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

Abstract

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.

Keywords: Anticipatory postural adjustments; Falls; Neuromechanics; Posture; Reactive postural adjustments.

Fig. 1

Four combinations of surface translations (anticipated and unanticipated, anterior and posterior) were applied during standing via a dual-belt instrumented treadmill. Participant responses were assessed across three time periods: pre-perturbation (750 ms before onset), early post-perturbation (0–750 ms), and late post-perturbation (750–1500 ms)

Fig. 2

EMG linear envelopes for the medial gastrocnemius (MG), soleus (SOL), and tibialis anterior (TA) in response to treadmill-belt perturbations beginning 750 ms prior to perturbation onset through 1500 ms after perturbation onset. Vertical dashed lines indicate perturbation onset (0 ms, left) and the end of the early post-perturbation epoch (+ 750 ms, right)

Fig. 3

Integrated EMG (iEMG) for the medial gastrocnemius (MG), soleus (SOL), and tibialis anterior (TA) during the pre-perturbation (-750 ms to 0 ms, A), early post-perturbation (0 ms to 750 ms, B), and late post-perturbation (750 ms to 1500 ms, C). AA: Anticipated Anterior; UA: Unanticipated Anterior; AP: Anticipated Posterior; UP: Unanticipated Posterior. Single asterisks (*) indicate significant pairwise effects of anticipation, hashtags (#) indicate significant effects of direction, and double asterisks (**) indicate significant interactions between anticipation and direction

Fig. 4

Center of mass (CoM) displacements estimated via anterior-posterior sacrum position in response to treadmill-belt perturbations. Significant pairwise effects of anticipation are shown by colored single asterisks (*), wherein the color refers to the condition with larger displacement