Effect of different preparatory states on the reflex responses of ankle flexor and extensor muscles to a sudden drop of support surface during standing in humans.

The present study hypothesized that if subliminal facilitation of the ankle flexor motoneurons is related to preparatory state to a mechanical perturbation while standing, different standing conditions such as standing with eyes-closed or touching a bar should have different influences on the facilitation levels. While the subjects were maintaining an upright stance posture, sudden drops of support surface were applied several times in the following four different conditions: (1) standing with eyes-open, (2) with eyes-closed, (3) with eyes-open and warning of the drop, and (4) with eyes-open and touching a bar with both hands. The results demonstrated that the EMG responses in the ankle flexor TA muscle increased in the eyes-closed condition, while it reduced in the conditions, such as that the subjects had prior warning or touched a bar. The preparatory state of supraspinal neural centers was suggested to be related to the observed variation in the EMG responses.