# Repeatability of scapular motion tracking in individuals with high body mass index

## TL;DR

This study shows that tracking scapular motion is repeatable in individuals with high BMI, though calibration methods affect accuracy.

## Contribution

The study evaluates the repeatability of scapular motion tracking in high BMI individuals using two calibration methods.

## Key findings

- Scapular motion tracking is feasible and repeatable for upward and internal rotation in high BMI individuals.
- Single calibration underestimates upward and internal rotation compared to double calibration, especially at higher elevations.
- Results support using acromial marker clusters for tracking in populations with potential soft tissue artifact.

## Abstract

Scapular motion tracking is essential for understanding shoulder kinematics, but its utility in individuals with high body mass index (BMI) requires further investigation. The purpose of this study was to define repeatability of skin-based scapular motion tracking in individuals with BMI ≥ 30 using two calibration procedures.

Nine participants (mean BMI (SD): 42.1 (7.9)) completed two sessions involving planar arm elevations and functional arm-focused tasks. Scapular motion was tracked with the acromial marker cluster, and scapular kinematics were calculated from single and double calibration procedures. Repeatability of discrete scapular angles at 15° increments of humeral elevation was evaluated using intraclass correlation coefficients (ICCs), and minimal detectable change (MDC). Differences in angle waveforms were also explored with statistical parametric mapping.

Findings suggest that scapular motion tracking is feasible and repeatable for upward rotation and internal rotation in individuals with high BMI (ICCs = 0.50-0.90; MDCs = 5 to 15°). The single calibration method underestimated upward and internal rotation as compared to the double calibration method, particularly at higher humeral elevations.

Although BMI was used as a proxy for body composition, this study supports scapular motion tracking with the acromial marker cluster in populations where soft tissue artifact may be more pronounced.