Wobble-board instability re-orthogonalizes postural geometry in older adults through exogenous constraint

Abstract Postural control is expressed as intermittent organization of center-of-pressure (CoP) motion on a saddle-shaped manifold typically aligned with the anteroposterior (AP) and mediolateral (ML) axes. When task demands reorient postural focus, this saddle rotates away from the AP–ML alignment yet preserves orthogonal axes that indicate directions of greatest and least fractal temporal correlations in sway. Preserved orthogonality appears to reflect a balance between endogenous fractal fluctuations and exogenous task demands. Perturbation, however, seems to erode older adults’ endogenous fractal support, prompting heavier, less nuanced reliance on exogenous constraints. We recruited older and younger adults to coordinate with tasks in orthogonal directions—performing the Trail Making Test (TMT) with forward (AP) focus while standing quietly or on a wobble board that induced ML perturbations. The wobble board weakened endogenous fractal support for inter-axial orthogonality in both groups while expanding the range of fractality. A forward focus during TMT reinstated endogenous fractal support in younger adults—except on the wobble board—and narrowed the fractality range. In older adults, TMT increased orthogonality only via exogenous influence, without reinstating endogenous fractal support. Thus, deviations of the saddle topology from the classical AP–ML orientation under competing task alignments reveal that aging entails greater reliance on exogenous supports as endogenous fractality weakens, even as perturbations can increase the range or magnitude of fractal temporal correlations. Neither aging nor perturbed posture so much loses fractal complexity as loses the capacity to exploit endogenous fractality to maintain stable topologies.