Modulation of pre-programmed muscle activation and stretch reflex to changes of contact surface and visual input during movement to absorb impact.

The purpose of this study was to examine the extent of modification of the preactivation and stretch reflex response in ankle joint muscles to different contact surfaces and visual input during movement to absorb impact. Experimental movements like landing were performed using a special sliding apparatus. Seven subjects made landings on the hard surface (Hard-S) of a metal force platform or soft surface (Soft-S) of a foam cushion with eyes open or closed. The electromyographic activities from the medial gastrocnemius (MG), soleus (Sol), and tibialis anterior (TA) muscles, contact force, and ankle joint angle were recorded. The preactivation levels of MG and TA to Hard-S increased compared to Soft-S. After foot contact, dorsiflexion velocity, impulse, and responses of the stretch reflex in MG and Sol were significantly larger on Hard-S than Soft-S. With eyes closed, there were trends of decrease in the preactivation. Although the dorsiflexion velocity and impulse showed no significant differences between both visual conditions, the stretch reflex responses with eyes closed were larger than those with eyes open for both surfaces. These results suggest that the preactivation is modulated to different surface and the reflex gain is enlarged by visual suppression.