Motor Control of Landing from a Jump in Simulated Hypergravity

On Earth, when landing from a counter-movement jump, muscles contract before touchdown to anticipate imminent collision with the ground and place the limbs in a proper position. This study assesses how the control of landing is modified when gravity is increased above 1 g . Hypergravity was simulated in two different ways: (1) by generating centrifugal forces during turns of an aircraft ( A300 ) and (2) by pulling the subject downwards in the laboratory with a Subject Loading System ( SLS ). Eight subjects were asked to perform counter-movement jumps at 1 g on Earth and at 3 hypergravity levels (1.2, 1.4 and 1.6 g ) both in A300 and with SLS . External forces applied to the body, movements of the lower limb segments and muscular activity of 6 lower limb muscles were recorded. Our results show that both in A300 and with SLS , as in 1 g : (1) the anticipation phase is present; (2) during the loading phase (from touchdown until the peak of vertical ground reaction force), lower limb muscles act like a stiff spring, whereas during the second part (from the peak of vertical ground reaction force until the return to the standing position), they act like a compliant spring associated with a damper. (3) With increasing gravity, the preparatory adjustments and the loading phase are modified whereas the second part does not change drastically. (4) The modifications are similar in A300 and with SLS , however the effect of hypergravity is accentuated in A300 , probably due to altered sensory inputs. This observation suggests that otolithic information plays an important role in the control of the landing from a jump.