Corrective postural responses evoked by completely unexpected loss of ground support during human walking.

Understanding reactive responses to unexpected perturbation is fundamental to research on falls and their prevention. In this study, seven healthy young males walked along a walkway with and without a completely unexpected loss of ground support that was made by removing a wooden board (6.5 cm high) mounted on the walkway. Electromyography (EMG), ground reaction forces, and knee and ankle kinematics were recorded and comprehensively analyzed to investigate the corrective postural response to the perturbation. Three sequential strategies were observed. First, the fastest response was the reflexive muscle activity of the perturbed ankle, which we argue was evoked and enhanced by the absence of the expected somatosensory afferents at the expected heel contact. We also demonstrated that rapid soleus activity partially contributed to absorbing the impact of the actual touchdown. Second, after the touchdown, we argue that the central nervous system may reset the gait rhythm to permit continued walking by delaying the subsequent take-off. As a result, the duration of the total stance phase was identical to that recorded during normal walking. Third, we observed an adaptive locomotion to surmount the hole; both knees were more flexed than normal in order to allow the subject to withdraw the perturbed leg from the hole.