The Validity and Usability of Markerless Motion Capture and Inertial Measurement Units for Quantifying Dynamic Movements

ABSTRACT Purpose Motion capture technology is quickly evolving, providing researchers, clinicians, and coaches with more access to biomechanics data. Markerless motion capture and inertial measurement units (IMUs) are continually developing biomechanics tools that need validation for dynamic movements before widespread use in applied settings. This study evaluated the validity of a markerless motion capture, IMU, and red, green, blue, and depth (RGBD) camera system as compared with marker-based motion capture during countermovement jumps, overhead squats, lunges, and runs with cuts. Methods Thirty adults were recruited for this study (sex: 18 females, 12 males; age: 25.4 ± 8.6 yrs; height: 1.71 ± 0.08 m; weight: 71.6 ± 11.5 kg). Data were collected simultaneously with four motion capture technologies (i.e., Vicon, marker-based; Theia/Optitrack, markerless; APDM Opals, IMUs; and Vald HumanTrak, RGBD camera). System validity for lower and upper body joint angles was evaluated using bias, root mean squared error (RMSE), precision, maximum absolute error, and intraclass correlation coefficients. System usability was descriptively analyzed. Results Overall, markerless motion capture had the highest validity (sagittal plane RMSE: 3.20°–15.66°; frontal plane RMSE: 2.12°–9.14°; transverse plane RMSE: 3.160°–56.61°), followed by the IMU system (sagittal plane RMSE: 8.11°–28.37°; frontal plane RMSE: 3.26°–16.98°; transverse plane RMSE: 5.08°–116.75°), and lastly the RGBD system (sagittal plane bias: 0.55°–129.48°; frontal plane bias: 1.35°–52.06°). Conclusions Markerless motion capture and IMUs have moderate validity for joint kinematics, whereas the RGBD system did not have adequate validity. Markerless systems have lower data processing time, require moderate technical expertise, but have high data storage size. IMUs are easier to use, can collect data in any location, but require participant set-up. Overall, individuals using motion capture should consider the specific movements, testing locations, and technical expertise available before selecting a system.