Modifications of audio-spinal facilitation during gait in normal man

An unexpected loud sound--sufficient to elicit an audio-spinal response in lower limb muscles during standing--has been delivered during the gait cycle in 40 healthy volunteers. Raw EMG bursts were recorded in the right trapezius (Tra), and in soleus (Sol) and tibialis anterior (TA) muscles bilaterally before being converted into envelope curves which were measured. The results indicated that the step cycle was not modified. On the other hand, EMG bursts time-locked to the sound appeared in Tra during gait. Audio-spinal responses similar to those seen during standing were present during gait only in the flexor TA provided it was silent. In Sol, responses were absent regardless of whether it was active or not. Some later and not time-locked EMG reinforcements were however seen, provided Sol was active. The audio-spinal responses during gait differed thus markedly from those seen at rest in the standing position. It is suggested to consider intervention of the central pattern generator which is already known to alter dramatically the responses to peripheral stimuli during gait. Another possibility might be that the descending influences are modified in supraspinal structures but this interpretation is less likely as responses are not cyclically modified in Tra.