Bases for the selection of alternate foot placement during straight- and turning-gait.

Humans typically plan foot placement strategy multiple steps in advance when walking across complex terrain. Planning steps in advance is beneficial for both upright stability and forward progress, but one drawback is that new obstacles can make planned foot placement location unsafe between planning and execution, necessitating a rapid shift to foot placement that impacts both stability and progress. This study investigates the selection of alternate foot placement during both straight- and turning-gait. Thirteen healthy young adults walked along a virtually projected walkway with precision footholds oriented in either a straight line or with a single 60°, 90°, or 120° step or spin turn to the right. Step turns are turns where the rotation of the body occurs while the outside limb is in contact with the ground, whereas spin turns pivot over the inside limb. On a subset of trials, participants were required to rapidly avoid stepping on select footholds. Hierarchical bootstrapping analyses revealed stereotyped alternate foot placement strategies across turn angles that differed between step turns and spin turns. Specifically, alternate foot placement for step turns aligned with the person's approach trajectory when they identified a new obstacle, regardless of turn angle; whereas alternate foot placement for disrupted spin turns aligned with the person's future turn trajectory. We conclude that when humans are forced to rapidly alter previously developed motor plans for foot placement, they utilize a rapid stereotyped behavior that changes based on the demand of the turn.