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

Postural steadiness during quiet standing shows substantial inter-individual 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 years), with data from 31 healthy young adults (19-22 years) included as a reference. Sway variability was quantified using center-of-pressure (CoP) root mean square displacement (CoP RMS ) and mean velocity (CoP MV ). In older adults, CoP RMS and CoP MV exhibited comparable inter-individual variability but were largely uncorrelated, indicating dissociable displacement- and velocity-related components. In contrast, young adults showed a positive coupling between CoP RMS and CoP MV . Further analyses of horizontal center-of-mass acceleration and sagittal-plane ankle-hip kinematics in older adults revealed that larger CoP RMS values were associated with slower, ankle-dominant behavior resembling a single-link inverted pendulum, whereas larger CoP MV values were linked to faster sway and stronger ankle-hip anti-phase 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 inter-individual variability in postural control.