Scapula abduction

Level of inferior angles of scapula, abduction-adduction... 

A combination of glenohumeral flexion and shoulder abduction is the primary body-control motion used to affect terminal device opening and closing or elbow flexion and extension. Typically, a total excursion of 10 cm (4 in) and upward of 222 N (50 Ibf) of force are possible using these motions. Elbow lock control is affected by a complex shoulder motion which involves downward rotation of the scapula combined with simultaneous abduction and slight extension of the shoulder... 

The scapulothoracic joint movements are related to scapular motions. The motions are medial and lateral movements of the scapula on the thorax (abduction-adduction), elevation of the scapula, and upward and downward rotation of the scapula (tilt), all relative to the glenoid fossa. 

The humerus impinges on the acromial arch at 90 degrees. To prevent impingement and permit abduction to 180 degrees, the scapula must rotate. 

The fibers of the deltoid, attached from the scapula to the humerus, contract to abduct the arm to 90 degrees, at which point they are maximally contracted. As the fossa rotates upward, it maintains the deltoid in position for maximal contraction, allowing the humerus to continue to 180 degrees. 

The scapulohumeral rhythm is a free-flowing and synchronous movement of the scapula and humerus. During abduction, the scapula rotates as the humerus elevates. For every 15 degrees of abduction, humeral elevation accounts for 10 degrees and scapular rotation accounts for 5 degrees. Dysfunction of humeral elevation or of scapular rotation can disturb this rhythm and interfere with shoulder function. Dysfunction of clavicular motion can also interfere with this rhythm. 

The clavicle moves during most shoulder activity. Dysfunctions in clavicular motion can interfere with normal shoulder movement. The combined axial rotation of the sternoclavicular joint (30 degrees) and the acromioclavicular joint (30 degrees) allows the normal 60 degrees of rotation of the scapula on full abduction of the shoulder. 

The scapulothoracic plane separates the body of the scapula and the subscapularis muscle from the thoracic surface, consisting of the superficial aspect of the serratus anterior muscle which overlies the ribs. This gliding plane allows the scapula and the glenoid cavity to tilt anteriorly and posteriorly around the rib cage during shoulder movements. In addition, the scapulothoracic articulation has an important role in shoulder abduction. 

In addition to the subacromial gliding plane, the scapulothoracic plane facilitates movement of the scapula relative to the chest wall and rotation of the scapula during abduction and adduction of the arm. 

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