Concurrent validation of OpenCap for identifying ACL re-injury risk factors during a drop jump test in a healthy cohort

TL;DR

This study tested a new low-cost motion analysis system called OpenCap for identifying ACL re-injury risk factors during drop jumps.

Contribution

The study evaluates the concurrent validity of a modified OpenCap system for kinematic and kinetic analysis compared to traditional marker-based systems.

Findings

OpenCap showed strong correlations for knee kinematics but had high RMSE in frontal-plane measurements.

Hip and knee moments showed strong correlations but wide agreement limits for knee moment asymmetry.

SPM identified significant differences during ground contact phases, suggesting limitations in OpenCap's accuracy.

Abstract

3D motion analysis (3DMA) can help identify patients at increased risk of ACL re-injury, but traditional marker-based systems have limited clinical accessibility. OpenCap, a novel, low-cost, markerless system, aims to enhance accessibility to 3DMA. This study evaluated the concurrent validity of a modified OpenCap version using a 2-DOF knee model for kinematics, while kinetics and ground reaction forces were derived using the native 1-DOF model, compared to a marker-based system. Twenty-four healthy participants performed 240 drop jumps, with data simultaneously captured by both systems. Root mean square error (RMSE), mean absolute error (MAE), maximum error, Pearson correlation, Bland-Altman plots, and statistical parametric mapping (SPM) were used to analyze inter-system differences. RMSE exceeded 6° for frontal-plane knee kinematics with strong waveform correlations (r > 0.90). Transverse-plane hip moments showed normalized MAE < 1% with weak to strong negative correlations. Sagittal-plane knee moments had normalized MAE of 5.6% and strong correlations (r > 0.90). Vertical GRFs showed normalized MAE > 6% and strong correlations (r > 0.90). SPM identified significant differences across most ground contact phases, and Bland-Altman analyses showed wide agreement limits for knee moment asymmetry at initial contact. OpenCap currently cannot be recommended for ACL re-injury risk assessment but demonstrated potential for increasing 3DMA accessibility.

The online version contains supplementary material available at 10.1038/s41598-026-44758-0.

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