Inter-session repeatability of Theia3D markerless motion capture gait kinematics

The reliability of kinematic gait data obtained using marker-based motion capture has been questioned due to its sensitivity to marker placement and soft tissue artefact. Markerless motion capture systems are free from these errors associated with marker-based systems and may be able to measure gait kinematics more reliably. This work aimed to determine the reliability of kinematic gait data obtained using Theia3D markerless motion capture, based on an established method. Seven healthy adult participants performed three sessions of ten over-ground walking trials in their own clothing while eight synchronized and calibrated cameras recorded video. Theia3D software was used to process the videos and obtain three-dimensional pose estimates of a multibody model scaled to each of the participants. The established method removes inter-subject variability and estimates average inter-session and inter-trial errors, and the ratio between them. Kinematic patterns across all sessions within individuals were consistent, particularly in the sagittal plane. Inter-trial errors were similar to those previously reported, and the average across all joints was 2.61°. Inter-session errors were similar or smaller than the inter-operator errors reported in previous reliability studies, and the average across all joints was 2.75°. Error ratios were lower than those reported elsewhere, with the largest being 1.1 and the average being 1.06, indicating that multiple sessions increased the total variability of subjects′ joint angle patterns by 6% over their natural inter-trial variability. These results indicate that gait kinematics measured using Theia3D markerless motion capture are more reliable than those measured using marker-based methods and are largely unaffected by multi-session methodologies.