Postural threat influences reactive stepping characteristics at the single-step threshold, but not when a step is necessary.

Following a loss of balance, proper reactive responses are needed to avoid a fall. Efforts have focused on how physiological factors affect these responses, but few have addressed psychological states. One psychological factor of particular interest is fear of falling (FoF); while prevalent in older adults, the mechanisms by which it increases fall risk are not fully understood. The purpose of this study was to use a well-established, postural-threat manipulation (standing at a height) to evaluate how such threat affects reactive stepping behavior during a perturbation protocol designed to identify the perturbation magnitude at which stepping became necessary for balance recovery. Ten healthy, young adults completed a single-step threshold (SST) test using a waist-mounted spring scale device to provide progressively-ordered anterior and posterior perturbations while standing on level ground and on a 1-m-high platform. Stepping kinematics and dynamic stability (i.e., margin of stability, MoS) were assessed. For a given direction, standing at a height (increased postural threat) did not significantly alter SST, although a trend toward lower posterior SST at height was observed (p = 0.057). However, increased postural threat substantially altered the timing and execution of the recovery step performed at SST. Participants initiated steps 200-300 ms earlier and took shorter steps at raised height (p < 0.05), yet achieved greater MoS at foot contact. Under threat, MoS was reduced to a lesser extent before stepping, with participants often stepping before MoS < 0, indicating that steps were initiated at a larger safety margin relative to instability. Findings suggest that within the current paradigm, postural threat influences when recovery steps are initiated and how they are executed at SST, even when the perturbation magnitude required to evoke stepping remains largely unchanged. Future work should evaluate whether older adults with FoF adopt such strategies and whether this impacts fall risk.