Strength curves for multiple-joint single degree of freedom exercises

226

Abstracts

independent observations to assess model performance. The model will be used to recommend and loads, and floor and shoe materials to prevent slip.

STRENGTH

CURVES

SINGLE

FOR IMULTIPLE-JOINT

J. G. ANDREWS (The University

DEGREE

cart handle heights

OF FREEDOM

EXERCISES

of Iowa, Iowa City. IA 52242. U.S.A.)

The purposes of this study were (1) to show that the concept of a strength curve for a single joint. one degree of freedom (DOF) exercise can be generalized to include multiple-joint, one DOF exercises, (2) to define the strength curve for such exercises, and (3) to illustrate how this definition applies with a simpleexample. Using an arbitrarily chosen, characteristic angular variable 07 as the one DOF exercise coordinate, the strength curve is defined as the plot of the system joint torque T vs the exercise coordinate 07. Here, T is the algebraic sum of the X L 2 resultant joint torque components T (i = I,. . , N) that (a) act on the distal body segment at each of the subject’s N active joints (i.e. joints that change their configurations during the exercise), and (b)correspond to the N angular variables ei that define the configurations of these N active body joints.

A SYSTEM

FOR ANALYSIS

OF POSTURES

AND FORCES

IN MANUAL

WORK

T. J. ARMSTRONG, M. ROBERTSON, B. BUCHHOLZ, B. S. JOSEPH, C. WOOLLEY and B. SILVERSTEIN (Center for Ergonomics, 1205 Beal, The University of Michigan, Ann Arbor, MI 48109, U.S.A.) Cumulative Trauma Disorders, such as carpal tunnel syndrome and tendinitis. are associated with repetitive work, certain postures and forceful exertions. This paper describes a system designed for analysis of work postures and forces. A video tape recorder is used to record work activities. The tapes are played back in slow motion so that the postural attributes can be determined for each joint. Forces are estimated from surface electromyography. Medial forearm EMG’s are calibrated for each test of each subject by having the subject make graded exertions in all anticipated postures. The EMG signals are integrated and superimposed on the video tape so that they can be synchronized with postures. To facilitate analysis a digital time signal is also superimposed on the video tape.

BASEBALL

PLAYERS

CANNOT

A. TERRY BAHILL (Systems and Industrial

KEEP THEIR

Engineering,

University

EYES ON THE BALL of Arizona.

Tucson, AZ 85721, U.S.A.)

We have measured the head and eye movements ofathletes trying to hit a ball moving along a computer controlled trajectory. We conclude that baseball and tennis players do not ‘keep their eye on the ball’; it is physiologically impossible. Usually they can keep up with the ball only for the first 907” of its flight. Sometimes they track the ball over the first portion of its trajectory, guess its future position, make an anticipatory saccadic eye movement to this predicted location, and then resume tracking. The size and timing of the anticipatory saccade varies from person to person. We have used this data to test a closed-loop control-system model for the head and eye movement control systems.

MECHANICAL

LOADING

AND SKELETAL

REMODELING

A. A. BIEWENER, S. SWARTZ and M. LEE (University

of Chicago,

DURING Chicago,

GROWTH IL, U.S.A.)

Using the chick tibia as our model, these experiments are designed to factor out the importance of differences in strain history on this bone’s remodeling response during growth from 4 to 17 weeks of age. The animals were trained and run on a treadmill at 30 9, of their top speed (constant stride frequency over this age range) for 15 min at five days week-‘. In cico strain recordings were made using both single element and rosette gauges at six sites on the tibia (right tibia: proximal/medial, P/M, cranial midshaft, Cr/M, caudal midshaft, Cd/M, media midshaft, M/M; left tibia: cranial distal, Cr/D, and caudal distal, Cd/D). Recordings were made at 5, 11 and 17 weeks age. Periodic fluorescent bone labels were administered to the animals to correlate loading measurements and age with changes in bone structure. Strain levels were found to vary significantly with gauge location. For instance at 5 weeks, running at 0.96 m s-*, the following strain levels (in microstrain) were recorded: - 710, P/M: + 330, Cr/M; - 1550, Cd/M; -930, M/M; - 1025, Cr/D; + 130, Cd/D. The distribution of loading was similar over a range of speed, but as the animals changed from a walk to a run strain magnitude rose sharply (e.g. 54% at Cd/M). These strains shoHc the chick tibia to be loaded primarily in bending (but in opposite directions at the midshaft and distal