Bones and Joints of the Shoulder - the Shoulder Provides a Wide Range of Motion Through Various Plains Which Make It One of the More Complicated Joints in the Body

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- Bones and Joints of the Shoulder - The shoulder provides a wide range of motion through various plains which make it one of the more complicated joints in the body. The muscles surrounding the shoulder act as the joints primary source of stability, along with secondary support from ligaments and the joint capsule. Due to the nature of the shoulder and its instability it is predisposed to overuse conditions, especially in overhead activity. The joints of the shoulder include the glenohumeral (GH) joint , acromioclavicular (AC) joint, and sternoclavicular (SC) joint. The bones that make up these joint are the humerus, scapula, clavicle, and manubrium of the sternum. (1)

- Muscles of the Shoulder o Muscles that Act on the Scapula . The muscles that act on the scapula serve to provide the shoulder with two basic functions. to move the scapula’s glenoid fossa to allow the shoulder increased ROM to fixate the scapula on the thorax to provide the rotator cuff with a stationary base of Figure 3 origin. (1) . The muscles that act on the scapula are: . Latissimus Dorsi Action – depression of the shoulder girdle . Levator Scapulae Action – elevation, downward rotation, extension of the cervical spine, rotation of the cervical spine

. Rhomboid Major Action – Scapular retraction, scapular Figure 4 elevation, downward rotation of the scapula . Rhomboid Minor Action – scapular retraction, scapular elevation . Trapezius (upper 1/3) Action – elevation of the scapula, upward rotation of the scapula, rotation of cervical side to opposite direction, extension of cervical spine . Trapezius (middle 1/3) Action – retraction of the scapula, fixation of the thoracic spine

. Trapezius (lower 1/3) Action – depression of the scapula, retraction of the scapula, fixation of the thoracic spine . Serratus Anterior Figure 5 Action – upward rotation, protraction, depression (lower fibers), elevation (upper fibers), fixation of the scapula on the thorax . Pectoralis Major Action – depression of the shoulder girdle (clavcular fibers) . Pectoralis Minor Action – forward (anterior) tilting

Figure 6 Muscles that Act on the Humerus

. Deltoid (anterior third) Action – flexion, abduction, horizontal adduction, internal rotation . Deltoid (middle third) Action – abduction and flexion . Deltoid (posterior third) Action – extension, horizontal abduction, abduction, external rotation . Latissimus Dorsi Action – extension, internal rotation, adduction . Pectoralis Major

Action – adduction, horizontal adduction, flexion, internal rotation . Teres Major Action – extension, internal rotation, adduction . Biceps Brachii Action – flexion and abduction . Coracobrachialis Action – flexion and adduction . Triceps Brachii Action – extension (long head) and adduction Figure 7 o The Rotator Cuff . Supraspinatous Action – abduction, external rotation, humeral head stabilization . Infraspinatous Action – external rotation, horizontal abduction, humeral head stabilization . Teres Minor Action – external rotation and horizontal abduction . Subscapularis Action – internal rotation and humeral head stabilization

(Muscles and actions were obtained from Starkey and Ryan, 2002)

- THE LABRUM o The labrum is a ring of fibrocartilage that runs around the outside border of the glenoid cavity. It serves to deepen the cavity and increase the articular surface of the humeral head on the glenoid cavity. (2)

Figure 8