Balance reactions to light touch displacements when standing on foam.

We hypothesized that standing on an unstable surface would increase the relevance of light touch to standing balance, such that unexpected displacement of a touch reference would result in more consistent expression of balance corrections, compared to standing on a firm surface. Ten healthy participants stood on a foam block atop a force plate without vision, while lightly touching a reference. The touch plate was unexpectedly displaced forwards 10 times. Responses in tibialis anterior (TA) were observed more frequently across the 10 trials compared with standing on a firm surface. However, the responses evoked in trials 2-10 were functionally distinct from those following the first trial. We suggest the first trial responses represent balance corrective responses induced by the slip of the finger relative to the reference. In contrast, the subsequent responses in TA are likely related to an arm-tracking reaction that emerges, indicating a rapid repurposing of the tactile feedback.