Compensation for the effect of soft tissue artefact on humeral axial rotation angle.

Soft tissue artefact (STA) is caused by the relative displacement of markers or sensors mounted on the skin surface with respect to the underlying bones, and is a major source of error in the kinematic measurement of human movement. In particular, the humeral axial rotation (HAR) is affected by STA. The aim of this study was to propose a method for compensating for STA and to validate its effectiveness. In the proposed method, the HAR angle was calculated by a second-order regression using three independent variables converted from the Cardan angles of the shoulder joint. The calculated HAR angle (HAR-r) was compared with the angle calculated from the direction of the longitudinal axis of the forearm (HAR-f). Highly linear correlations were found between HAR-r and HAR-f when the elbow joint was flexed at 90°. The elbow flexion/extension motion had little influence on the HAR-r, whereas HAR-f became unstable when the elbow joint approached its full extension. Because HAR-r effectively compensates for the STA and is independent of the elbow flexion/extension, the regression method is suitable for the movement analysis of the upper limbs.