# Monocular 3D Sway Tracking for Assessing Postural Instability in Cerebral Hypoperfusion During Quiet Standing

## TL;DR

This paper introduces a low-cost monocular imaging system that accurately measures 3D postural sway during quiet standing, enabling assessment of balance control related to cerebral hypoperfusion, with potential applications in clinical settings.

## Contribution

The study presents a novel monocular system for 3D sway tracking that is accurate, inexpensive, and suitable for assessing postural instability in neurovascular conditions.

## Key findings

- High agreement with commercial motion capture system (error 0.00015 mm)
- Detected sway differences related to cerebral hypoperfusion
- System effectively monitors neuromuscular balance in simple environments

## Abstract

Postural instability is prevalent in aging and neurodegenerative disease, decreasing quality of life and independence. Quantitatively monitoring balance control is important for assessing treatment efficacy and rehabilitation progress. However, existing technologies for assessing postural sway are complex and expensive, limiting their widespread utility. Here, we propose a monocular imaging system capable of assessing sub-millimeter 3D sway dynamics during quiet standing. Two anatomical targets with known feature geometries were placed on the lumbar and shoulder. Upper and lower trunk 3D kinematic motion was automatically assessed from a set of 2D frames through geometric feature tracking and an inverse motion model. Sway was tracked in 3D and compared between control and hypoperfusion conditions in 14 healthy young adults. The proposed system demonstrated high agreement with a commercial motion capture system (error $1.5 \times 10^{-4}~\text{mm}$, [$-0.52$, $0.52$]). Between-condition differences in sway dynamics were observed in anterior-posterior sway during early and mid stance, and medial-lateral sway during mid stance commensurate with decreased cerebral perfusion, followed by recovered sway dynamics during late stance with cerebral perfusion recovery. This inexpensive single-camera system enables quantitative 3D sway monitoring for assessing neuromuscular balance control in weakly constrained environments.