The effect of gait approach velocity on the broken escalator phenomenon

Walking onto a stationary surface previously experienced as moving generates an after-effect commonly known as the "broken escalator" after-effect (AE). This AE represents an inappropriate expression of the locomotor adaptation necessary to step onto the moving platform (or escalator). It is characterised by two main biomechanical components, an increased gait approach velocity (GAV) and a forward trunk overshoot on gait termination. We investigated whether the trunk overshoot and other biomechanical measures are the direct inertial consequence of the increased GAV or whether these are the result of an independent adaptive mechanism. Forty-eight healthy young adults walked onto a movable sled. They performed 5 trials with the sled stationary at their preferred walking velocity (BEFORE trials), 5 with the sled moving (MOVING or adaptation trials), and 5 with the sled stationary again (AFTER trials). For the AFTER trials, subjects were divided into four groups. One group was instructed to walk slowly ("slower"), another with cueing at the BEFORE pace ("metronome"). The third group walked without cueing at the BEFORE pace ("normal"), and the fourth, fast ("faster"). We measured trunk pitch angle, trunk linear horizontal displacement, left shank pitch angular velocity and surface EMG from lower leg and trunk muscles. In the AFTER trials, an AE was observed in these biomechanical measures for all gait speeds, but these were not strongly dependent on GAV. An AE was present even when GAV was not different from that of BEFORE trials. Therefore, we conclude that, although contributary, the trunk overshoot is not the direct consequence of the increased GAV. Instead, it appears to be generated by anticipatory motor activity "just in case" the sled moves, herewith termed a "pre-emptive" postural adjustment.