Postural instability recruits shorter-timescale processes into the non-Gaussian cascade processes

Healthy human postural sway exhibits strong intermittency, reflecting a richly interactive foundation of postural control. From a linear perspective, intermittent fluctuations might be interpreted as engagement and disengagement of complementary control processes at distinct timescales or from a nonlinear perspective, as cascade-like interactions across many timescales at once. The diverse control processes entailed by cascade-like multiplicative dynamics suggest specific non-Gaussian distributional properties at different timescales. Multiscale probability density function (PDF) analysis showed that when standing quietly, stable sand-filled loading of the upper extremities would elicit non-Gaussianity profiles showing a negative-quadratic crossover between short and long timescales. Unstable water-filled loading of the upper extremities would elicit simpler monotonic decreases in non-Gaussianity, that is, a positive-quadratic cancellation of the negative-quadratic crossover. Multiple known indices of postural sway governed the appearance or disappearance of the crossover. Finally, both loading conditions elicited Levy-like distributions over progressively larger timescales. These results provide evidence that postural instability recruits shorter-timescale processes into the non-Gaussian cascade processes, that indices of postural sway moderate this recruitment, and that postural control under unstable loading shows stronger statistical hallmarks of cascade structure.