Fast muscle responses to an unexpected foot-in-hole scenario, evoked in the context of prior knowledge of the potential perturbation.

This study investigated the effect of prior knowledge of the potential loss of support during walking on muscle responses to the potential perturbation. Four conditions were tested; non-instructed control (NC), non-instructed perturbed (NP), instructed control (IC) and instructed perturbed (IP). Participants were perturbed by having them step into a hidden hole (8.5 cm) in a walkway during the NP and IP trials. Participants had no prior knowledge of the potential perturbation under the NC and NP conditions, but under the instructed conditions, participants were informed that there might be a hole in the walkway. A cautious landing strategy was observed in the IC trials. The participants exhibited flat-footed landings (plantar angle: NC: 13.7 +/- 2.8 degrees; IC: 8.5 +/- 5.2 degrees) and a prolonged double support phase (NC: 138 +/- 18 ms; IC: 161 +/- 17 ms) when they had prior knowledge of the possible hole. When the participants encountered a hole, we saw triggering of fast muscle responses in the ipsilateral plantarflexors and knee extensor, as well as in the contralateral dorsiflexors and knee flexors. This pattern was interpreted as a stop walking synergy. The opposite muscle activation pattern, which was thought of as a resume walking synergy, was induced when no hole was presented and actual foot contact occurred at the expected instant. The latencies between the onsets of muscle responses and the expected heel contact were shorter under the IP condition than under the NP condition (ipsilateral soleus: NP: 78 +/- 13 ms, IP: 64 +/- 14 ms; contralateral biceps femoris: NP: 94 +/- 25 ms; IP: 76 +/- 17 ms). Our results demonstrate that reactive muscle responses to perturbations depend on the anticipatory state with respect to potential perturbations.