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1 Centre for Neuroscience, University of Alberta, Edmonton, AB, Canada
2 Department of Rehabilitation Medicine, Keio University School of Medicine, Tokyo, Japan
3 Centre for Neuroscience, University of Alberta, Edmonton, AB, Canada; Department of Physiology, University of Alberta, Edmonton, AB, Canada
* To whom correspondence should be addressed. E-mail: richard.stein{at}ualberta.ca.
The purpose of this paper was to study spinal inhibition during several different motor tasks in healthy human subjects. The short-latency, reciprocal inhibitory pathways from the common peroneal (CP) nerve to the soleus muscle and from the tibial nerve to the tibialis anterior (TA) muscle were studied as a depression of ongoing voluntary EMG activity. First, the effect of stimulus intensity on the amount of inhibition was examined to decide an appropriate stimulation to study the task-dependent modulation of inhibition. Then, the inhibition at one level of stimulation (1.5 X motor threshold: MT) was investigated during standing, walking, and running. The change in slope of inhibition vs. EMG level, which approximates the fraction of ongoing activity that is inhibited, decreased with CP stimulation from 0.52 during standing to 0.30 during fast walking (6 km/h) to 0.17 during running at 9 km/h. Similarly, the slope decreased with tibial nerve stimulation from 0.68 (standing) to 0.42 (fast walking) to 0.35 (running at 9 km/h). All differences, except the last one, are highly significant (P < 0.01, Students t-test). However, the difference between walking (0.42) and running (0.36) at the same speed (6 km/h) was not significant with tibial nerve stimulation and only significant at P < 0.05 with CP nerve stimulation (0.30, 0.20). Also, the difference between standing (0.52) and slow walking (3 km/h; 0.41) with CP stimulation was not significant, but was significant (P < 0.01) with tibial nerve stimulation (0.68, 0.49). In conclusion, our findings indicate that spinal reciprocal inhibition decreases substantially with increasing speed, and only changes to a lesser extent with task.
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