Understanding Shoulder Anatomy and Muscle Functions
Anatomy of the Shoulder
Trapezius
| Muscle | Origin | Insertion | Action | Innervation |
|---|---|---|---|---|
| Levator Scapulae | Transverse processes of cervical vertebrae 1-4 | Vertebral border of scapula near superior angle | Elevates scapula | Cervical nerves (C3-C4) and dorsal scapular nerve (C5) |
| Trapezius | Occipital bone, ligamentum nuchae, and spinous processes of thoracic vertebrae | Acromion and scapular spine | Upper: elevates/upwardly rotates the scapula. Middle: retracts scapula. Lower: depresses/upwardly rotates scapula. When scapula is fixed, can extend the neck. | Spinal accessory nerve (Cranial Nerve XI) |
| Coracobrachialis | Coracoid process | Medial margin of shaft of humerus | Adduction and flexion at the shoulder | Musculocutaneous nerve (C5-7) |
| Deltoid | Clavicle, acromion, scapular spine | Deltoid tuberosity of the humerus | Abducts the shoulder; anterior portion: flexion and medial rotation; posterior part: extension and lateral rotation of the humerus | Axillary nerve (C5-6) |
| Latissimus Dorsi | Spinous processes of inferior thoracic and all lumbar vertebrae, ribs 8-12, and thoracolumbar fascia | Floor of intertubercular sulcus of the humerus | Extension, adduction, and medial rotation at the shoulder | Thoracodorsal nerve (C6-8) |
| Teres Major | Inferior angle of the scapula | Medial lip of intertubercular sulcus of humerus | Extension and medial rotation of the shoulder | Lower subscapular nerve (C5-6) |
| Rhomboid Major | Spinous processes of superior thoracic vertebrae | Vertebral border of scapula from spine to inferior angle | Adducts and performs downward rotation of the scapula | Dorsal scapular nerve (C5) |
| Rhomboid Minor | Spinous processes of superior thoracic vertebrae | Vertebral border of the scapula | Adducts and performs downward rotation of the scapula | Dorsal scapular nerve (C5) |
| Serratus Anterior | Anterior and superior margins of ribs 1-8/9 | Anteromedial surface of the vertebral border of the scapula | Protracts shoulder, upward rotation of the scapula | Long thoracic nerve (C5-7) |
| Subclavius | First rib | Clavicle, inferior border | Depresses and protracts shoulder | Nerve to subclavius (C5-6) |
| Infraspinatus | Infraspinous fossa of scapula | Greater tubercle of the humerus | Lateral rotation at the shoulder | Suprascapular nerve (C5-6) |
| Teres Minor | Lateral border of the scapula | Greater tubercle of humerus | Lateral rotation and adduction at shoulder | Axillary nerve (C5) |
| Supraspinatus | Supraspinous fossa of scapula | Greater tubercle of humerus | Abducts the shoulder | Suprascapular nerve (C5) |
| Subscapularis | Subscapular fossa of scapula | Lesser tubercle of humerus | Extension and medial rotation at shoulder | Subscapular nerve (C5-6) |
| Pectoralis Major | Cartilages of ribs 2-6, body of sternum, and inferior, medial portion of clavicle | Crest of greater tubercle and lateral lip of intertubercular sulcus of humerus | Adduction, flexion, and medial rotation at shoulder | Pectoral nerves (C5-T1) |
| Pectoralis Minor | Anterior surface and superior margins of ribs 3-5 and fascia covering associated external intercostal muscles | Coracoid process of scapula | Depresses and protracts shoulder, downwardly rotates scapula. Elevates ribs when scapula is stabilized. | Medial pectoral nerve (C8-T1) |
| Triceps Brachii | Lateral head: superior lateral margin of humerus; Long head: infraglenoid tubercle of scapula; Medial head: posterior surface of humerus, inferior to radial groove | Olecranon of ulna | Extends elbow, but long head extends and adducts the shoulder | Radial nerve (C6-8) |
| Biceps Brachii | Short head: coracoid process; Long head: supraglenoid tubercle | Radial tuberosity via flattened tendon to aponeuroses of deep fascia of forearm | Flexion of elbow and shoulder, weak supinator | Musculocutaneous nerve (C5-6) |
Shoulder Movements
| Movement | Degrees | Position | Axis | Fixed Arm | Moving Arm |
|---|---|---|---|---|---|
| Shoulder Flexion | 0-180 | Supine lying, arm at side | Over greater tuberosity of humerus | Parallel to tabletop | In midline of arm |
| Shoulder Extension | 0-60 | Prone or sitting, arm at side | Over greater tuberosity of humerus | Parallel to tabletop or vertical if sitting | In midline of arm |
| Shoulder Abduction | 0-180 | Supine, arm at side | Just lateral to tip of coracoid process | Parallel to midline of trunk | In midline of arm |
| External Rotation | 0-90 | Supine with arm 90° elbow flexed, forearm perpendicular to table and pronated | Over olecranon | Remains perpendicular to tabletop | In midline of forearm |
| Internal Rotation | 0-70 | As above | As above | As above | As above |
| Horizontal Flexion | 0-135 | ||||
| Horizontal Extension | 0-130 |
Capsular Pattern at the Shoulder: External rotation is the most limited, followed by abduction and then internal rotation.