"What if": the use of biomechanical models for understanding and treating upper extremity musculoskeletal disorders.

To aid understanding of the working of the upper extremity, several musculoskeletal models of the shoulder and arm have been developed. These models comprise the full shoulder girdle, which implies that the thoracohumeral link is formed by a scapular and clavicular segment. These models are based upon limited anatomical parameter sets and work on the assumption of a general control principle. Upper Extremity models have proven to be useful for different categories of applications, such as quantification of the load on musculoskeletal structures, or the evaluation of changes in the musculoskeletal structure on function and mechanical integrity ("what if" questions). Although these models are increasingly used, validation has long been a difficult issue. With the development of instrumented endoprostheses, a new method for model validation has come within reach. Up till now results have indicated that to obtain 'true' force values, models should be scaled, and should allow for cocontraction. Musculoskeletal models will be finding their way in education and in clinical decision making. On the longer run individualized models might become important for application to individual patients, although scaling will for some time remain a difficult issue.