Event-driven intermittent control in human balancing on an unstable and unrestrained platform

Abstract

Purpose The use of an intermittent control strategy by the central nervous system (CNS) in controlling the human quiet stance is established in the literature. An intermittent control strategy is a strategy in which the variables of interest are monitored continuously but corrective action is taken intermittently and only when the variables exceed certain threshold criteria. It is unclear whether an intermittent control strategy is employed in balancing tasks such as standing and balancing on an unrestrained, unstable platform. Methods Given the more demanding nature of balancing on an unstable platform, we hypothesize that the CNS might employ a continuous control strategy when first exposed to this task. We study a simple, unrestrained anterior-posterior rocker balancing task, and with the help of a study with human subjects and numerical simulations, we test this hypothesis. Data from a total of 22 human subjects undertaking a quiet standing and a rocker alancing task were recorded and analysed. For numerical simulations, a Rolling Disk coupled with an Inverted Pendulum (RDIP) model with continuous and intermittent control representing the rocker balancing and a Simple Inverted Pendulum (SIP) model to represent the quiet standing task were utilised. In both models, a simple threshold function used in prior literature is employed. Results:Analysis of centre-of-pressure (COP) data collected from human-subject trials with the rocker balancing task reveal non-uni-modal distribution, two distinct slopes in the power spectrum density, and parameter trends in the diffusion stabilogram consistent with the presence of an intermittent control strategy. Furthermore, the trends observed in parameters between the quiet-standing and rocker balancing tasks are consistent with those obtained from numerical simulations of SIP and RDIP models. Conclusion:The data and analysis from the experiments and simulations provide evidence for the presence of an event-driven intermittent control strategy in rocker balancing with reduced intermittency and higher postural fluctuations compared to quiet standing.