Accuracy and Validity of 3D Markerless Motion Capture Compared to Marker-Based Systems for Lower-Limb Biomechanical Assessment: A Systematic Review.

Marker-based motion capture systems are considered the gold standard for biomechanical analysis of movements associated with anterior cruciate ligament (ACL) injury risk; however, their cost and technical requirements limit their use for large-scale athlete screening. Markerless motion capture has emerged as a potential alternative, using pose estimation algorithms or depth cameras to quantify movement without reflective markers. This systematic review evaluated the accuracy and validity of markerless motion capture systems for measuring lower-limb kinematics during jump-landing tasks commonly used in ACL injury screening. MEDLINE, Embase, and Web of Science were searched from 1990 to March 2025 for studies comparing markerless and marker-based systems in healthy participants. Extracted outcomes included Bland-Altman plots, root mean square error, mean absolute error, Pearson's correlation coefficient, coefficient of multiple correlation, and intraclass correlation coefficient. Across studies, markerless systems demonstrated moderate to high validity for several lower-limb kinematic measures, particularly in the sagittal plane, although validity varied across joints, movement phases, and task complexity. These findings suggest markerless motion capture shows potential for biomechanical assessment in ACL injury screening, but further validation is required before widespread implementation.