The interactions of the nine muscles that cross the glenohumeral joint are complex and each muscle contributes to more than one movement. Additionally, movement at the glenohumeral joint can take place in all directions where associated muscles can act as synergists in one type of movement and then antagonists in another type of movement. Therefore six movements of the joint and humerus will be considered; flexion, extension, abduction, adduction, medial rotation and lateral rotation.