Proceedings of The Physiological Society
University College London December 2005 (2006) Proc Physiol Soc 1, C5
Limb muscle responses to vestibular stimulation during treadmill walking in man
Iles, J F; Tanner, Rachel;
1. Zoology, University of Oxford, Oxford, United Kingdom.
Bipolar binaural galvanic vestibular stimulation (GVS) induces medium latency (ML) responses in leg and trunk muscles and sway towards the anode side in standing subjects. During walking, GVS applied early in stance induces ML responses in ankle muscles of the stance limb (Iles et al. 2004) and path deviation towards the anode (Bent et al. 2004). Mean ML emg increases seen during walking were 75% and 73% in soleus and tibialis anterior, respectively, but with considerable inter-subject variability (soleus range 14-182%, coefficient of variation 74%, n=12; tibialis anterior range 0-248%, coefficient of variation 102%, n=11). Walking subjects with their eyes shut made left hand contact with a front platform so that they could maintain their position on the treadmill. We reasoned that variation in hand contact area and pressure might cause variation in response to GVS because hand contact reduces leg muscle ML responses during standing and replaces them with arm muscle responses (Britton et al. 1993). We have studied ML response amplitude in 12 control subjects. In three subjects we measured stance leg ML response amplitude (in a window 120-250 ms after GVS onset) with eyes shut and light fingertip contact with the platform. This was compared with eyes shut and firm grasp of one or both side-rails parallel to the treadmill belt. In a fourth subject we compared eyes open and no hand contact with eyes open and firm grasp. In all four cases firm grasp abolished the GVS induced excitatory ML responses in both soleus and tibialis anterior. To examine the relative importance of vision and touch we studied all the permutations of vision and contact in each of two subjects, analysing the data with ANOVA and the Bonferroni multiple comparisons post test. Grasping the side-rails produced a large and significant diminution compared to light touch or no hand contact. ML responses (at 140 ms) were recorded in the upper limbs during firm grasp of both hands with eyes closed. With the GVS anode on the left, the right triceps brachii was excited and the left inhibited. Excitatory responses could be elicited at all times during the stride. The effects of providing support through the arms during walking are similar to those seen during standing: responses in the legs are reduced and ML responses appear in arm muscles. This may be related to reports of reduced body sway resulting from hand contact during treadmill walking (Dickstein & Laufer, 2004).
Where applicable, experiments conform with Society ethical requirements