Changes in shoulder muscle function with humeral position: A graphical description Philippe Favre, MSc*, Hilaire A.C. Jacob, PhD, Christian Gerber, MD Laboratory for Orthopaedic Research, Department of Orthopaedics, Balgrist, University of Zurich, Zurich, Switzerland Summary 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  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 elec- tromyographic 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 illus- trate the use of this data, particularly to clarify clinical issues. Ó 2009 Journal of Shoulder and Elbow Surgery Board of Trustees. The study of muscle moment arms gives insight into the role of a muscle (abductor or adductor, anterior or posterior flexor, internal or external rotator). 2,3 Knowledge of the muscle moment arms is also one of several important factors to consider when a tendon transfer is envisaged. 3,4,19 The muscle’s line of action changes with humeral position, and so does the corresponding moment arm. The function of the muscle is thereby affected by the position of the humerus. 37 Several electromyographic (EMG) studies 1,5,29,36 have determined muscle activity in a given task with changes in position of the humerus, but without knowledge of the momentary length of muscle moment arms, the specific contribution of each muscle cannot be assessed. Studies that have considered the change of muscle moment arms with the position of the humerus were restricted to one rotation of the humerus, usually either scapular plane elevation or axial rotation, and only to a single directional component of the moment arm. 24,30,35 However, because each muscle can possibly give rise to rotation components about 3 orthogonal axes simultaneously (eg, abduct, flex, and internally rotate the humerus at the same time) and because many functional shoulder disorders also involve anterior/posterior flexion and internal/external rotation, it is important to describe all 3 corresponding moment arm components simultaneously. Because of the large range of motion of the gleno- humeral joint in everyday activities, a comprehensive understanding of the complex function of a shoulder muscle is called for. Stability of the glenohumeral joint in all possible positions of the humerus can only be achieved by an interplay of several glenohumeral muscles, equili- brating an external force or moment, while at the same time balancing each other’s redundant actions. The whole set of glenohumeral muscles should, therefore, be taken into consideration rather than a few selected ones. *Reprint requests: Philippe Favre, MSc, Forchstrasse 340, 8008 Zurich, Switzerland. E-mail address: pfavre@research.balgrist.ch (P. Favre). J Shoulder Elbow Surg (2009) 18, 114-121 www.elsevier.com/locate/ymse 1058-2746/2009/$36.00 - see front matter Ó 2009 Journal of Shoulder and Elbow Surgery Board of Trustees. doi:10.1016/j.jse.2008.06.010