Jackson, D.K., Newman, D.J., Bloomberg, J.J., "Changes in Astronaut Lower Limb and Body Mass-Center Kinematics in Downward Jumping Following Space Flight," 66th Aerospace Medical Association Meeting, Anaheim, California, May, 1995.

INTRODUCTION. Astronauts exposed to microgravity conditions in space flight exhibit postural and gait instabilities upon return to earth. Effects include changes in lower limb intersegmental power transfer and decrements in head stability during gait, as well as instability in the landing phase of "drop" experiments. The present study investigated changes in jumping performance postflight, including joint kinematics, body mass center motion and joint power transfer.

METHODS. The crews of several space shuttle missions were tested both pre- and postflight with a series of 6 two-footed hops down from a 30 cm. high step. A video-based 3-D motion analysis system permitted calculation of body segment positions and joint angular displacements.

RESULTS. Analysis of lower limb joint angles found that hip and knee flexion following jump landing was greater postflight than preflight, and recovery toward equilibrium posture was delayed. Body center of mass (COM) motion in the saggital plane after impact showed greater maximum downward displacements combined with larger forward overshoot of the final equilibrium position and fore-aft COM oscillations. Phase-plane descriptions of joint kinematics exhibited greater flexion rates postflight following impact, followed by smaller extension rates than were observed preflight. Estimates of joint energy transfer from area swept out by the phase portrait indicate that the power absorbed by the knee joint upon impact is decreased postflight. This observation, combined with the higher postflight joint flexion rates, implies a reduction in the peak knee torque generated postflight.

CONCLUSIONS. The postflight increase in lower limb joint angle excursions and slower recovery toward equilibrium indicate a decrease in the "bandwidth" of the postural control system. Evidence for postflight reductions in peak joint torques is consistent with reduced control gains and decreased system bandwidth. Larger postflight overshoots and oscillations in COM fore-aft position indicate lower relative stability of the postural control system. Greater joint angle excursions, decreased peak joint moments, and lower relative stability imply reductions in both the stiffness and viscous components of the lower limb impedance as a consequence of microgravity exposure.

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