Symposium on Olympic Sports Medicine
Clinics in Sports Medicine Vol. 2, No. 1, pp. 55-69, March 1983
The relationship between resistance exercises and muscle strength has been known for centuries. In ancient Greece, Milo the wrestler used progressive resistance exercises to improve his strength. His original method consisted of lifting a calf each day until it reached its full growth and this technique provides probably the first example of progressive resistance exercises.
Today, it is well documented in the literature that the size of skeletal muscle is affected by the amount of muscular activity performed. Increased work by a muscle can cause that muscle to undergo compensatory growth (hypertrophy), whereas disuse leads to wasting of the muscle (atrophy).
This information has stimulated the medical and sports professions, especially coaches and athletes, to try many combinations and techniques of muscle overload. These attempts to produce a better means of rehabilitation or a physiologic edge in sporting activities have only scratched the surface of the cellular mechanisms and physiologic consequences of muscular overload.
Muscular strength may be defined as the force that a muscle group can exert against a resistance in a maximal effort. In 1948, Delorme (35) 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 distinguished between exercise regimens for producing muscle hypertrophy and those for producing muscle power. (66) He concluded that the number of repetitions for each set of exercise determines the different characteristics of the various training procedures.
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, the Oxford technique, the double and triple progressive super set systems, and many others. Each system's effectiveness has been supported and refuted by numerous investigations.
Berger concluded that six to seven repetitions three times a week was best for developing dynamic strength (13). Research conducted by Steinhause emphasized the need to increase the intensity, not the amount, of work in order to develop maximum strength. (88)
In more recent studies pertaining to exercise, Pipes and Wilmore compared isokinetic training with isotonic strength training in adult men (77). According to their findings with isokinetic contractions at both low and high speeds, the isokinetic training procedure demonstrated marked superiority over the isotonic methods. (Note: This study was not a valid study and do not accept its merit).
In 1972, I introduced the dynamic variable resistance (DVR) exercise principles in 1968, which resulted in the variable resistance exercise equipment (3-7). For the first time biomechanical principles were employed in the design of exercise equipment.
Owing to ambiguity in the literature concerning certain physiologic terms and differences in laboratory procedures, the following terms are defined:
In most existing exercise equipment today and in the previously cited research, resistive training was performed with "tools" that lack intelligence. That means the equipment was "unaware" that a subject was performing an exercise on it. For example, the equipment employed in the study conducted by Pipes and Wilmore assumed certain velocities on the isokinetic modality used. However, verification of the speed was impossible since a closed loop feedback and sensors were not used, as they do not exist on the equipment employed (Note: This was unacceptable study in the field). However, with the advent of miniaturized electronics in computers, it is possible today to join exercise equipment with the computer's artificial intelligence. For the first time it is possible for the equipment to adapt to the user rather than for the user to adapt to the equipment.
Another 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. 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.
Hellebrandt and Houtz shed some light on the mechanism of muscle training in an experimental demonstration of the overload principle. They found that the repetition of contractions that place little stress on the neuromuscular system had little effect on the functional capacity of the skeletal muscles; however, they 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.
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 are results of rotational motion of the links, which are made of 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 performance are force, displacement, and 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 athletic performance, 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.
Since most athletic events are ballistic movements 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.
Resistive Excercising Methods
There is a significant difference between various resistive training methods. As far as isotonic and isokinetic exercises are concerned, for example, in the isotonic exercises the inertia, that is, the initial resistance, has to be overcome first, then 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 else 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 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.
In isotonic resistive training, if more than one repetition is to be used, one must use submaximal overload on the initial contractions in order to complete the required repetitions. Otherwise, the entire regimen will not be completed, owing to fatigue. Berger and Hardage studied this problem by training two groups of men with 10 RM. One group trained following the standard Berger technique while the other group used one repetition maximum for each of the ten repetitions. This was accomplished by progressively reducing the weight for the next repetition in a manner that paralleled the fatigue of the muscle. The results showed that the intensity of the work seemed to be the important factor in strength increases, since the maximal overload group showed significantly greater strength gains than did the standard 10 RM group.
