Resistive Exercising Methods

There are significant differences in the manner of execution of the various resistive training methods. In isotonic exercises, the inertia, which is the initial resistance, must be overcome before the execution of the movement progresses. The weight of the resistance cannot be heavier than the maximum strength of the weakest muscle acting in a particular movement or the movement cannot be completed. Consequently, the amount of force generated by the muscles during an isotonic contraction does not maintain maximum tension throughout the entire range of motion. In an isokinetically loaded muscle, the desired speed of movement occurs almost immediately and the muscle is able to generate a maximal force under a controlled and specifically selected speed of contraction. The use of the isokinetic principle for overloading muscles to attain their maximal power output has direct applications in the fields of sport medicine and athletic training. Many rehabilitation programs utilize isokinetic training to recondition injured limbs of athletes to their full range of motion. The unfortunate drawback to this type of training is that the speed is constant and there are no athletic activities that are performed at a constant velocity. The same disadvantage applies to normal human activities.

In isotonic resistive training, if more than one repetition is to be used, a submaximal load must be selected for the initial contractions in order to complete the required repetitions. Otherwise, the entire regimen would not be completed, owing to the inability to perform or to fatigue.

A modality that can adjust the resistance so that it parallels fatigue to allow a maximum effort for each repetition would be a superior type of equipment. This function could be accomplished by manually removing weight from the bar while the subject trained. This is neither convenient nor practical. With the aid of the modern computer, the function can be performed automatically.

Another drawback with many isotonic types of resistive exercises is that the inertia resulting from the motion changes the resistance depending on the acceleration of the weight and of the body segments. In addition, since overload on the muscle changes due to both biomechanical levers and the length-tension curve, the muscle is able to achieve maximal overload only in a small portion of the range of motion. To overcome this shortcoming in resistive training, some strength training devices have been introduced that have "variable resistance" mechanisms in them. However, these "variable resistance" systems increase the resistance in a linear fashion and this linearity may not truly accommodate the individual. When including inertial forces to the variable resistance mechanism, the accommodating resistance can be canceled by the velocity of the movement.

There seem to be unlimited training methods and each is supported and refuted by as many "experts." In the past, the problem of accurately evaluating the different modes of exercise was rendered impossible because of the lack of adequate diagnostic tools. For example, with isotonic exercises, the investigator does not know exactly the muscular effort nor the speed of movement but knows only the weight that has been lifted. When a static weight is lifted, the force of inertia provides a significant contribution to the load and cannot be quantified by feel or observation alone. In the isokinetic mode, the calibration of the velocity is assumed and has been poorly verified. The rotation of a dial to a specific speed setting does not guarantee the accuracy of subsequently generated velocity. In fact, discrepancies as great as 40 percent have been observed when verifying the bar velocity.