MECHANISM FOR REFLEX REVERSAL DURING WALKING IN HUMAN TIBIALIS ANTERIOR MUSCLE REVEALED BY SINGLE MOTOR UNIT RECORDING

1. A reversal in the sign of a cutaneous reflex during walking was rec ently described in the human. Such reversals were most clearly seen in muscles that were active in two parts of the step cycle, such as the tibialis anterior (TA). The current study determined whether the rever sal resulted from differential activation of a single group of motor u nits. 2. Single motor units were recorded from the TA muscle of health y human subjects while they walked on a treadmill with a splint that l imited motion at the ankle joint. The majority of motor units from whi ch recordings were made (43 out of 46) were active in both the swing p hase and the transition from swing to stance, indicating that the two bursts of activity from the TA muscle do not represent the activity of two separate populations of motor units. 3. The firing behaviour of t hree motor units was observed during walking steps when stimuli were a pplied to the posterior tibial nerve during either the swing phase or the transition from swing to stance. The post-stimulus time histograms indicated that the same motor unit was excited during the swing phase , and inhibited during the transition from swing to stance. 4. The res ults support the hypothesis that there are parallel excitatory and inh ibitory pathways from cutaneous afferents to single motoneurones of th e TA muscle. A shift in balance between the two pathways as a function of the step cycle most probably generates the reflex reversal observe d.