OpenCap Monocular: 3D Human Kinematics and Dynamics From a Single Smartphone

OpenCap Monocular enables accurate 3D human movement and musculoskeletal analysis using just a single smartphone video. Traditional motion capture systems require expensive labs, multiple cameras, reflective markers, and force plates—making them impractical for routine clinical use or large-scale studies. OpenCap Monocular removes these barriers by estimating 3D kinematics and kinetics from one static phone camera. The system combines monocular computer vision with physics-based optimization and musculoskeletal modeling to produce biomechanically realistic joint angles, ground reaction forces, and muscle dynamics. Starting from a single RGB video, OpenCap Monocular refines 3D pose estimates, enforces physically plausible motion (such as accurate foot–ground contact), and computes musculoskeletal forces without any wearable sensors or lab equipment. Validated against gold-standard motion capture and force plates, OpenCap Monocular achieves low kinematic error and accurately estimates clinically meaningful metrics such as knee extension moments during sit-to-stand and knee adduction moments during walking—key indicators for frailty and knee osteoarthritis. By deploying the pipeline through a smartphone app and secure cloud computing, OpenCap Monocular makes high-quality biomechanical analysis fast, free, and accessible, opening the door to remote studies, home-based assessments, and routine clinical evaluation of human movement at scale.