Based on these findings, it would seem appropriate to assume that a modality that can adjust the resistance so that it parallels fatigue to allow the maximum RM for each repetition would be superior to the currently available equipment. Berger accomplished this function by removing weight from the bar while the subject trained. This is neither the most convenient nor the most practical method. With the aid of the modern computer, this function can be performed automatically.
Another drawback with current isotonic types of resistive exercises is that with the aid of inertia, due to the motion, the resistance changes depending on the acceleration of the weight and the body segments. In addition, since overload on the muscle changes due to both biomechanical levers and the length-tension curve, the muscle can obtain maximal overload only in a small portion of the range of motion. To overcome this shortcoming in resistive training, several companies have manufactured strength training devices that have "variable resistance" mechanisms in them. However, these "variable resistance" systems increase the resistance in a linear fashion and this linearity does not truly accommodate the individual. When including inertial forces to the variable resistance mechanism, the accommodating resistance might be canceled by the velocity of the movement.
There seem to be unlimited training methods and each system is supported and refuted by as many "experts." In the past, the problem of validly evaluating the different modes of exercise was rendered impossible because of the lack of the proper diagnostic tools. For example, in the isotonic type of exercise the investigator does not know exactly the muscular effort and the speed of movement but knows only the weight that has been lifted. When a static weight is lifted, the force of inertia is 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 very poorly verified. The rotation of a dial to a specific location does not guarantee the accuracy of subsequently generated velocity. In fact, discrepancies as great as 40 per cent are found when verifying the velocity of the bar.
The Intelligent Exercise Machine
In all of the previous descriptions of exercise equipment, the user has had to determine the amount of resistance and the number of repetitions desired. The reason the user made the choices was, of course, that the exercise equipment itself was inherently incapable of any intellectual participation. However, with the advent of computers, it became possible to design exercise equipment with artificial intelligence enabling the computerized machine to select the best exercise method based on each individual user. Thus, the user need not be an expert in any biologic, physiologic, or exercise area, since the exercise machine is programmed with information from many scientific fields, thus correctly benefiting the different individual users.
The exercise machine described herein is the result of the application of many unique, innovative features and mechanisms to the long-established fields of resistive exercise or training for athletics, rehabilitation, and physical fitness. The underlying principle behind these innovations is that of a computer-controlled feedback or servomechanism that is able to maintain any desired pattern of force and motion throughout the range of each exercise, regardless of the magnitude or rate of force applied by the person exercising. The advantages of an intelligent feedback-controlled mechanism over existing resistive exercise mechanisms are many.
First, all systems that employ weights as the mechanism for resistance have major drawbacks in four or more areas, as follows:
The biomechanical considerations are the most important for exercise equipment and have been previously explained. Inertia is the property of resisting any change in motion; because of this property, it requires a greater force to begin moving weights than it does to keep them moving in a constant manner. Similarly, when the person exercising slows his motion at the end of an exercise movement, the weights tend to keep moving until slowed by gravity. This phenomenon reduces the required force at the end of a motion sequence. This property becomes especially pronounced as acceleration and deceleration increase, effectively reducing the useful range of motion of weight-based exercise equipment. The risk of injury is obvious in weight-based exercise equipment. When weights are raised during the performance of an exercise, they must be lowered to their original resting position before the person using the equipment can release the equipment and stop exercising. Injury could easily result if the weights fell back to their resting position accompanied by the concomitant motion of the bar or the handle attached to the weights. If the person exercising happened to lose his grip, or was unable to hold the weights owing to exhaustion or imbalance, serious injuries could and have resulted.
Finally, while being raised or lowered, weights, whether on exercise equipment or free standing, offer resistance only in the direction opposite to that of gravity. This resistance can be redirected by pulleys and gears but still remains unidirectional. In almost every exercise performed, the muscle or muscles being trained by resistance in one direction are balanced by a corresponding muscle or muscles that could be trained by resistance in the opposite direction. With weight-based systems, a different exercise, and often a different mechanism, is necessary to train these opposing muscles .
Exercise mechanisms that employ springs, torsion bars, and the like are able to overcome the inertia problem of weight-based mechanisms and can partially overcome the unidirectional force restriction by both expanding and compressing the springs. However, the serious problem of safety remains. An additional problem is the fixed, nonlinear resistance.