Function of the Supraspinatus Muscle
The supraspinatus muscle primarily compresses the humeral head into the glenoid fossa to stabilize the glenohumeral joint and assists with shoulder abduction, particularly in the early phases (0-60 degrees), though it contributes less abduction torque than commonly believed and the deltoid can largely compensate for its loss. 1, 2
Primary Functions
Joint Stabilization (Dominant Role)
- The supraspinatus acts primarily as an active stabilizer of the shoulder joint by compressing the humerus into the glenoid cavity, preventing superior migration of the humeral head during arm elevation. 1
- This compression force is critical because it counteracts the superior-directed shear forces generated by the deltoid muscle, particularly at lower abduction angles. 2
- The supraspinatus generates substantial force (117-205 N during abduction) not primarily for movement but to stabilize the joint and neutralize antagonistic forces. 2
Shoulder Abduction (Secondary Role)
- The supraspinatus assists with glenohumeral abduction synergistically with the deltoid muscle, working most optimally between 0 and 60 degrees of abduction based on its lever arm mechanics. 1
- Despite common teaching, the supraspinatus contributes only approximately 25% of total abduction torque, while the middle deltoid contributes 35-65% and the subscapularis contributes 30%. 2
- The supraspinatus is activated prior to movement onset but not earlier than other shoulder muscles including infraspinatus, deltoid, and upper trapezius, contradicting the traditional claim that it "initiates" abduction. 3
Biomechanical Considerations
Moment Arm Efficiency
- The supraspinatus remains a more effective shoulder abductor than other rotator cuff muscles (infraspinatus, subscapularis) despite generating less force, due to its superior moment arm for abduction. 2
- The rotator cuff muscles are more effective abductors at lower abduction angles, while the deltoids become more effective at higher angles. 2
Load Requirements and Vulnerability
- In shoulders with larger critical shoulder angles (38° vs. normal 33°), the supraspinatus must generate 13-33% more force (15-23 N increase) to stabilize the arm during abduction between 33-37 degrees of elevation. 4
- This increased load requirement in abnormal shoulder anatomy predisposes the supraspinatus to overload and tendinopathy. 4
Functional Redundancy
- With complete loss of supraspinatus function, the deltoid can almost completely compensate by increasing its force by only one-third of the lost supraspinatus force, resulting in only a 6% decrease in glenohumeral elevation. 5
- Elimination of supraspinatus force actually decreases subacromial pressures by 8%, indicating the muscle does not depress the humeral head during elevation as traditionally taught. 5
Clinical Implications
Pathology Recognition
- Tenderness to palpation over the supraspinatus is a predictor of shoulder pain and impingement syndrome. 6
- Tendinopathy of the supraspinatus is commonly detected on ultrasound in patients with hemiplegic shoulder pain and rotator cuff pathology. 6
- Undersurface (articular-side) supraspinatus tears result from repetitive eccentric stress, particularly in overhead athletes with scapular dyskinesis and rotator cuff weakness. 7
Treatment Considerations
- Scapular retraction enhances supraspinatus force production during humeral elevation compared to scapular protraction. 2
- The "full can" exercise (scaption with external rotation) is preferred over "empty can" (scaption with internal rotation) as it produces similar supraspinatus activity with less impingement risk. 2
- Strengthening should target the entire rotator cuff and scapular stabilizers, not just the supraspinatus in isolation, given the integrated function of these muscle groups. 8, 7