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 stability and progress. This study investigates the bases for 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{degrees}, 90{degrees}, or 120{degrees} turn to the right. On a subset of trials, participants were required to rapidly avoid stepping on select footholds. We found stereotyped alternate foot placement strategies across turn angles that differed by turn strategy. Alternate foot placement for step turns supported the walkers instantaneous kinematics but not the planned change in trajectory; whereas foot placement for disrupted spin turns supported the walkers anticipated change in 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.