Quantifying the Shoulder Rhythm and Comparing Non-Invasive Methods of Scapular Tracking for Overhead and Axially Rotated Humeral Postures

The present research quantified the shoulder rhythm for arm postures that represent the right-handed reachable workspace and compared 3 methods of scapular tracking: acromion marker cluster (AMC), stylus and scapular locator. The shoulder rhythm models can be incorporated into existing and future shoulder biomechanical models to determine shoulder geometry when simulating postures experienced in workplaces and thus have ergonomic implications for correctly identifying risk factors. The results of this research also provide guidance for future studies involving scapular tracking. Fourteen male and 14 female participants performed static arm postures spread over 5 elevation angles: 0, 45, 90, 135, 180 degrees, three elevation planes: 0, 45, 90 degrees to the frontal plane and, three axial rotations: maximum internal, neutral, and maximum external rotation. Kinematic data was recorded using a Vicon MX20+ motion-tracking system. Bone rotations were calculated using Euler angles and continuous prediction models were generated to estimate scapular and clavicular orientations based primarily on thoracohumeral relative orientations. Methods of scapular tracking were compared using repeated measures analysis of variance. Participant characteristics did not influence any of the scapular or the clavicular angles (p>.05). Axial rotation did not influence scapular retraction/protraction and elevation plane did not influence clavicular elevation (p>.05). Elevation angle was the largest contributor to lateral rotation and posterior tilt of the scapula and all clavicular angles. Plane of elevation was the largest contributor to scapular protraction. Using the stylus as the gold standard, the locator and the AMC underestimated lateral rotation, with a maximum difference of 11 degrees and 9 degrees between the locator and the stylus and AMC and the stylus measurements, respectively. The AMC and the locator overestimated posterior tilt at overhead postures and underestimated it at low elevation angles. The maximum difference between the AMC- and the locator- and the stylus-measured tilt was 10 degrees. The scapular locator consistently overestimated protraction by approximately 5 degrees. The AMC underestimated protraction in the frontal plane at low elevation angle but overestimated it at all other postures and the overestimation increased with plane of elevation, internal rotation and elevation angle. Overall, it is recommended to use AMC rather than the scapular locator to measure scapular position

Three-dimensional comparison of static and dynamic scapular motion tracking techniques

The final publication is available at Elsevier via http://dx.doi.org/10.1016/j.jelekin.2013.09.011. © 2014. This manuscript version is made available under the CC-BY-NC-ND 4.0 license http://creativecommons.org/licenses/by-nc-nd/4.0/The shoulder is complex and comprised of many moving parts. Accurately measuring shoulder rhythm is difficult. To classify shoulder rhythm and identify pathological movement, static measures have been the preferred method. However, dynamic measures are also used and can be less burdensome to obtain. The purpose of this paper was to determine how closely dynamic measures represent static measures using the same acromion marker cluster scapular tracking technique. Five shoulder angles were assessed for 24 participants using dynamic and static tracking techniques during humeral elevation in three planes (frontal, scapular, sagittal). ANOVAs were used to identify where significant differences existed for the factors of plane, elevation angle, and tracking technique (static, dynamic raising, dynamic lowering). All factors were significantly different for all shoulder angles (p<0.001), except for elevation plane in scapulothoracic protraction/retraction (p=0.955). Tracking techniques were influential (p<0.001), but the grouped mean differences fell below a clinically relevant 5° benchmark. There was large variation in mean differences of the techniques across individuals. While population averages are similar, individual static and dynamic shoulder assessments may be different. Caution should be taken when dynamic shoulder assessments are performed on individuals, as they may not reflect those obtained in static scapular motion tracking.Natural Sciences and Engineering Research Council || Canada Foundation for Innovatio