Explaining Parkinsonian postural sway variabilities using intermittent control theory

Postural impairment due to neuro-degenerative disorders such as Parkinson's Disease (PD) leads to restricted gait patterns, fall-related injuries, decreased mobility, and loss of functional independence. Though several clinical and posturographic studies have attempted to reveal the complex pathophysiology involved in PD, the diversity of Parkinsonian population makes them unclear and sometimes even contradictory. For instance, studies related to the Center of Pressure (CoP) sway during quiet stance in PD patients highlight both increase and reduction of magnitude in contrast to age-matched healthy individuals. A possible explanation for this contradiction is presented in this article. While the presence of intermittent control has been observed in postural control in human quiet stance, we hypothesize that one of the factors that affects postural instability in PD might be the increase in intermittency in active feedback control. Using a simulation model representing the Anterior-Posterior dynamics of human quiet standing, the intermittent control strategy is first contrasted against continuous control strategy in terms of stability, energy efficiency and settling time, thus establishing the inherent advantages of an intermittent control strategy. Further, the ability of the intermittent control strategy to explain several clinical observations in PD is demonstrated. An experimental pilot study is also conducted to support the simulation study, and several body sway parameters derived from recordings of CoP are presented. The presented results are in close agreement with reported clinical observations and may also prove useful for the assessment of disease progression and future fall risk.