How many muscles are in the shoulder joint?

Shoulder Joint Muscles

The shoulder joint is stabilized and moved by multiple muscle groups that work in coordinated fashion, with the primary dynamic stabilizers being the four rotator cuff muscles (supraspinatus, infraspinatus, subscapularis, and teres minor), the three deltoid divisions (anterior, middle, and posterior), and numerous scapular stabilizers including the serratus anterior, trapezius, rhomboids, and levator scapulae 1, 2, 3.

Primary Muscle Groups

Rotator Cuff Muscles (4 muscles)

• Supraspinatus: Initiates and assists shoulder abduction, particularly effective at lower abduction angles 2
• Infraspinatus: Primary external rotator, generates forces two to three times greater than supraspinatus during scapular plane abduction 2
• Subscapularis: Primary internal rotator, contributes approximately 30% of abduction torque and acts as a stabilizer during external rotation 2, 4
• Teres minor: Assists with external rotation and posterior stability 1

Deltoid Muscles (3 divisions)

• Anterior deltoid: Generates approximately 323 N of force during abduction, though has limited abduction effectiveness at low angles due to small moment arm 2
• Middle deltoid: Most powerful abductor contributing 35-65% of abduction torque, generating approximately 434 N of force 2
• Posterior deltoid: Assists with extension and external rotation 2, 4

Scapular Stabilizers

• Serratus anterior: Critical for scapular upward rotation (45-55°), posterior tilt (20-40°), and external rotation (15-35°) during humeral elevation; prevents scapular winging 2
• Trapezius (upper, middle, lower fibers): Works synergistically with serratus anterior for scapular control 1
• Rhomboids: Scapular retraction and stabilization 1
• Levator scapulae: Scapular elevation 1

Additional Shoulder Muscles

• Pectoralis major: Internal rotation and adduction 4
• Latissimus dorsi: Adduction, extension, and internal rotation; acts as stabilizer during flexion 4
• Teres major: Adduction and internal rotation 3
• Biceps brachii: Anterior shoulder stability and elbow flexion 3
• Coracobrachialis: Shoulder flexion and adduction 1

Functional Coordination

The shoulder requires simultaneous coordinated activation of agonist and antagonist muscles for both movement and stability 3, 4. Cross-correlation analysis demonstrates highly significant positive correlations (p < 0.001) between deltoid and rotator cuff groups, deltoid and adductor groups, and adductor and rotator cuff groups during functional tasks 3.

Force Distribution During Abduction

• Middle deltoid: 434 N 2
• Anterior deltoid: 323 N 2
• Subscapularis: 283 N 2
• Infraspinatus: 205 N 2
• Supraspinatus: 117 N 2

These forces serve not only to produce movement but also to stabilize the joint and neutralize antagonistic effects 2. The rotator cuff neutralizes superior-directed forces from the deltoid at lower abduction angles, while the deltoid becomes more effective at higher angles 2.

Clinical Relevance

Scapular dyskinesis significantly contributes to rotator cuff injury, as poor scapular coordination disrupts normal shoulder mechanics and increases injury risk 1, 2. Scapular retraction increases subacromial space width and enhances supraspinatus force production compared to protraction 2.

The infraspinatus, subscapularis, and latissimus dorsi act as stabilizers during flexion; the subscapularis stabilizes during external rotation; and both subscapularis and supraspinatus stabilize during extension 4. This coordinated muscle activity is essential for glenohumeral joint stability, which relies primarily on active muscle control rather than passive capsular structures 5, 6.