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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