BIPEDAL REFLEX COORDINATION TO TACTILE STIMULATION OF THE SURAL NERVEDURING HUMAN RUNNING

1. Cutaneous reflex responses were elicited during human running (8 km /h) on a treadmill by electrical stimulation of the sural nerve at the ankle. Stimulus trains (5 pulses of 1 ms at 200 Hz) at three nonnocic eptive intensities, which were 1.5, 2.0, and 2.5 times perception thre shold (PT), were delivered at 16 phases of the step cycle. For 11 subj ects the surface electromyographic (EMC) activity of both the ipsilate ral and contralateral long head of the biceps femoris (iBF and cBF, re spectively), the semitendinosus (IST and cST), the rectus femoris (iRF and cRF), and the tibialis anterior (iTA and cTA) were recorded. 2. D uring human running, nonnociceptive sural nerve stimulation appears to be sufficient to elicit large, widespread and statistically significa nt reflex responses, with a latency of similar to 80 ms and a duration of similar to 30 ms. These reflex responses seem to be an elementary property of human locomotion. This is indicated by the occurrence of t he responses in all subjects, the consistency of most of the reflex pa tterns across the subjects and, apart from a small amount of habituati on, the reproducibility of the responses during the course of the expe riment. 3. The responses are modulated continuously throughout the ste p cycle such that their magnitude does not in general covary with the background locomotor activities. This is observed most clearly in iST, iTA, and cTA for which statistically significant reflex reversals are demonstrated, and in cRF and cTA for which the responses are gated du ring most of the step cycle. 4. The response magnitude generally incre ases as a function of increasing intensity, whereas the phase-dependen t reflex modulation is intensity independent.5. A functional dissociat ion within the ipsilateral hamstring muscles is demonstrated: the iBF and iST show an antagonistic reflex pattern (facilitatory and suppress ive, respectively) during the periods of synergistic background locomo tor activity in the step cycle. Contralaterally, however, the cBF and cST are reflexively activated as close synergists during these periods . 6. The reflex responses and their phase-dependent modulation are dif ferent for the homologous muscles in the two legs. Yet, some similarit ies are observed. These are present rather with respect to the phase o f the corresponding leg than with respect to the phase of the stimulat ed leg. Both observations suggest that the phase-dependent reflex modu lation is controlled separately in the ipsilateral and contralateral l egs. 7. The response simultaneity in all investigated muscles supports the notion of a coordinated cutaneous interlimb reflex during human r unning. This reflex coordination is intensity independent (within the range of nonnociceptive stimulation) and different from the locomotor coordination. The possible functional significance underlying the bipe dal reflex coordination is discussed. It is suggested that tactile cut aneous feedback may be used to move the perturbed leg away from the st imulus, with the general constraint of preserving both the cadence and the balance at all times during the step cycle. The contralateral res ponses are thought to play an important, supportive role in promoting a smooth transition between these potentially conflicting requirements .