Errors associated with utilizing prescribed scapular kinematics to estimate unconstrained, natural upper extremity motion in musculoskeletal modeling

Musculoskeletal modeling is capable of estimating physiological parameters that cannot be directly measured, however, the validity of the results must be assessed. Several models utilize a scapular rhythm to prescribe kinematics, yet it is unknown how well they replicate natural scapular motion. This study evaluated kinematic errors associated with a model that employs a scapular rhythm using 2 shoulder movements: abduction and forward reach. Two versions of the model were tested: the original MoBL ARMS model that utilizes a scapular rhythm, and a modified MoBL ARMS model that permits unconstrained scapular motion. Model estimates were compared against scapulothoracic kinematics directly measured from motion capture. Three-dimensional scapulothoracic resultant angle errors associated with the rhythm model were greater than 10° for abduction (mean: 16.4°, max: 22.4°) and forward reach (mean: 11.1°, max: 16.5°). Errors generally increased with humerothoracic elevation with all subjects reporting greater than 10° differences at elevations greater than 45°. Errors associated with the unconstrained model were less than 10°. Consequently, use of the original MoBL ARMS model is cautioned for applications requiring precise scapulothoracic kinematics. These findings can help determine which research questions are suitable for investigation with these models and assist in contextualizing model results.