Soleus H-reflex excitability during standing with stable and unstable light touch

Light touch reduces sway when participants stand with their eyes closed. Unexpected displacement of the touch reference induces a "false-positive" balance correction in most participants. However, this response is only observed with the first displacement, with subsequent displacements eliciting an arm-extension response. This suggests that the sensorimotor processes involved in stabilizing sway from light touch are rapidly adapted within a single trial. We hypothesized that this change in sensorimotor behaviour would be reflected in changes in the excitability of the soleus H-reflex. Soleus H-reflexes and M-waves, along with background muscle activity, were obtained from ten healthy participants during five standing conditions: 1) eyes open (EO), 2) eyes open with touch (EOT), 3) eyes closed (EC), 4) eyes closed with touch (ECT), and 5) eyes closed touching a reference that has been unexpectedly and repeatedly displaced (ECP). Postural sway was evaluated in each condition, inferred from the motion of the centre of pressure recorded with the use of a force plate. As expected, H-reflexes were suppressed, while sway parameters were increased, during the EC condition, when compared with EO, EOT and ECT. In contrast, H-reflexes during the ECP condition were suppressed comparable to the EC condition, while the sway parameters were not similarly increased. We suggest that the suppression of the H-reflex during the ECP condition reflects an inferred change in the sensorimotor task; specifically switching from utilizing the light touch reference as an external spatial reference, to engaging the touch reference as a target to be maintained.