INFLUENCE OF PLANTAR CUTANEOUS AFFERENTS ON EARLY COMPENSATORY REACTIONS TO FORWARD FALL

The influence of cutaneous afferents in the compensatory reactions to a forward fall was investigated. Modification of cutaneous afferent activity was obtained in two different ways: first, by varying the initial pedal support conditions, secondly by anesthetizing the plantar foot sole. The initial pedal support conditions were: 1) bipedal posture, 2) unipedal posture, with contact and 3) unipedal posture, without contact. Nine healthy subjects participated in the control (without anesthesia) experimental session, of which three subjects participated in a session where the plantar sole was anesthetized. The compensatory reactions to a perturbation of balance of a subject initially with a bipedal stance, showed synchronized EMG activity in both Soleus muscles, starting on average at 59 ms, and a burst of EMG activity in the Tibialis Anterior of the starting foot after 200 ms. When the subject was in unipedal posture, the EMG responses on the side which was without support, showed several modifications: the EMG burst in Soleus was strikingly depressed, the response in Tibialis Anterior appeared earlier (mean latency 90 ms) and its magnitude was enhanced. When this foot was in contact with a rigid support, the Soleus showed a short burst of activity and the activity in Tibialis Anterior started at a mean latency of 150 ms. The results of the anesthesia session showed that: in the bipedal posture, the Soleus response was depressed and its duration shortened, the Tibialis Anterior response increased in magnitude and the onset was earlier; in the unipedal posture with contact, the magnitude of Soleus response was lower than the control value; in unipedal posture, the motor pattern was similar to that observed in the control unipedal condition. The plantar cutaneous afferents play a marked role in postural control regulation, as is evidenced by the comparison between the EMG responses in control situations and in sessions with anesthesia of the foot sole. © 1990 Springer-Verlag.