Ageing alters postural sway responsivity to the centre of mass movement

Research suggests that ageing involves a 'loss of complexity' in postural control, reflected in reduced fractal temporal correlations of sway fluctuations. However, this view may be simplistic, overlooking the potential complexity of reorganization that compensates for age-related decline. To explore this, we studied CoM-CoP coupling in young and older adults standing on a rigid or foam surface with eyes open or closed. The coordination between the centre of mass (CoM) and centre of pressure (CoP) may reflect how the multi-segmental postural system is organized. We found CoM played a primary role in driving postural adjustments, with CoP reacting to, rather than influencing, the CoM. Older adults showed larger CoM-to-CoP effects than young adults at timescales linked to transitions from long-latency responses to compensatory adjustments, especially under unstable or vision-restricted conditions. Foam surface instability accentuated short-range CoP responsivity to CoM in young adults and extended that responsivity across the entire time course in older adults. Notably, variability in CoP's temporal structure moderated the task dependence of CoM-to-CoP effects. These results refine the 'loss of complexity' view, showing that ageing entails a more responsive postural system rather than a simple decline in complexity.