Interactions between initial posture and task-level goal explain experimental variability in postural responses to perturbations of standing balance

Abstract

Postural responses to similar perturbations of standing balance vary widely within and across subjects. Here, we identified two sources of variability and their interactions by combining experimental observations with computational modeling: differences in posture at perturbation onset across trials, and differences in task-level goals across subjects. We first collected postural responses to unpredictable backward support-surface translations during standing in 10 young adults. We found that maximal trunk lean in postural responses to backward translations were highly variable both within (mean of ranges 28.3°) and across subjects (range of means 39.9°). Initial center of mass (COM) position was correlated with maximal trunk lean during the response but this relation was subject specific (R² = 0.29 - 0.82). We then used predictive simulations to assess causal relations and interactions with task-level goal. Our simulations showed that initial posture explains the experimentally observed intra-subject variability with a more anterior initial COM position increasing the use of the hip strategy. Differences in task-level goal explain observed inter-subject variability with prioritizing effort minimization leading to ankle strategies and prioritizing stability leading to hip strategies. Interactions between initial posture and task-level goal explain observed differences in intra-subject variability across subjects. Our findings suggest that variability in initial posture due to increased sway as observed in older adults might increase the occurrence of less stable postural responses to perturbations. Insight in factors causing movement variability will advance our ability to study the origin of differences between groups and conditions.

Adaptations in Reactive Balance Strategies in Healthy Older Adults After a 3-Week Perturbation Training Program and After a 12-Week Resistance Training Program

“…The maximal trunk lean angle θ trunk , max is a measure of the reliance on a hip strategy for a specific trial. However, θ trunk , max depends on the perturbation magnitude and, as we recently demonstrated, the initial posture of the subject (Van Wouwe et al, 2020 ). xCOM 300 ms after perturbation onset ( xCOM 300 ms ) captures the effect of both the perturbation magnitude and initial posture (Van Wouwe et al, 2020 ).…”

“…However, θ trunk , max depends on the perturbation magnitude and, as we recently demonstrated, the initial posture of the subject (Van Wouwe et al, 2020 ). xCOM 300 ms after perturbation onset ( xCOM 300 ms ) captures the effect of both the perturbation magnitude and initial posture (Van Wouwe et al, 2020 ). Therefore, the relation between θ trunk , max and xCOM 300 ms established based on several perturbation trials better quantifies an individual's reliance on a hip strategy than θ trunk , max .…”

“…To assess reactive balance performance during standing we quantified the step incidence for specific perturbation directions and magnitudes, where perturbations were applied as support-surface translations using a CAREN platform (Motek Medical, Netherlands) (Van Wouwe et al, 2020 ). Participants stood barefoot on the movable platform with their feet at shoulder width looking forward and wore a safety harness to catch them in case of a loss of balance.…”

“…A correction of the force plate data was performed to remove forces and moments registered due to the inertia of the force plate (Roberts et al, 2019 ). We corrected for these forces and moments by subtracting the forces and moments registered while the platform was moving without any load on it from the data acquired with the subject on the platform (Van Wouwe et al, 2020 ).…”

“…The reliance on a hip strategy was quantified by the relation between the xCOM position 300 ms after perturbation onset ( xCOM 300 ms ) and the maximal trunk lean angle θ trunk , max during non-stepping responses K hip (Van Wouwe et al, 2020 ). The maximal trunk lean angle θ trunk , max is a measure of the reliance on a hip strategy for a specific trial.…”