Adaptation of foot muscle activation and stabilization strategies in steps with unexpected heights

The intrinsic foot muscles are known to contribute to postural control during upright stance and locomotion. However we know little about the role of these muscles in stabilization following unexpected perturbations. We conducted an experiment in which subjects were trained to repeatedly step at a particular height with visual and audio cues removed. After a random-length period of stepping, subjects were perturbed by an unexpected change in step height. Activation increases of Flexor Digitorum Brevis (FDB) and Soleus (SOL) were larger in surprise step downs than surprise step ups, while the opposite was true for Abductor Hallucis (AH). Additionally, since AH activation increased only after ground contact in surprise down steps, it may be comparatively more driven by muscle stretchreflex than FDB and SOL, whose activation did increase prior to ground contact. These results suggest that individual foot muscle responses to unexpected perturbations are task and loading specific.