Dynamics of the body centre of mass during actual acceleration across transition speed

Judged by whole body dynamics, walking and running in humans clearly differ. When walking, potential and kinetic energy fluctuate out-of-phase and energy is partially recovered in a pendulum-like fashion. In contrast, running involves in-phase fluctuations of the mechanical energy components of the body centre of mass, allowing elastic energy recovery. We show that, when constantly accelerating across the transition speed, humans make the switch from walking to running abruptly in one single step. In this step, active mechanical energy input triples the normal step-by-step energy increment needed to power the imposed constant acceleration. This extra energy is needed to launch the body into the flight phase of the first running step and to bring the trunk into its more inclined orientation during running. Locomotor cycles immediately proceed with the typical in-phase fluctuations of kinetic and potential energy. As a result, the pendular energy transfer drops in one step from 43% to 5%. Kinematically, the transition step is achieved by landing with the knee and hip significantly more flexed compared to the previous walking steps. Flexion in these joints continues during the first half of stance, thus bringing the centre of mass to its deepest position halfway through stance phase to allow for the necessary extension to initiate the running gait. From this point of view, the altered landing conditions seem to constitute the actual transition.