Online. Feedback Control of Exercise
Published on Saturday, August 10, 1996 by Gideon Ariel
Feedback Control of Exercise
A newer form of exercise equipment can determine the level of effort by the person, compare it to the desired effort, and then adjust accordingly. The primary advantage of this resistive mechanism is that the pattern of resistance or the pattern of motion is fully programmable. The concept of applying a pattern of resistance or motion to training and rehabilitation was virtually impossible until the invention of computerized feedback control. Prior to the introduction of computerized feedback control, fitness technology could provide only limited modes of resistance and motion. Barbells or weights of any type provide an isotonic or constant resistance type of training only when moved at a constant velocity. Typically, users are instructed to move the weights slowly to avoid the problem of inertia resulting from the acceleration or deceleration of mass. Weights used with cams or linkages which alter the mechanical advantage can provide a form of variable resistance. However, the pattern is always fixed and the varying mechanical advantage causes a variation in velocity that increases inertial effects. Users must move the weights slowly to preserve the resistance pattern. Another deficiency with these types of equipment is that they do not approximate the body or limb movement pattern of a normal human activity.
An exercise machine controlled by a computer possesses several unique advantages over other resistive exercise mechanisms, both fixed and feedback controlled. The most significant of these advances is the introduction of software to the human/computer feedback loop. The computer and its associated collection of unique programs can regulate the resistance to vary with the measured variables of force and displacement as well as modify the resistance according to data obtained from the feedback loop while the exercise progresses. This modification can, therefore, reflect changes in the pattern of exercise over time. The unique program selection can effect such changes in order to achieve a sequential or patterned progression of resistance for optimal training effect. The advantage of this capability over previous systems is that the user can select the overall pattern of exercise and the machine assumes responsibility for changing the precise force level, the speed of movement, and the temporal sequence to achieve that pattern.
The first resistive training and rehabilitation device to employ computerized feedback control of both resistance and motion during exercise was the Ariel Computerized Exercise System (55). For the first time, a machine dynamically adapted to the activity being performed rather than the traditional approach of modifying the activity to conform to the limitations of the machine. Biomechanical results previously calculated could be used to program the actual patterns of motion for training or rehabilitation. The equipment utilizes a passive hydraulic resistance mechanism operating in a feedback-controlled mode under control of the system’s computer. A simplified functional description of this mechanism and its operation is described in the following paragraphs.
A hydraulic cylinder is attached to an exercise bar through a mechanical linkage. As the bar is moved, the piston in the hydraulic cylinder moves which pushes oil from one side of the cylinder, through a valve, and into the other side of the cylinder. When the valve is fully open there is no resistance to the movement of oil and, thus, no resistance in the movement of the bar. As the valve is closed, it becomes harder to push the oil from one side of the cylinder to the other and, thus, harder to move the bar. When the valve is fully closed, oil cannot flow and the bar will not move. In addition to the cylinder, the resistance mechanism contains sensors to measure the applied force on the bar and the motion of the bar.
To describe the operation of the computerized feedback loop, assume the valve is at some intermediate position and the bar is being moved at some velocity with some level of resistance. If the computer senses that the bar velocity is too high or that bar resistance is too low, it will close the valve by a small amount and then check the velocity and resistance values again. If the values are incorrect, it will continue to regulate the opening of the valve and continually check the results until the desired velocity or resistance is achieved. Similar computer assessments and valve adjustments are made for every exercise. Thus, an interactive feedback loop between the computer and the valve enable the user to exercise at the desired velocity or resistance. The feedback cycle occurs hundreds of times a second so that the user experience no perceptible variations from the desired parameters of exercise.
There are a number of advantages in a computerized feedback controlled resistance mechanism over devices that employ weights, springs, motors, or pumps. One significant advantage is safety. The passive hydraulic mechanism provides resistance only when the user pushes or pulls against it. The user may stop exercising at any time and the exercise bar will remain motionless. Another advantage is that of bi-directional exercise. The hydraulic mechanism can provide resistance with the bar moving in each direction, whereas weights and springs provide resistance in only one direction. Opposing muscle groups can be trained in a single exercise. Two additional problems associated with weight training, noise and inertia, are also eliminated because the hydraulic mechanism is virtually silent and full resistance can be maintained at all speeds.
