ALTERED ASTRONAUT LOWER-LIMB AND MASS CENTER KINEMATICS IN DOWNWARD JUMPING FOLLOWING SPACE-FLIGHT

Astronauts exposed to the microgravity conditions encountered during s pace flight exhibit postural and gait instabilities upon return to ear th that could impair critical post flight performance. The aim of the present study was to determine the effects of microgravity exposure on astronauts' performance of two-footed jump landings. Nine astronauts from several Space Shuttle missions were tested both preflight and pos tflight with a series of voluntary, two-footed downward hops from a 30 -cm-high step. A video-based, three-dimensional motion-analysis system permitted calculation of body segment positions and joint angular dis placements. Phase-plane plots of knee, hip, and ankle angular velociti es compared with the corresponding joint angles were used to describe the lower limb kinematics during jump landings. Thp position of the wh ole-body center of mass (COM) was also estimated in the sagittal plane using an eight-segment body model. Four of nine subjects exhibited ex panded phase-plane portraits postflight, with significant increases in peak joint flexion angles and flexion rates following space flight. I n contrast, two subjects showed significant contractions of their phas e-plane portraits postflight and three subjects showed insignificant o verall changes after space flight. Analysis of the vertical COM motion generally supported the joint angle results. Subjects with expanded j oint angle phase-plane portraits postflight exhibited larger downward deviations of the COM and longer times from impact to peak deflection, as well as lower upward recovery velocities. Subjects with postflight joint angle phase-plane contraction demonstrated opposite effects in the COM motion. The joint kinematics results indicated the existence o f two contrasting response modes due to microgravity exposure. Most su bjects exhibited ''compliant'' impact absorption postflight, consisten t with decreased limb stiffness and damping, and a reduction in the ba ndwidth of the postural control system. Fewer subjects showed ''stiff' ' behavior after space flight, where contractions in the phase-plane p ortraits pointed to an increase in control bandwidth. The changes appe ared to result from adaptive modifications in the control of lower lim b impedance. A simple 2nd-order model of the vertical COM motion indic ated that changes in the effective vertical stiffness of the legs can predict key features of the postflight performance. Compliant response s may reflect inflight adaptation due to altered demands on the postur al control system in microgravity, while stiff behavior may result fro m overcompensation postflight for the presumed reduction in limb stiff ness inflight.