Tools To
Assist Measurement and Training
of Human Fitness and Exercise Levels
For centuries, many devices have been created specifically
for strength development. These devices include treadmills, bicycle ergometers, rowing
machines, skiing simulators, as well as many of the more traditional resistive exercises
with dumb bells, bar bells, and commercially available weight equipment. Each type of
exercise has some advantages but none are designed to cope with the difficulties inherent
with the gravitational effects which affect the multi-linked human body performing on
various exercise equipment.
All systems that employ weights as the mechanism for
resistance have major drawbacks in four or more areas, as follows: (1) biomechanical
considerations, (2) inertia, (3) risk of injury, and (4) uni-directional resistance. The
biomechanical parameters are extremely important for human performance and should be
incorporated into exercise equipment. The biomechanical factors were discussed previously.
Inertia is the resistance to changes in motion. In other words, a greater force is
required to begin moving weights than is necessary to keep them moving. Similarly, when
the exercising person slows at the end of a movement, the weights tend to keep moving
until slowed by gravity. This phenomenon reduces the force needed at the end of a motion
sequence. Inertia 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 most 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. If the person exercising loses his/her grip, or is unable to hold the weights
owing to exhaustion or imbalance, the weights fall back to their resting position serious
injuries can and have occurred. 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 which employ springs, torsion bars, and
the like are able to overcome the inertia problem of weight-based mechanisms and,
partially, to compensate 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 that is characteristic of springs and is
usually unacceptable to most exercise equipment users.
The third resistive mechanism commonly employed in existing
exercise equipment is a hydraulic mechanism. Hydraulic devices are able to overcome the
inertial problem of weights, the safety problem of both weights and springs, and, with the
appropriate selection or configuration, the unidirectional problem. However, previous
applications of the hydraulic principle have demonstrated a serious deficiency that has
limited their popularity in resistive training. This deficiency is that of a fixed or a
preselected flow rate through the hydraulic system. With a fixed flow rate, it is a well
established fact that resistance is a function of the velocity of the piston and, in fact,
varies quite rapidly with changes in velocity. It becomes difficult for a person
exercising to select a given resistance for training due to the constraint of moving
either slower or faster than desired in order to maintain the resistance. Additionally, at
any given moment, the user is unsure of just what the performing force or velocity
actually is.
In the field of rehabilitation (54) especially, isokinetic
or constant velocity training equipment is a relatively new fitness technology that has
enjoyed wide acceptance. These mechanisms typically utilize active or passive hydraulics
or electric motors and velocity-controlling circuitry. The user or practitioner selects a
constant level of velocity for exercise and the mechanism maintains this velocity while
measuring the force exerted by the subject. Although demonstrating significant advantages
over weight-based systems, isokinetic systems possess a serious limitation. There are
virtually no human activities that are performed at a constant velocity. Normal human
movement consists of patterns of acceleration and deceleration. When a person learns to
run, ride a bike, or write, an acceleration/deceleration sequence is established that may
be repeated at different rates and with different levels of force, but always with the
pattern unique to that activity. To train, rehabilitate, or diagnose at a constant
velocity is to change the very nature of the activity being performed and to violate most
biomechanical performance principles.