Detecting Postural Instability in Parkinson’s Disease from IMU-based Objective Measures

Parkinson’s Disease (PD) is a progressive neurological disorder that affects both motor and non-motor functions. A critical aspect of PD is postural instability, which is associated with a higher risk of falls and a significant decline in mobility. The onset of postural instability is frequently identified through subjective clinical assessments such as the pull test, which aligns with the transition from Stage II to Stage III on the Hoehn and Yahr (H&Y) scale, a measure of general motor impairment. This study aims to pinpoint this transition by providing an objective quantification of the pull test, to prevent falls and manage severe motor symptoms via early detection. Sixty objective metrics were characterised from acceleration data using inertial measurement units (IMU) during pull tests conducted with 30 PD patients to differentiate between H&Y Stages II and III. Significant differences in feature distributions were identified, with two features achieving a ROC AUC of 0.95: chest to lumbar maximum acceleration ratio, and power ratio of 8-12 Hz versus total acceleration power. This paper demonstrates the ability of certain IMU-derived features to distinguish between H&Y Stages II and III, presenting more straightforward and objective measurements compared to conventional pull test metrics. Furthermore, the study explores the inconsistencies in the administration of the pull test by different examiners and shows that even with these inconsistencies, objective measures can still effectively differentiate between stages. To the best of our knowledge, this work represents the first extensive attempt to objectively quantify the pull test for this type of classification.