Decomposing Marker-to-Keypoint Discrepancy in Stereo Markerless Motion Capture: A Comparison of Three Pose-Estimation Architectures Against OptiTrack

When a markerless system is validated against marker-based motion capture, the result is usually a single number: the average distance between the two systems at a given joint. That number is read as the accuracy of the pose estimator, but it also contains everything else in the measurement chain—where the markers were placed, how well the systems were synchronised, and how good the calibration was. We asked whether differences and rankings between pose estimators remain interpretable once this uncertainty is accounted for. In a single-participant methodological case study, three checkpoints from MMPose, an open-source pose-estimation toolbox (ViTPose, RTMPose-m, HRNet-w32), were compared against an OptiTrack reference using a calibrated stereo pair of industrial cameras in recordings of the elbow and wrist. The distance between the marker-pair midpoint and the triangulated keypoint had a median of 23.9 to 47.1 mm. Calibration, reconstruction scale, temporal offset and median sampling error gave the group medians a combined standard uncertainty of 4.5–14.9 mm (a lower bound, excluding marker placement). No difference between the three checkpoints was resolvable above the uncertainty of the measurement chain, and the leading checkpoint changed when the calibration was resampled. As supporting mechanisms, marker placement produced an offset comparable to the measured distance, whereas the temporal offset barely changed it. These numbers are specific to this participant and protocol; the transferable contribution is the validation and uncertainty-analysis framework.