Organization of stabilizing reflex responses in tibialis anterior muscles following ankle flexion perturbations of standing man.

Reflex activity in human ankle muscles in response to 36 deg./s dorsi-flexion rotations of the feet was investigated in subjects standing upright and when leaning back so as to preactivate ankle flexor muscles. Short latency stretch reflex activity in soleus and inhibition in tibialis anterior muscles occurred at 50 ms from ankle rotation onset. Two prominent bursts of tibialis activity followed at 83 and 131 ms. and preceded large stabilizing ankle torques. Head movements commenced 20 ms after foot rotations and acquired accelerations exceeding 100 deg./s2 within 60 ms. It is suggested that the tibialis anterior activity is either a vestibulospinal reflex resulting from the head movement, or a stretch reflex only present during standing, since this activity was not observed when seated subjects received identical foot rotations.