Disentangling stability and flexibility degrees in Parkinson’s disease using a computational postural control model

Abstract

Background Impaired postural control in Parkinson’s disease (PD) seriously compromises life quality. Although balance training improves mobility and postural stability, lack of quantitative studies on the neurophysiological mechanisms of balance training in PD impedes the development of patient-specific therapies. We evaluated the effects of a balance-training program using functional balance and mobility tests, posturography, and a postural control model. Methods Center-of-pressure (COP) data of 40 PD patients before and after a 12-session balance-training program, and 20 healthy control subjects were recorded in four conditions with two tasks on a rigid surface (R-tasks) and two on foam. A postural control model was fitted to describe the posturography data. The model comprises a neuromuscular controller, a time delay, and a gain scaling the internal disturbance torque. Results Patients’ axial rigidity before training resulted in slower COP velocity in R-tasks; which was reflected as lower internal torque gain. Furthermore, patients exhibited poor stability on foam, remarked by abnormal higher sway amplitude. Lower control parameters as well as higher time delay were responsible for patients’ abnormal high sway amplitude. Balance training improved all clinical scores on functional balance and mobility. Consistently, improved ‘flexibility’ appeared as enhanced sway velocity (increased internal torque gain). Balance training also helped patients to develop the ‘stability degree’ (increase control parameters), and to respond more quickly in unstable condition of stance on foam. Conclusions Projection of the common posturography measures on a postural control model provided a quantitative framework for unraveling the neurophysiological factors and different recovery mechanisms in impaired postural control in PD. Electronic supplementary material The online version of this article (10.1186/s12984-019-0574-0) contains supplementary material, which is available to authorized users.

Postural control learning dynamics in Parkinson’s disease: early ‎improvement with plateau in stability, and continuous progression in ‎flexibility and mobility

“…This study investigated the motor learning dynamics of the postural control in people with PD, using the unidimensional measures of stability and flexibility degree that we proposed in a previous study [30]. The found improvement at mid-training, and from mid-to post-training for clinical measures of functional mobility implies a relatively constant improvement in mobility.…”

“…RMS provides a measure of sway amplitude, which is normally larger in PD patients [35]. MV also reflects the degree to which patients regulates the spontaneous sway in a flexible manner [30,36]. Higher MV reflects higher flexibility.…”

“…Greater f95 (smaller ∆t c ) indicates higher stiffness. However, these measures are the overall outcome of interconnected underlying neurophysiological mechanisms, and therefore were projected onto a postural control model to separate stability and flexibility degree [30].…”

“…Based on the COP-based sway measures, the parameters of a patient-specific postural control model of PD were estimated through an optimization algorithm (i.e. K P , K D , K I , K n , τ d ) [30]. From the model point of view, K P and K n exclusively quantifies the stability and flexibility degree, respectively, that contribute to the spontaneous sway.…”

“…Furthermore, the framework showed to be sufficiently sensitive to balance-training programs [29,30], and as such paved the path for future studies of postural control learning dynamics, using unidimensional and meaningful assessment measures.…”