Surprisingly little information exists of the mechanics in the steps initializing the walk-to-run transition (WRT) in humans. Here we assess how mechanical work of the limbs (vertical and horizontal) and the individual joints (ankle, knee and hip) are modulated as humans transition from a preferred constant walking velocity (WLK) to a variety of running velocities (RUN; ranging from a sprint to a velocity slower than WLK). WRTs to fast RUNs occur nearly exclusively through positive horizontal limb work, satisfying the goal of forward acceleration. Contrary to our hypothesis, however, positive mechanical work remains above that of WLK even when decelerating. In these WRTs to slow running, positive mechanical work is remarkably high and is comprised nearly exclusively of vertical limb work. Vertical-to-horizontal work modulation may represent an optimization for achieving minimal and maximal RUN velocity, respectively, while fulfilling an apparent necessity for energy input when initiating WRTs. Net work of the WRT steps was more evenly distributed across the ankle, knee and hip joints than expected. Absolute positive mechanical work exhibited a clearer modulation towards hip-based work at high accelerations (> 3 m s–2), corroborating previous suggestions that the most proximal joints are preferentially recruited for locomotor tasks requiring high power and work production. In WRTs to very slow RUNs, high positive work is nevertheless done at the knee, indicating that modulation of joint work is not only dependent on the amount of work required but also the locomotor context.
Neville J. Pires, Brendan S. Lay, and Jonas Rubenson