Which muscles are typically weak after an anterior shoulder dislocation?

Muscle Weakness After Anterior Shoulder Dislocation

After anterior shoulder dislocation, the posterior rotator cuff muscles—specifically the infraspinatus and teres minor (external rotators)—are typically weak, along with the supraspinatus and scapular stabilizers. 1, 2

Primary Muscles Affected

Posterior Rotator Cuff Weakness

• The infraspinatus and teres minor demonstrate the most significant weakness and atrophy following anterior shoulder dislocation. 3, 2
• Patients with anterior instability have smaller muscle cross-sectional area of the posterior rotator cuff (infraspinatus + teres minor) compared to the anterior rotator cuff (subscapularis), with a subscapularis-to-infraspinatus ratio of 1.18 versus 0.79-0.93 in other instability patterns. 2
• This creates a muscular imbalance where the anterior structures (subscapularis) are relatively stronger than the weakened posterior external rotators. 1, 2

Supraspinatus Involvement

• The supraspinatus commonly shows weakness and atrophy after anterior dislocation. 3, 4
• Physical examination reveals focal weakness with decreased range of motion during abduction with external or internal rotation. 1

Scapular Stabilizers

• Scapular stabilizer muscles demonstrate weakness and contribute to ongoing instability. 1
• Scapular dyskinesis—poor coordination of scapular movements during arm elevation—compounds the problem by failing to maintain proper humeral head positioning. 5

Biomechanical Mechanism

The weakness pattern reflects the injury mechanism itself:

• Weakened posterior shoulder musculature combined with overdeveloped or relatively stronger anterior musculature creates the biomechanical environment for anterior instability. 1
• The posterior rotator cuff (external rotators) experiences repetitive eccentric stress during activities, leading to fatigue and injury. 1
• This muscular imbalance prevents the humeral head from staying centered in the glenoid fossa during arm motion, perpetuating instability. 1, 5

Age-Related Considerations

Younger Patients

• More likely to have labroligamentous injury and persistent instability after dislocation. 1
• Demonstrate weakness in rotator cuff muscles combined with ligamentous laxity as the primary pathology. 5

Older Patients

• More likely to have associated rotator cuff tears (supraspinatus, infraspinatus) in addition to weakness. 1, 6
• May present with persistent pain and disability from combined rotator cuff pathology and nerve injuries. 6

Clinical Examination Findings

• Deltoid and rotator cuff atrophy on inspection. 4
• Tenderness over the rotator cuff insertion sites. 4
• Limited passive range of motion. 4
• Weakness specifically on abduction and external rotation testing. 4
• Pain during external rotation maneuvers. 1

Rehabilitation Focus

Strengthening must target the external rotators (infraspinatus, teres minor), supraspinatus, and scapular stabilizers to restore the transverse force couple balance. 1, 3

• Training should use progressive resistance for rotator cuff muscles and deltoid three times weekly for 8 weeks minimum. 3
• Focus on muscle strength, coordination, and endurance training of the rotator cuff complex. 3
• Re-establish proper shoulder and spine mechanics while restoring full range of motion. 1
• Most patients improve with this approach unless they have abnormal skeletal anatomy or multidirectional instability. 3