generated muscular forces must occur at the right time for optimum results. For this reason, the strongest weightlifter cannot put the shot as far as the experienced shotputter Although the weightlifter possesses greater muscular force, he has not trained his brain centers to produce the correct forces at the appropriate time. Older individuals may be unable to walk up and down stairs or perform many of the daily, routine functions which had been virtually automatic before the deterioration produced by weakness, disease, or merely age.