THE CONTROL OF LANDING FROM A COUNTER-MOVEMENT JUMP IN HYPER-GRAVITY

It is well known that during a self-initiated fall or a drop-landing on Earth, the lower limb muscles are activated before ground contact (Melvill Jones & Watt 1971, Santello, 2005). The sensory inputs from the proprioceptive, otolithic and visual systems provide critical information to estimate the instant of touchdown, and the magnitude of the impact. The timing and amplitude of the muscular pre-activation is though to be modulated accordingly to these estimated parameters (Santello, 2005). However, there is still controversy about the modulation of this pre-activation: some authors (Santello & McDonagh, 1998) claim that it is independent of the height of the drop whereas others (Liebermann & Hoffman, 2004) have observed an increase of the pre-activation time with increased drop height. Here we explore the motor control of the counter movement jump (CMJ) in real hyper-gravity conditions up to 1,6g during ESA-parabolic flights. The question is if/how the central nervous system (CNS) predicts the time of touchdown, the characteristics of the ground reaction forces and controls the landing when gravity is increased. One can make the hypothesis that the impact with the ground will be increased, but it is unclear how the CNS will anticipate the moment of ground contact when the pull-down acceleration is increased and how the energy of the body at touchdown will be absorbed to avoid rebound on the ground. We expect that increased gravity will increase muscular amplitude while muscular pre-activation timing will remain. In order to verify these hypotheses, we recorded CMJs in the A300 zero-g during increased gravity fields obtained by turns of the airplane (1.2-1,4-1,6g). The ground reaction forces (GRF), the kinematics of the lower-limb segments and the electrical activity of the lower-limb muscles were measured in 9 subjects during 2 parabolic flight campaigns (55th-56th); a total of 7-13 jumps per subject were recorded in each hyper-gravity condition. With increased gravity, the height of the jump is decreased, but the vertical GRF peak at landing and the whole body stiffness are increased. The range of motion of each joint during landing decreased with hyper-gravity, leading to a decreased vertical downward displacement of the centre of mass. All muscles were pre-activated and the amplitude of the muscular activity increased with increased gravity. In conclusion the CNS seems able to take into account the increased downward acceleration by increasing the stiffness of the body together with an increased pre-programmed activity of the lower limb muscles. Future analysis will determine the specific effect of real hyper and hypo-g conditions on the landing pattern as well as on the factors explaining the EMG modifications observed on our preliminary results.