Changes in shoulder muscle function with humeral position: A graphical description

A graphical description of the change in the role played by each of the scapulohumeral muscles with respect to spatial joint position is presented. Moment arms were collected from a biomechanical model using the tendon travel method. Data cover elevation and flexion in a space between the frontal plane and a plane of elevation 60 degrees anterior to this. Segments of a given muscle were seen possibly to exhibit antagonistic moment components in relation to others, emphasizing the importance of muscle segmentation in biomechanical models. Graphical description of muscle function in conjunction with electromyographic studies enables a more complete assessment of active muscle function in relation to arm motion and position. In cases of attenuated muscular function, this also offers a means of detecting which muscle is involved and which other muscles possess compensating potential. Two examples illustrate the use of this data, particularly to clarify clinical issues.