The middle fibres of the deltoid are responsible for the next 15-90 degrees.The first 0-15 degrees of abduction is produced by the supraspinatus.Abduction (upper limb away from midline in coronal plane):.Biceps brachii weakly assists in forward flexion. Flexion (upper limb forwards in sagittal plane) – pectoralis major, anterior deltoid and coracobrachialis.Extension (upper limb backwards in sagittal plane) – posterior deltoid, latissimus dorsi and teres major.The shoulder joint is an extremely mobile joint, with a wide range of movement possible: This resists superior displacement of the humeral head. Coracoacromial ligament – extends between the acromion and coracoid process of the scapula, forming an arch-like structure over the shoulder joint (coracoacromial arch).It holds the tendon of the long head of the biceps in the intertubercular groove. Transverse humeral ligament – extends between the two tubercles of the humerus.It supports the superior part of the joint capsule. Coracohumeral ligament – extends from the base of the coracoid process to the greater tubercle of the humerus.They act to stabilise the anterior aspect of the joint. Glenohumeral ligaments (superior, middle and inferior) – extend from the humerus to the glenoid fossa, reinforcing the joint capsule.Ligaments play an important role in stabilising the shoulder joint: The synovial membrane lines the inner surface of the joint capsule and produces synovial fluid to reduce friction between the articular surfaces. The joint capsule is lax – permitting greater mobility (particularly abduction). It extends from the anatomical neck of the humerus to the border or ‘rim’ of the glenoid fossa. The joint capsule is a fibrous sheath which encloses the structures of the joint. Fig 1 – The articulating surfaces of the shoulder joint.
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