Postural instability, a leading cause of fall-related injuries and fatalities, is a common hazard for patients with Parkinson’s disease, as well as in some otherwise healthy older people. However, the underlying neuromechanical causes of postural instability remain poorly understood.
To develop a deeper understanding, Suzuki et al. created a model of intermittent time-delayed feedback control to study how we sway when we we try to stand quiet. The model focuses on the intermittent switching between inactivation and activation of feedback actions as controlled by the central nervous system (CNS), and treating the intermittent switching as a key mechanism of postural stability.
In their model, the researchers used postural sway data from healthy young adults, healthy elderly adults, as well as patients with Parkinson’s disease.
“Nonlinear characteristics of temporal patterns of sway play a key role because, contrary to popular belief, wider sway amplitudes are not necessarily associated with balance instability,” author Taishin Nomura said.
To show how stabilization is maintained or lost, they incorporated a Bayesian parameter inference to reproduce a diverse set of spontaneous sway data that accounts for constant transient changes, including feedback time delays and sensory uncertainties the CNS must overcome to maintain postural stability.
In the most severe cases of Parkinson’s disease, the researchers found that loss of intermittent control in the ankles significantly reduced stabilization, especially for patients with more rigid ankles. This is contrary to the traditional theory of control that favors stiffer ankles for postural stability.
“Switching between two unstable dynamics at an appropriate sequence of timings makes the overall dynamics stable,” Nomura said.
Future studies should consider joint flexibility as part of the treatment for postural stability.
Source: “Postural instability via a loss of intermittent control in elderly and patients with Parkinson’s disease: A model-based and data-driven approach,” by Yasuyuki Suzuki, Akihiro Nakamura, Matija Milosevic, Kunihiko Nomura, Takao Tanahashi, Takuyuki Endo, Saburo Sakoda, Pietro G. Morasso, and Taishin Nomura, Chaos (2020). The article can be accessed at https://doi.org/10.1063/5.0022319.
This paper is part of the Dynamical Disease: A Translational Perspective special topic, learn more here.
