Motor control of landing in an unsteady environment

When landing from a jump or a drop, muscles contract before touchdown to anticipate imminent collision with the ground, soften ground contact and allow to return to a stable standing position without stepping or rebounding.

This study assesses the effect of the unsteadiness of the environment on the motor control of landing. The 'unsteady environment' was induced by asking participants to perform drop landings inside an aircraft that underwent trajectories parallel to Earth's surface. The participants also performed the same task in a 'steady environment' in our laboratory.

Ground reaction forces, lower limb joints' movements and the activity of lower limb muscles were recorded. The stability of the landing was assessed by the vertical and anterior-posterior stability indexes, center of pressure measures and by the coefficient of variation of kinetic and kinematic parameters.

On one hand, participants slowdown their joint movements and reduce the knee joint excursion during landing, probably to avoid excessive movements that may induce imbalance. On the other hand, the stability of the landing is reduced while the variability of the movement is increased, illustrating a less stable and less consistent landing. In addition, whatever the environment, landing parameters associated with increased stiffness (i.e., increased impact forces and decreased joint range of motion) are correlated with decreased landing stability.

Overall, landings in the'unsteady environment' appear to be more cautious but less stable and less finely tuned. Since the stability of the landing is not directly influenced by the steadiness of the environment, this more cautious behavior could be, at least in part, related to the fear/apprehension induced by sudden acceleration variations of the frame of the aircraft.