Assessing postural instability during cerebral hypoperfusion using sub-millimeter monocular 3D sway tracking

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. By physically embedding anatomical targets with known \textit{a priori} geometric models, 3D central and upper body kinematic motion was automatically assessed through geometric feature tracking and 3D kinematic motion inverse estimation from a set of 2D frames. Sway was tracked in 3D and compared between control and hypoperfusion conditions. The proposed system demonstrated high agreement with a commercial motion capture system (error $4.4 \times 10^{-16} \pm 0.30$~mm, $r^2=0.9773$). Significant differences in sway dynamics were observed in early stance central anterior-posterior sway (control: $147.1 \pm 7.43$~mm, hypoperfusion: $177.8 \pm 15.3$~mm; $p=0.039$) and mid stance upper body coronal sway (control: $106.3 \pm 5.80$~mm, hypoperfusion: $128.1 \pm 18.4$~mm; $p=0.040$) commensurate with cerebral blood flow (CBF) perfusion deficit, followed by recovered sway dynamics during late stance governed by CBF recovery. This inexpensive single-camera system enables quantitative 3D sway monitoring for assessing neuromuscular balance control in weakly constrained environments.