Muscular strength is defined as the maximum force that can be developed by a muscle or muscle group against resistance. Resistance can take one of two forms in athletic training: constant resistance or variable resistance.
Constant resistance is a form of training where the resistance directed against the target muscle or muscle group does not vary through the range of athletic movement. The lifting of free weights is an example of constant resistance. In an exercise involving constant resistance, the muscular power generated and the effect of the resistance on the muscle are not constant. An example of the effect of constant resistance is found in the simple curl of a free weight using a conventional dumbbell. At the start of the exercise, the muscles of the arm, primarily the bicep, pull the weight upward. There is a point, as the dumbbell is lifted, where the resistance on the muscle is greatest; at the movement before and after that maximal point, the resistance force is much less. At the conclusion of the exercise, there is little or no resistance being applied to the bicep.
Variable resistance requires differing degrees of force to be applied to the target muscle to create constant resistance, compelling the muscle to work harder to meet the demands of the exercise. In a variable resistance arm curl, the forces applied to the bicep are variable at each stage of the curl in order to maintain the same resistance throughout the trajectory of movement.
Variable resistance exercise is designed to achieve maximum muscular involvement. Inherent in the effectiveness of variable resistance exercise is the relationship between the intensity required of the athlete and the volume of resistance work involved. Resistance exercises are typically carried out through the use of specialized machines that are designed to permit control of the entire force-generating movement.
The machines may be designed for the training of any muscle group in the body. Machines that isolate the pectoral muscles through a seated bench press, deltoid machines, and shoulder extension machines are three of a multitude of models commercially available. The common features of variable resistance training machines are the presence of cables, pulleys, or other devices to create variability, coupled with the placement of the user in a fixed position to ensure that the user cannot recruit other muscle groups to assist in the completion of the prescribed movements.
Variable resistance training also endeavors to capitalize on the fact that the force that is inherent in the development of greater strength has itself two aspects: the internal forces that are generated within the joint to produce strength and the external force produced by the resistance.
Variable resistance training success is predicated on the principle of regulating the amount of work per unit of training time. The variable resistance training is intended to harness the strength of the user in a coordinated movement. Repetition of those coordinated movements is a means of refining the relationship between the brain and the muscles it directs in the exercise. The repetitions also serve to help exclude the interference by external stimuli in the performance of the movement. This progression through the exercise that subjects the body to variable forces ultimately enhances athletic skill.
One of the world's best known variable exercise machines is the Universal line, developed in the early 1970s by Dr. Gideon Ariel, a specialist in bio-mechanics. The Universal machines are constructed on the basis of two principles that guide their application in muscle development. The first principle is that resistance exercise is most effective when a multiple number of joints are required to complete the exercise. The second principle is the notion of explosive repetitions—to achieve the maximum level of mental concentration and to obtain the maximum firing levels of muscle fiber for optimal performance.
The engagement of a multiple number of joints in resistance training requires the body develop the strength to counteract the forces applied by the machine throughout the entire range of motion in each repetition. When those repetitions must be performed as rapidly as possible, without sacrificing adherence to proper form or otherwise compromising the desired range of motion, the athlete will be compelled to react with greater speed. The faster that the athlete must react to the resistance directed into the target muscle, the faster the individual muscles will fire. This approach mirrors the other training methods that are used by athletes to develop the capabilities the fast-twitch fibers present in each muscle.