Let your feet do the walking: constraints on the stability of bipedal coordination

In this paper we consider whether the behaviour of the neural circuitry tha t controls lower limb movements in humans is shaped primarily by the spatio temporal characteristics of bipedal gait patterns, or by selective pressure s that are sensitive to considerations of balance and energetics. During th e course of normal locomotion, the full dynamics of the neural circuitry ar e masked by the inertial properties of the limbs. In the present study, par ticipants executed bipedal movements in conditions in which their feet were either unloaded or subject to additional inertial loads. Two patterns of r hythmic coordination were examined. In the in-phase mode, participants were required to flex their ankles and extend their ankles in synchrony. In the out-of-phase mode, the participants flexed one ankle while extending the o ther and vice versa. The frequency of movement was increased systematically throughout each experimental trial. All participants were able to maintain both the in-phase and the out-of-phase mode of coordination, to the point at which they could no longer increase their frequency of movement. Transit ions between the two modes were not observed, and the stability of the out- of-phase and in-phase modes of coordination was equivalent at all movement frequencies. These findings indicate that, in humans, the behaviour of the neural circuitry underlying coordinated movements of the lower limbs is not constrained strongly by the spatiotemporal symmetries of bipedal gait patt erns.