The measurement of in vivo joint angles during a squat using a single camera markerless motion capture system as compared to a marker based system

• A single Kinect holds great potential as a clinical tool to assess 3D motion.
• We compared joint angles between a Kinect and a marker-based motion capture system.
• The Kinect reliability and accuracy agreed well with the marker-based system.

Markerless motion capture may have the potential to make motion capture technology widely clinically practical. However, the ability of a single markerless camera system to quantify clinically relevant, lower extremity joint angles has not been studied in vivo. Therefore, the goal of this study was to compare in vivo joint angles calculated using a marker-based motion capture system and a Microsoft Kinect during a squat. Fifteen individuals participated in the study: 8 male, 7 female, height 1.702 ± 0.089 m, mass 67.9 ± 10.4 kg, age 24 ± 4 years, BMI 23.4 ± 2.2 kg/m2. Marker trajectories and Kinect depth map data of the leg were collected while each subject performed a slow squat motion. Custom code was used to export virtual marker trajectories for the Kinect data. Each set of marker trajectories was utilized to calculate Cardan knee and hip angles. The patterns of motion were similar between systems with average absolute differences of <5 deg. Peak joint angles showed high between-trial reliability with ICC > 0.9 for both systems. The peak angles calculated by the marker-based and Kinect systems were largely correlated (r > 0.55). These results suggest the data from the Kinect can be post processed in way that it may be a feasible markerless motion capture system that can be used in the clinic.