Foreknowledge of postural demands of an upcoming step modulates rapid stepping behaviour

Interacting in a dynamic world often requires rapid steps towards visual stimuli. We recently showed that rapid steps under low postural demands (i.e. stepping anterolateral) were facilitated by short-latency (∼120 ms) bursts of muscle activity termed express visuomotor responses (EVRs). However, when stepping introduced high postural demands (i.e. stepping anteromedial), EVRs were first suppressed, and then mediolateral anticipatory postural adjustments (ML-APAs) were generated. In these prior experiments, participants had certainty that the upcoming step would introduce low or high postural demands. Here, we manipulated such foreknowledge to investigate its influence on EVR and ML-APA expression during step initiation.

Healthy young adults (n = 20) stepped to visual targets over a range of step eccentricities in blocks of foreknowledge present (anterolateral or anteromedial steps, involving low or high postural demands) or foreknowledge absent conditions (anterolateral and anteromedial steps). We measured bilateral EMG from gluteus medius, vertical ground-reaction forces and step reaction times.

For anterolateral steps, we observed greater EVRs, reduced ML-APAs and shorter reaction times in foreknowledge present vs. absent conditions. Conversely, for anteromedial steps, we observed small EVRs, larger and more ML-APAs and longer RTs under foreknowledge.

Our results suggest that foreknowledge of low postural demands during anterolateral steps lets people safely launch EVRs while suppressing ML-APAs, prioritizing shorter RTs over balance. In contrast, foreknowledge of high postural demands during anteromedial steps leads people to prioritize balance, expressing ML-APAs and delaying step initiation. Our findings are relevant for individuals with impaired balance control, in whom prioritization of balance may impact rapid stepping.