Neurobiomechanics Characterization of Postural Control in Chronic Stroke - A Linear and Non-linear Approach

Abstract

Postural control reflects the dynamic balance between stability and adaptability, which is often compromised following a stroke. The Human Movement System theory highlights the need for advanced tools to capture the complexity of motor systems. However, non-linear analyses and culturally adapted instruments remain underutilised in clinical contexts. Main Objectives: To investigate non-linear methods for analysing motor variability in individuals' post-stroke, adapt and validate a culturally relevant clinical assessment tool, and characterise the complexity and adaptability of the feedforward phase of gait initiation using linear and non-linear approaches. Methods: Three studies were conducted. The first study consisted of a scoping review analysing non-linear measures applied to kinematic, kinetic, and electromyographic data in post-stroke populations across different motor tasks. The second, a cross-sectional study, involved the cross-cultural adaptation and validation of the Mini-BESTest for European Portuguese by evaluating its psychometric properties in 100 individuals with sensorimotor impairments. The third cross-sectional study analysed the feedforward phase of gait initiation in 17 individuals' post-stroke and 16 healthy controls, employing motion capture systems and force platforms to assess the dynamics of the CoP and joint angle variability using linear and non-linear measures. Results: The complexity of movement patterns was analysed in 14 studies, with entropy emerging as the most used non-linear measure across tasks such as reaching, arm tracking, elbow flexion, wrist and knee extension, and gait (Study I). The Mini-BESTest demonstrated excellent psychometric properties, confirming its suitability for clinical use in Portuguese-speaking contexts (Study II). Study III identified, post-stroke, greater CoP displacement, lower velocity, and reduced complexity in CoP dynamics, alongside increased displacement and amplitude of body segments (head, trunk, pelvis, and limbs) in all planes. Exacerbated rotational patterns in the transverse plane were also observed, reflecting reduced complexity in non-linear parameters and greater variation in linear measures. Conclusions: Postural control dysfunctions after stroke highlight the importance of non-linear approaches, which provide insights into the adaptability and complexity of motor control. The validated Mini-BESTest has proven to be an effective clinical assessment tool. These advancements underpin the development of more innovative and effective rehabilitation strategies, with significant potential to enhance stability and adaptability in post-stroke individuals.