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The intermittent control during human quiet upright stance is a new hypothesis which claims that the active neural feedback control generating the ankle muscle torque is switched off and on intermittently at appropriate timings. The intermittent strategy is capable of providing compliant posture while ensuring robust stability. Contrastingly, impairment of postural reflexes in patients with Parkinson’s disease (PD) causes postural instability. Here we hypothesize that the instability in PD patients might be due to a loss of appropriate intermittent activations in the feedback muscle torque during stance. In order to provide evidence for this hypothesis, we characterized stochastic postural sway patterns measured as changes in center of pressure (CoP) and activities of ankle muscles during quiet stance in healthy young and elderly subjects as well as PD patients. To this end, sway patterns and associated ankle muscle activities were quantified by several indices including the CoP sway area, scaling factors of double-power-law power spectra of the sway, as well as levels and patterns of the muscle activations. Hierarchical cluster analysis was performed to suggest that the sway patterns could be classified into two major types. The first type consisted mainly of sway and muscle activation patterns from healthy subjects and some PD patients with the mild level of severity, and they showed features indicating the intermittent control. The second type, consisting mainly of PD patients with relatively severe levels of motor symptoms, was accompanied with non-intermittent but tonic muscle activities and sway areas either smaller or larger than those in the first type. Moreover, the major two types were further classified into several subtypes with distinguishable characteristics. Results suggested that a loss of the intermittent activations in the ankle muscles could be a cause of the postural instability for a population of PD patients.

A Classification of Postural Sway Patterns During Upright Stance in Healthy Adults and Patients with Parkinson’s Disease