Do perturbation-evoked responses result in higher reaction time costs depending on the direction and magnitude of perturbation?

To date, little work has focused on whether cognitive-task interference during postural response execution is influenced by the direction and/or magnitude of the perturbation applied. Hypothetically, the increased difficulty associated with a backward loss of balance could necessitate increased allocation of cognitive resources to counteract destabilizing forces. The current study investigated these relationships using a paradigm in which individuals performed a cognitive task (auditory Stroop task during quiet stance; baseline condition). In certain trials, a translation of the support surface was concurrently evoked (magnitude: small or large; direction: forward or backward) which required a postural response to maintain balance. Ten healthy young adults completed four blocks of these experimental trials (26 randomized trials/block). Postural stability during balance recovery was evaluated using the margin of stability (MoS), while Stroop task performance was based on reaction time cost (RTC) and differences between experimental conditions. Results showed no effect of perturbation direction on RTC, but there was an observed MoS increase at peak extrapolated center of mass excursion following a small perturbation evoked concurrently with the cognitive task. No effect of cognitive-task performance was detected for MoS during stepping strategies (followed large perturbations). Instead, increased RTC were observed relative to the fixed base of support responses. In general, young adults adopted a "posture-first" strategy, regardless of perturbation direction, reinforcing the importance of cognition in the maintenance of upright balance.