Adaptive effects of spaceflight as revealed by short-term partial weight suspension

Background: Human performance and adaptation to altered loading levels is i nvestigated. A previous astronaut jumping study demonstrated significantly altered landing performance following spaceflight, complementing reports of postflight postural and gait instabilities. A dynamic model indicated that leg stiffness changes accounted for the kinematic differences due to adapt ation in open-loop modulation of leg impedance. Muscular atrophy or altered stretch reflexes and Vestibular sensing could not be discounted. Hypothesi s: We hypothesize that partial weight unloading can cause modulation of leg impedance and altered jump landing performance similar to longer-term micr ogravity exposure, while controlling for muscle atrophy and altered gravice ptor inputs. Lower-body impedance changes after partial weight unloading su pport the hypothesis that postflight differences result primarily from modu lation of leg impedance due to reduced postural control demands in microgra vity. Methods: Six subjects performed six baseline 30-cm downward jumps fro m a platform, followed by 10 jumps under 60% body weight unloading (the ada ptation sequence), and then six additional jumps under full-body loading (t ermed "adapted jumps"). Joint and mass center kinematics were compared for the baseline and adapted jump landings. A second order model evaluated chan ges in vertical leg impedance. Results: Three subjects exhibited significan t increased joint angles and rates. Vertical ground reaction forces showed more heavily damped responses after adaptation. Model fits to mass center m otion indicated reduced leg stiffness. Conclusions: Post-adaptation perform ance is similar to that of four astronaut subjects who demonstrated reduced postflight leg stiffness. The new study strongly suggests adaptive control of lower limb impedance to accommodate altered gravity levels which can be induced by minimal unloading exposure. Partial weight unloading provides a simple, inexpensive analog to spaceflight for certain postural and movemen t studies.