The quantification on human effort and motion for the upper limbs by means of an Exoskeletal Kinematometer
Ramsey, Jerry D.
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The primary purpose of this investigation was to develop instrumentation and computational procedures for the quantification of human effort involved in three dimensional planar movement of the upper limbs, and to evaluate these quantitative measures. A device for monitoring the kinematic motion of the upper limbs, the Exoskeletal Kinematometer, was developed to provide instantaneous angular displacement data. This device consists of a shoulder mount, two arm mounts upon which are fastened linkages that follow the arm movement, and potentiometers which measure the angular displacement at the joints of rotation. The angular displacement data served as input to a series of computational procedures which were developed to evaluate various mechanical measures of human effort. The specific measures of interest were: 1. The velocity and acceleration at the center of mass of each arm segment, 2. The changes in mechanical energy of the entire arm system, 3. The force and torque at each arm joint, and 4. The linear and angular impulse at each arm joint. These mechanical indices of effort were determined for each of four subjects under various experimental levels of weight carried, distance moved, angle of abduction, and task duration. Three levels of weight (0, .4, and .8 kilograms), three angles of abduction (0, 20, and 40 degrees), two distances (10 and 20 centimeters), and three durations (0, 2, and 4 minutes) were selected as important parameters found in many "light" or "bench-work" industrial jobs. In addition to assessing the characteristics of the mechanical indices cited above, these measures were correlated with the physiological measures of the same task as reported in research conducted during a parallel investigation. Within the limits of this study, the findings evolving from this investigation are summarized below. Angular impulse or the torque-time product, was determined to be the best single mechanical measure of human effort. Both the total angular impulse, which includes the angular impulse at the shoulder, elbow, and wrist, and the shoulder angular impulse could be supported as equally good measures that show sensitivity to the major experimental variables of this task, i.e., subject, weight, distance, and angle of abduction. The average torque at the shoulder showed high correlation to shoulder angular impulse and as such was a comparable indicator of human effort. Both force and the force-time product of linear impulse appeared to be less desirable measures of human effort because of their insensitivity to the effects of distance and angle. The predictive equations for force and linear impulse contained only the variables of weight and subject's arm mass. The mechanical measures of total energy, maximum velocity at the hand, and maximum acceleration at the hand show reasonable potential as indices of human effort, but are especially influenced by the variables of subject and distance. Substitutions of total body weight for arm volume and body height for arm length resulted in only minor losses in accuracy for the predictive equations developed in this analysis. All of the mechanical measures had a higher correlation with the physiological measure of ventilation rate/body surface area than with the measures of heart rate or oxygen consumption. The mechanical measures showed higher correlation with the physiological measures for the dynamic task than for the equivalent static task. The correlations between the mechanical and physiological measures of the task substantiate that total and shoulder angular impulse, shoulder torque, and total mechanical energy are the better of the mechanical indices of human effort.