Effects of walking velocity on relative phase dynamics in the trunk in human walking.

The nature of coordination changes and stability features in the relative phase dynamics of the trunk were examined in seven healthy subjects, while walking velocity on a treadmill was gradually increased and decreased. Predictions from Schöner et al (J. Theor. Biol. 142, 359-391, 1990) regarding transition mechanisms in quadrupedal walking generalized to pelvis-thorax phase relations in bipedal walking, in that more continuous transitions with and without loss of stability were observed when walking velocity was manipulated as a control parameter. Relative phase changed from more in-phase (about 25 degrees) at lower velocities to more out-of-phase (about 110 degrees) at higher velocities. Stability analysis of relative phase demonstrated the existence of more than one stable coordination pattern ('multistability'). Total ranges of motion in pelvis, thorax, and trunk, as well as stride length were larger at the decreasing velocity range was compared to the increasing velocity range, showing dependence on direction of control parameter manipulation ('hysteresis effect'). The nature of these transitions identifies phase relations in the trunk in human walking as lower symmetry dynamics, a finding consistent with the proposed dynamics of the quadrupedal walking mode. These results suggest the existence of different coordination patterns (multistability) in the human bipedal walking mode and question traditional distinctions in only two modes (walking and running) in human gait.