Mechanism of Abduction at the Shoulder Joint

Overview

Full overhead abduction (0°–180°) involves coordinated movement at multiple joints of the shoulder complex:

• Glenohumeral joint
• Sternoclavicular joint
• Acromioclavicular joint
• Scapulothoracic articulation

The glenohumeral joint alone cannot achieve 180° of abduction.

Phases of Abduction

Phase 1: 0°–90° (Glenohumeral joint)

• Occurs entirely at the glenohumeral joint
• Initiated and sustained by supraspinatus and deltoid (both involved throughout 0°–90°)
• Supraspinatus and deltoid together constitute a “force couple” — supraspinatus pulls the humeral head medially into the glenoid while deltoid elevates the arm

Phase 2: 90°–120° (Lateral rotation of humerus required)

• At 90° abduction, the greater tubercle of the humerus impinges on the undersurface of the acromion
• Abduction beyond 90° requires lateral rotation of the humerus to carry the greater tubercle posteriorly, clearing it from under the acromion
• Infraspinatus and teres minor produce this lateral rotation

Phase 3: 120°–180° (Scapular rotation required)

• Abduction above 120° requires forward/upward rotation of the scapula on the chest wall
• Produced by synergistic action of:
  • Serratus anterior — rotates inferior angle of scapula laterally
  • Upper and lower fibres of trapezius — rotates spine of scapula downward, glenoid upward
• These two muscles act as a “force couple” turning the glenoid cavity upwards

Scapulohumeral Rhythm

For every 15° of total shoulder elevation:

• 10° occurs at the glenohumeral joint
• 5° occurs at the scapulothoracic articulation

This gives a 2:1 ratio of glenohumeral to scapulothoracic movement.

Overall contributions to 180° overhead movement:

• Glenohumeral joint: 100°–120°
• Scapulothoracic (scapular rotation): 50°–60°

Disruption of this rhythm (e.g., rotator cuff pathology, frozen shoulder) leads to altered scapulohumeral rhythm — excessive scapular shrugging during attempted overhead movements.

Sternoclavicular and Acromioclavicular Contribution

• During early elevation: sternoclavicular joint moves maximally
• During terminal elevation: acromioclavicular joint moves maximally
• Clavicular rotation around its long axis is used in overhead movements

Muscles Summary for Full Abduction

Phase	Joint	Muscles
0°–90°	Glenohumeral	Supraspinatus + Deltoid
90°–120°	Glenohumeral	+ Lateral rotators (infraspinatus, teres minor)
120°–180°	Scapulothoracic	Serratus anterior + Trapezius (upper + lower fibres)