Postural reflexes act to maintain balance and upright stance in response to perturbations¹. Impairment of these postural responses in disorders such as Parkinson's disease results in postural instability, and commonly leads to falls, reduced walking confidence and diminished quality of life^2,3,⁴. In clinical practice, postural reflexes are typically assessed with the pull test, where an examiner briskly pulls the patient backward at the shoulders and visually grades the response^5,6,7,8. Postural instability is usually scored using the Unified Parkinson's Disease Rating Scale (UPDRS) (0 – normal to 4 – severe), as published by the International Movement Disorder Society⁵. This method has been used extensively in the assessment of individuals with Parkinson's disease but suffers poor reliability and very limited scaling (score/4)^6,7,9. Pull test scores often do not correlate with important clinical endpoints such as falls and the integer-based rating lacks sensitivity to detect fine postural changes^10,11.

Laboratory-based objective measures offer precise information about the nature of balance response by quantifying kinetic (e.g., the center of pressure), kinematic (e.g., joint goniometry/limb displacement) and neurophysiological (e.g., muscle recruitment) endpoints¹². These methods may identify abnormalities before postural instability is clinically evident and track changes over time, including responses to treatment^13,14.

Tools for Quantifying Postural Instability

Conventional techniques of dynamic posturography commonly employ moving platforms. Resulting postural responses are quantified using a combination of posturography, electromyography (EMG), and accelerometry^12,15,16. However, the bottom-up responses of platform perturbations – which evoke a response like slipping on a wet floor, are fundamentally different from the top-down postural responses of the clinical pull test – as may occur when being bumped in a crowd. Emerging evidence suggests truncal perturbations yield different postural characteristics to those of moving platforms^17,18,19. Accordingly, others have attempted truncal perturbations in the laboratory using complex techniques including motors, pulleys, and pendulums^15,20,21,22. Methods of measurement are often expensive and inaccessible and comprise of video-based motion capture that requires dedicated space in specialized laboratories^20,21. Ideally, an objective method to characterize pull test responses should have excellent psychometric properties, be easy to administer, simple to operate, widely accessible, and portable. This is important to facilitate widespread adoption of the technique as an alternative assessment tool to assess postural responses within research and potentially, clinical settings.

The Instrumented Pull Test

The aim of this protocol is to offer researchers a technique for objective assessment of postural responses to the pull test. A semi-portable and widely available electromagnetic motion capture system underpins the technique. The perturbation involves manual pulls that do not require specialized mechanical systems. This method has sufficient sensitivity to detect small differences in postural reaction times and response amplitudes; therefore, it is suited to capturing potential abnormalities rated from normal up to grade 1 postural instability according to the UPDRS (postural instability with unassisted balance recovery)⁵. This method may also be utilized to explore the effects of therapy on postural instability. The protocol described here is derived from that in Tan et al.²³.