Reliability and Validity of an Acromion Marker Cluster for Recording Scapula Posture at End Range Clavicle Protraction, Retraction, Elevation, and Depression

Acromion marker cluster (AMC) methods have been shown to accurately track scapula motion during humeral elevation below 90°, however, their accuracy has not been assessed in shoulder girdle motion such as clavicle protraction, retraction, elevation, and depression independent of humeral movement. The aim of this study was to examine the reliability and validity of the AMC method to record scapula orientation at end range clavicle protraction, retraction, elevation, and depression. The right scapulae of 22 female and 20 male asymptomatic volunteers were assessed with an AMC and scapula locator (SL) method during end range clavicle protraction, retraction, elevation, and depression (without humeral elevation) using an 8-camera 3D movement registration system. Measurements recorded from the AMC and SL measures showed fair to excellent agreement (ICC 0.4–0.92). While the AMC method overestimated and underestimated scapular motion in some planes compared to the SL, root mean square error between methods were low for scapular internal/external rotation (2.3–3.7°), upward/downward rotation (4.5–6.6°), and anterior/posterior tilt (3.2–5.1°), across all conditions. The AMC method was shown to be a reliable and valid measurement of scapula orientation at end range clavicle movements independent of humeral movement.