Measuring sagittal plane hip and knee joint kinematics during swing phase: reliability of a webcam-based approach

Purpose: Joint angle analysis during gait is crucial for identifying pathological conditions and estimating joint loading, thereby supporting clinical decision-making for injury prevention. Although various methods are available for analyzing joint angles, webcam-based motion capture systems (MoCap) are gaining attention due to their affordability and user-friendliness. This study aimed to evaluate and compare the inter-rater and intra-trial reliability of a webcam-based MoCap with that of a conventional inertial measurement unit (IMU)-based system. Methods: Gait analysis was conducted on 15 participants (6 males, 9 females; mean age: 28.1 ± 5.26 years). While participants walked a 3-meter distance, hip and knee joint angles in the sagittal plane were simultaneously recorded using both inertial measurement unit (IMU) sensors and a webcam-based MoCap. Inter-rater and intra-trial reliability were assessed using intraclass correlation coefficients (ICCs), and agreement between the two systems was evaluated using Bland-Altman analysis. Results: For intra-trial reliability, most IMU-based systems demonstrated excellent reliability (ICC > 0.8). Although slightly lower, the webcam-based MoCap also achieved substantial to almost perfect reliability (ICC = 0.652 - 0.838). Inter-rater reliability between the IMU and webcam-based MoCap generally showed moderate to substantial agreement (ICC = 0.466 - 0.696). Conclusions: These findings suggest that the webcam-based MoCap may serve as a viable alternative in settings where IMU systems are unavailable or impractical. Future studies should aim to refine webcam-based tracking algorithms to improve event detection, assess reliability across diverse populations and movement tasks, and further validate such systems against gold-standard marker-based 3D optical MoCap.