The Ariel Computerized Exercise System allows the user to set a pattern of continuously varying velocity or resistance. The pattern can be based on direct measurements of that individual’s motion derived from the biomechanical analysis or can be “designed” or created by the user with a goal of training or rehabilitation. During exercise, the computer uses the pattern to adjust bar velocity or bar resistance as the subject moves through the full range of motion. In this manner, the motion parameters of almost any activity can be closely duplicated by the exercise system allowing training or rehabilitation using the same pattern as the activity itself.
The software consists of two levels. One level of software is invisible to the individual using the equipment since it controls the hardware components. The second level of software allows interaction between the user and the computer. The computer programs necessary to provide the real-time feedback control, the data program and storage, and the additional performance manipulations are extensive. The software provides computer interaction with the individual operator by automatically presenting a menu of options when the system is activated.
Selection of the diagnostics option allows several parameters about that person to be evaluated and stored if desired. The diagnostic parameters include the range of motion, the maximum force, and the maximum speed that the individual can move the bar for the specific activity selected. The maximum force and maximum speed data can be determined at each discrete point in the range of movement as well as the average across the entire range. The diagnostic data can be used solely as isolated pre- and post-test measurements. However, the data can also be stored within the person’s profile so that subsequent actions and tests performed on the equipment can be customized to adjust to that specific individual’s characteristics.
The controlled velocity option permits the individual to control the speed of bar movement. The pattern of the velocity can be determined by the person using the equipment and these choices of velocity patterns include: (1) isokinetic, which provides a constant speed throughout the range of motion; (2) variable speed, in which the speed at the beginning of the motion and the speed at the end of the stroke are different with the computer regulating a smooth transition between the two values; and (3) programmed speed, which allows the user to specify a unique velocity pattern throughout the range of movement. For each of the choices, determination of the initial and final velocities are at the discretion of the individual through an interactive menu. The number of repetitions to be performed can be indicated by the person. It is possible to designate different patterns of velocity for each direction of bar movement.
The controlled resistance option enables the person to control the resistance or amount of force required to move the bar. The alternatives include: (1) isotonic, which provides a constant amount of force for the individual to overcome in order to move the bar; (2) variable resistance, in which the force at the beginning of the motion and the force at the end of the movement are different with the computer regulating a smooth transition between the two values; (3) programmed resistance, which permits the individual to specify a unique force pattern throughout the range of movement. An interactive menu enables the person to indicate the precise initial and final values, the number of repetitions to be used, and each direction of bar motion for the three choices.
The controlled work option allows the individual to determine the amount of work, in Newton/meters or joules, to be performed rather than the number of repetitions. In addition, the person can choose either velocity or resistance as the method for controlling the bar movement. As with the previous options, bi-directional control is possible.
The data storage capability is useful in the design of research protocols. The software allows an investigator to “program” a specific series of exercises and the precise manner in which they are to be performed, e.g. number of repetitions, amount of work, etc., so that the user need only select his or her name from the graphic menu and the computer will then guide the procedures. Data gathered can be stored for subsequent analysis. The equipment has the capacity to “program” a sequence of events, such as a series of different exercises; determination of that sequence is solely at the discretion of the research investigator or other user. Data storage is presented as an option; it is not a required mode of operation. The equipment is fully operational for all options irrespective of whether the data storage option is activated.
Numerous features further enhance the application of this advanced fitness technology. Individual exercise programs can be created and saved on the computer or a diskette. Users can perform their individual program at any time merely by loading it from computer memory or the diskette. Measurements of exercise results can be automatically saved and progress monitored by comparing current performance levels to previous ones. Performance can be measured in terms of strength, speed, power, repetitions, quantity of work, endurance and fatigue. Comparison of these quantities can be made for flexors versus extensors, right limb versus left limb, as well as between different dates and different individuals. Visual and audio feedback are provided during exercise to ensure that the subject is training in the proper manner and to provide motivation for optimal performance.
Accuracy of measurement is essential and it is deemed as one of the most important considerations in the software. Calibration of the equipment is performed dynamically and is a unique feature that the computerization and the feedback system allow. Calibration is performed using weights with known values and the procedure can be performed for both up and down directions. This type of calibration is unique since the accuracy of the device can be ascertained throughout the range of motion.