Capturing the frame of reference of shoulder muscle forces

We have adjusted and validated models of the lines of action of six human shoulder muscles. Compared to other models of the shoulder mechanism (e.g., 9, 11, 17, 24) ours is greatly simplified in that the scapula and its many muscles are largely neglected, and the action of each of six broad muscles is summarized by a single line segment. The close correspondence between measured and predicted moment direction over a wide range of postures suggests that this latter simplification was reasonable. We found that assignment of two via points (one fixed to the origin and one fixed to the insertion) was adequate to represent the measured actions, and our sensitivity analysis highlights the importance of via point placement. Because our model predicts the action of each muscle over a very large range of arm postures, it should be useful for future investigations of the control of muscle forces.