Exploring the role of applied force eccentricity after foot-contact in managing anterior instability among older adults during compensatory stepping responses.

Background: The specific mechanisms responsible for age-related decline in forward stability control remain unclear. Previous work has suggested reactive control of net ground reaction force (GRF) eccentricity may be responsible for age-related challenges in mediolateral stability control during the restabilisation phase of forward compensatory stepping responses.

Research Questions: Does reactive control of GRF eccentricity play a role in managing forward stability control during the restabilisation phase of a forward stepping response to external balance perturbation?

Methods: Healthy younger (YA) (n = 20) and older adults (OA) (n = 20) were tethered to a rigid frame, via adjustable cable. Participants were released from a standardised initial forward lean and regained their balance using a single step. Whole-body motion analysis and four force platforms were utilised for data acquisition. Forward instability was quantified as centre of mass (COM) incongruity - the difference between the first local peak and final stable anterior COM positions. The extent of GRF eccentricity was quantified as the sagittal-plane angle of divergence of the line of action of the GRF relative to the COM. Two discrete points during restabilisation were examined (P1 and P2), which have been suggested to be indicative of proactive and reactive COM control, respectively. Age-related differences in magnitude, timing and trial-to-trial variability of kinematic and kinetic outcome variables were analysed using two-factor ANOVAs with repeated-measures.

Results: OA exhibited greater COM incongruity magnitude and variability - both were reduced with trial-repetition. There were no age-related differences in the magnitude or timing of P2. Instead, OA exhibited a reduced magnitude of GRF eccentricity at P1. There was a positive correlation between AP COM incongruity magnitude and P1 magnitude.

Significance: Different from mediolateral stability control, the present results suggest that OA may experience forward stability control challenges as a function of insufficient preparatory lower limb muscle activation prior to foot-contact.