Prolonged exposure to microgravity modifies limb endpoint kinematics during the swing phase of human walking

Many neurophysiological and behavioural studies suggested that the lower limb endpoint might be a centrally represented variable within the gravity reference frame and effectively controlled during the locomotion. This investigation examines the effects of prolonged exposure to microgravity upon lower limb endpoint kinematics. Seven cosmonauts were tested before and the 2nd day after a 6 month orbital mission. The spatio-temporal patterns of the limb extremity (malleolus) were analysed during the swing phase of the walk using a motion analysis system. Paths and velocity profiles of endpoint displacements were computed and compared for both the pre- and post-flight walking sessions. The results indicated significant modifications in both spatial and temporal features the 2nd day after re-entry on Earth. The step was significantly lengthened, and its shape was altered, presenting higher vertical component compared with the pre-flight values. The acceleration duration was increased whereas the swing duration remained invariant whatever the walking session. These modifications in endpoint kinematics could partly but not completely be explained by peripheral factors like muscle atrophy or plasticity at the spinal level. We suggest that, apart of changes in peripheral level, central modifications concerning internal models of gravity, putatively used to plan limb motion, might have induced such locomotor changes.