Landing from an unexpected fall and a voluntary step.

The electromyographic (EMG) response of soleus to a sudden fall has previously been shown to consist of two peaks of activity, the first a response to release, and the second in relation to landing (Greenwood and Hopkins, 1976). We have now examined activity in soleus during falls at various reduced accelerations. The amplitude of the first peak is proportional to acceleration and is not present during falls at accelerations of less than about 2-0 ms-2. The timing of the second peak is related to the timing of landing whether the duration of the fall depends upon the height or upon the acceleration of the fall. Its timing depends upon knowledge of the height of the fall, rather than to ongoing visual information, and upon labyrinthine function. EMG activity in soleus was also examined during voluntary downward steps which could be unexpectedly increased in height. The moment of landing is preceded by an increase of EMG activity and is followed by a number of peaks of activity. These peaks were not seen after the moment of expected landing during unexpectedly lengthened steps and thus probably represent reflex activity due to the mechanical event of landing.