Substituting anticipatory locomotor adjustments online is time constrained

Two crucial, multi-articular strategies for anticipatory locomotor adjustments (ALA) are knee flexor generation to step over obstacles and hip flexor generation to step up. While lower limb control can be adapted online to modify an already planned obstacle avoidance, or to avoid the sudden appearance of an obstacle, it is not known whether a planned ALA can be substituted by different one online. The present objective was to study such ALA substitutions at two specific timepoints: the final planning stage and the initiation of ALA execution. Ten healthy, young adults (22.0 ± 1.7 years; 5 males) walked in a Virtual Environment (VE) representing the laboratory within a head mounted display. Two blocks of trials, one involving an initial VE with an obstacle (OB) and the other an initial VE with a platform (PL) (heights of 15% of lower limb length for both), were presented, where the initial VE could remain unchanged or be randomly switched between them at one of the two timepoints. The final VE always corresponded to the real environment. Lead limb kinematics, joint kinetics and energetics, as well as electromyography were measured. Repeated measures ANOVAs were used to compare across conditions. Foot clearance, knee flexor generation, and hip flexor generation all changed in the expected directions for the final VEs when requiring early substitution, but not when switched late. These findings show that volitional, locomotor strategies may be substituted at the end of the ALA planning phase, but not once execution is initiated.