Proceedings of The Physiological Society

University College London December 2005 (2006) Proc Physiol Soc 1, PC14

Poster Communications

Responses to stepping on an unexpectedly lowered support surface

van der Linden, Marleen; Marigold, Daniel; Hendricks, Henk; Dietz, Volker; Duysens, Jacques;

1. Rehabilitation Medicine, Radboud University Nijmegen Medical Centre, Nijmegen, Netherlands. 2. Gait and Posture Lab, University of Waterloo, Waterloo, ON, Canada. 3. Spinal Cord Injury Center, University Hospital Balgrist, Zurich, Switzerland. 4. SMK research, Sint Maartenskliniek, Nijmegen, Netherlands.

The aim of this study was to determine responses to an unexpected step-down, which resembles stepping on an unexpectedly low pavement. The question was asked whether the absence of positive feedback caused by the loss of expected ground support during walking in humans results in suppression (as described in foot-in-hole studies in cats (1)) or instead in an increase in muscle activity (as observed after a sudden drop of the support surface in human gait (2))? A walkway was built, which embedded a gravity-driven platform that could unexpectedly lower the ground support surface before heel contact by 5.0 cm. Surface electromyographic (EMG) data were collected bilaterally from the rectus femoris (RF), biceps femoris (BF), tibialis anterior (TA) and medial gastrocnemius (MG) muscles of twelve healthy young adults. Glasses blocked the lower part of the visual field. Stepping down unexpectedly (UD) was compared with unexpected level walking (UL, subjects did not know whether the surface was lowered or not), and with stepping down expectedly (ED). The results for both comparisons (UD-ED and UD-UL) were very similar. In the UD condition, some muscles (ipsi MG and contra TA, BF and RF) showed an increase while others (ipsi BF) a decrease in activity with latencies of 46-69 ms after expected heel contact (and 21-44 ms prior to actual touchdown). Following actual heel contact, extra activity was observed in all muscles with durations of 40 to 200 ms. The initial 40 ms following expected heel contact did not differ from control conditions, showing that muscle activity in this period is pre-programmed in origin. The early onset of the ensuing unloading responses suggests involvement of spinal circuitries. In contrast to the cat experiments (1), both increases and decreases in muscle activity were observed. The extra activation in muscles such as the MG prior to touchdown is functionally relevant since it prepares the ipsilateral limb for the delayed impact.

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