Facilitation of spinal reflexes assists performing but not learning an obstacle-avoidance locomotor task

The aim of this study was to investigate spinal reflex (SR) modulation during the performance and learning of a precision locomotor task. Healthy subjects had to minimize foot clearance when repeatedly stepping on a treadmill over a randomly approaching obstacle. The subjects walked with reduced vision and were informed about the approaching obstacle and task performance by acoustic warning and feedback signals, respectively. SRs were randomly evoked by tibial nerve stimulation (with non-nociceptive and nociceptive stimulus intensity) during the mid-stance phase in both normal and pre-obstacle stepping. Foot clearance and electromyographic activity of the tibialis anterior and biceps femoris muscles of the right leg were analysed. Only if a delay was introduced between warning signal and nerve stimulation, was the SR amplitude in both muscles enhanced prior to obstacle steps compared with normal steps for both stimulus intensities. Thus, the reflex enhancement depended on the subject's awareness of the approaching obstacle. Improved performance was reflected in a decreased foot clearance, but did not correlate with the course of SR amplitude. It is concluded that obstacle stepping is associated with a facilitation of SR pathways, probably by supraspinal drive. This facilitation might provide assistance in safe obstacle stepping, e.g. to compensate quickly if resistance is encountered.