Step-Aside Movement Characterization Protocol

The most comfortable and natural step width for a human is close to the shoulder width [18 (link), 19 ]. Thus, participants were asked to perform step-aside movements with their natural step widths at a comfortable speed. According to the definition in the relevant literature, foot placement is actively adjusted by the central nervous system at the time of gait initiation [20 (link)]. Since the purpose of this study was not to confirm this, foot placement was fixed to one step-width level. We applied three parallel lines of white pinstripe tape on the floor at shoulder-width distances as the step-aside guidelines. Participants looked ahead and obtained visual feedback on the step width by looking at a mirror placed in front of them.
In practice sessions for step-aside movements, all participants were asked to stand quietly on the left and middle guidelines, lift their right foot, and move it to the right guideline. Afterwards, they were asked to lift their left foot, move it to the middle guideline, and finally return to the quiet standing position (a to c in Fig 1, 1 to 5 in Fig 2). After 3 s of rest, step-aside movement to the left was performed. As indicated by Wu et al. (2014), there is an ever-present variability in motor execution [21 (link)]. Therefore, the participants were asked to perform two practice sessions before the experiment to become accustomed to their most comfortable movement speed. After being fully equipped and practiced, participants repeated the movements 12 times in each direction for a total of 24 steps. Data from the first and last steps in each direction were not included in the statistical analysis. The determination of the step count is explained in detail in the data analysis section.