Non-linear Centre of Pressure Analysis During Quiet Stance: Application to Mild Traumatic Brain Injury

Abstract

A quiet stance framework and a control system perspective were used to explore healthy balance and balance after mild traumatic brain injury. Linear and non-linear centre of pressure analyses were applied. The foundation was laid by reviewing literature to understand how balance is achieved, how it is represented as a control system, what factors are known to affect balance, and the cornerstone—how to choose appropriate measures to quantify balance. To understand how mild traumatic brain injury affects the brain, a scoping review of the evolution of symptoms and effects was used to form a conceptual description. Findings described phases of functional effects that resulted from neurometabolic cascade; consequently, balance and dual-task functional effects were determined to stem from widespread not focal changes in the brain. Subsequent studies were tailored to address gaps in knowledge. Linear and non-linear centre of pressure measures were first investigated in healthy young adults to determine what supplemental information could be provided by non-linear measures describing local stability and scaling. It was found that linear and non-linear measures were complementary in assessing balance system input-output, control, and integration. Furthermore, normative non-linear data were established for single leg and tandem stance. Subsequently, these measures were investigated in young adults and adolescents with recent mild traumatic brain injury based on the hypothesis that altered mechanisms affecting balance would be reflected by changes in these measures. In young adults, increased complexity of short-term scaling indicated subtle changes to balance control after injury. In adolescents, linear and non-linear measures also demonstrated changes to output, control, and temporal relations of balance. Altered balance was also demonstrated while concurrently performing a Stroop task. On the whole, changes to multiple aspects of balance supported the concept of widespread effects resulting from mild traumatic brain injury. Balance control in quiet stance was further explored using three-dimensional state space reconstruction of centre of pressure. Visual representations demonstrated that dynamic structure within centre of pressure reflected control characteristics. These control characteristics were still present after mild traumatic brain injury.