Fast responses to stepping-target displacements when walking.

It is well-known that goal-directed hand movements can be adjusted to small changes in target location with a latency of about 100 ms. We tested whether people make similar fast adjustments when a target location for foot placement changes slightly as they walk over a flat surface. Participants walked at 3 km/h on a treadmill on which stepping stones were projected. The stones were 50 cm apart in the walking direction. Every 5-8 steps, a stepping stone was unexpectedly displaced by 2.5 cm in the medio-lateral direction. The displacement took place during the first half of the swing phase. We found fast adjustments of the foot trajectory, with a latency of about 155 ms, initiated by changes in muscle activation 123 ms after the perturbation. The responses corrected for about 80% of the perturbation. We conclude that goal-directed movements of the foot are controlled in a similar way to those of the hand, thus also giving very fast adjustments.