The effects of anticipation of standing surface translations on distal leg muscle excitations

This study aimed to investigate the interactions between anticipation and direction of surface translations applied during standing on distal leg muscle excitations measured via surface electromyography (EMG) in the context of resultant changes in center of mass (CoM) displacements. Twenty young adults experienced four combinations of surface translations (i.e., anticipated anterior, unanticipated anterior, anticipated posterior, unanticipated posterior) via a dual-belt instrumented treadmill. For each condition, a 200 ms, 6 m/s² anterior or posterior perturbation was delivered either unexpectedly or after a three-second verbal countdown. Each condition was repeated three times in a randomized order. We recorded surface EMG from the MG, SOL, and TA, and motion capture collected pelvis displacements as a surrogate for CoM. Data were analyzed across three time periods: pre-perturbation, early post-perturbation, and late post-perturbation. Direction-dependent EMG responses were as hypothesized, with TA iEMG increasing by 81% following anterior perturbations and MG and SOL iEMG increasing by 60% and 32% respectively following posterior perturbations. As hypothesized, anticipated balance challenges elicited greater proactive iEMG for all muscles. However, only unanticipated anterior perturbations elicited greater reactive TA iEMG than anticipated anterior perturbations. These proactive neuromechanical adjustments, particularly for anterior surface translations that would precipitate a backward fall, appeared protective and effective based on resultant patterns of CoM displacements. These data serve as a reference for understanding how aging and disease impact proactive and reactive postural control, especially for populations who may have difficulty with both the planning and execution of corrective neuromuscular adjustments.