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Validation of the Transformer-Based Monocular System (Capture4D): A Real-Time Kinematic Analysis in Coaching/Teaching
Yue Zhao1, Shuo Wang2, Zan Gao3
1Sports Coaching College, Beijing Sport University, Beijing 100084, China.
Sensors (Basel, Switzerland)
|March 14, 2026
Summary
Capture4D, a new monocular system, accurately captures tennis motions using a single camera. This cost-effective solution offers practical motion analysis for sports coaching and training.
Area of Science:
- Biomechanics
- Computer Vision
- Sports Science
Background:
- Traditional optical motion capture (OMC) systems are expensive and restrictive.
- Monocular video-based methods offer accessibility but face accuracy limitations, especially in dynamic sports.
- Accurate human motion capture is vital for performance analysis and biomechanical studies.
Purpose of the Study:
- To validate Capture4D, a novel Transformer-based monocular system, for capturing diverse tennis strokes.
- To assess the accuracy and practicality of Capture4D compared to gold-standard OMC.
- To demonstrate the system's utility in biomechanical analysis and sports coaching.
Main Methods:
- Developed a universal biomechanical analysis framework (K0-K5) for twelve fundamental stroke types.
- Conducted validation experiments using Capture4D (single RGB camera) and Qualisys OMC with 9 high-level tennis players.
- Evaluated accuracy using Normalized Mean Per Joint Position Error (NMPJPE), RMSE, and MAE for 3D joint coordinates and angles.
Main Results:
- Capture4D demonstrated effective motion capture for tennis serves, with average NMPJPE between 69.5 mm and 88.3 mm, acceptable for coaching.
- Joint angle trajectories from Capture4D were comparable to OMC, indicating high accuracy.
- Capture4D offered significant advantages: ~50% reduction in setup time and ~80% cost savings compared to OMC.
Conclusions:
- Capture4D is a valid and practical monocular motion capture solution for tennis coaching and broader sports applications.
- The system's accuracy is sufficient for many coaching and teaching use cases, despite being slightly less precise than OMC.
- Capture4D democratizes motion analysis with its convenience, cost-effectiveness, and applicability in diverse environments where OMC is not feasible.
