Individual differences in postural control in older adults: two-dimensional structure of sway variability and multijoint dynamics

Postural steadiness during quiet standing shows substantial interindividual variability in older adults, yet the neuromechanical basis of this heterogeneity remains unclear. We recorded ground reaction forces during eyes-closed standing in 31 healthy older adults (65-73 yr), with data from 31 healthy young adults (19-22 yr) included as a reference. Sway variability was quantified using center-of-pressure (CoP) root mean square displacement (CoPRMS) and mean velocity (CoPMV). In older adults, CoPRMS and CoPMV exhibited comparable interindividual variability but were largely uncorrelated, indicating dissociable displacement- and velocity-related components. In contrast, young adults showed a positive coupling between CoPRMS and CoPMV. Further analyses of horizontal center-of-mass acceleration and sagittal-plane ankle-hip kinematics in older adults revealed that larger CoPRMS values were associated with slower, ankle-dominant behavior resembling a single-link inverted pendulum, whereas larger CoPMV values were linked to faster sway and stronger ankle-hip antiphase coordination characteristic of a double-link inverted pendulum. These findings indicate that postural steadiness in older adults is organized along at least two independent dimensions linked to distinct coordination strategies. Our findings provide a basis for interpreting commonly used CoP-based measures in relation to underlying control dynamics and offer a more mechanistic perspective on interindividual variability in postural control.NEW & NOTEWORTHY Postural steadiness in older adults exhibited a two-dimensional structure of interindividual variability defined by center-of-pressure displacement (CoPRMS) and velocity (CoPMV), whereas young adults showed a coupled, one-dimensional pattern. Moreover, in older adults, these two dimensions were associated with distinct coordination patterns: CoPRMS with ankle-dominant, single-link inverted pendulum-like behavior, and CoPMV with ankle-hip anti-phase oscillations characteristic of double-link inverted pendulum-like behavior.