Functions of the Supraspinatus and Infraspinatus Tendons
Supraspinatus Tendon Function
The supraspinatus primarily contributes to glenohumeral abduction and joint stabilization through compression rather than serving as the sole initiator of shoulder abduction. 1
Primary Biomechanical Roles
Glenohumeral abduction contribution: The supraspinatus generates approximately 25% of the total abduction torque during shoulder elevation, though it is more effective at lower abduction angles (0-45 degrees) due to its favorable moment arm. 2
Joint compression and stabilization: The supraspinatus produces more glenohumeral joint compression than torque, helping to stabilize the humeral head within the glenoid fossa during arm elevation. 3
Counterbalancing superior translation: The supraspinatus works with other rotator cuff muscles to neutralize the superior-directed force generated by the deltoid at lower abduction angles, preventing superior migration of the humeral head. 2
Anatomical Position and Clinical Relevance
The supraspinatus is located superiorly in the rotator cuff complex and occupies a narrow subacromial space where it passes beneath the coracoacromial arch. 4
The tendon attaches to the superior facet and superior half of the middle facet of the greater tuberosity, with the anterior half (approximately 12.6 mm) composed solely of supraspinatus fibers. 5
Impingement vulnerability: The supraspinatus is most susceptible to impingement during abduction between 70-120 degrees when the humerus is simultaneously abducted and internally rotated, combined with hypovascularity near its insertion site. 4
Muscle Activation Patterns
The supraspinatus activates prior to movement onset during shoulder abduction but does not initiate abduction earlier than infraspinatus, deltoid, or axioscapular muscles—making the common statement that "supraspinatus initiates abduction" misleading. 1
During scapular plane abduction, the supraspinatus requires larger force contributions near the beginning of motion (0-30 degrees), while the deltoid becomes more important near the end range. 6
Infraspinatus Tendon Function
The infraspinatus serves dual critical functions: external rotation of the humerus and substantial contribution to shoulder abduction, generating forces two to three times greater than the supraspinatus during scapular plane elevation. 2
Primary Biomechanical Roles
External rotation: The infraspinatus is the primary external rotator of the shoulder, controlling and stabilizing movements during the deceleration phase of throwing and overhead activities. 7
Abduction assistance: The infraspinatus contributes approximately 10% of total abduction torque and generates an estimated 205 N of force during abduction—significantly more than the supraspinatus (117 N)—though its moment arm makes it less effective as a pure abductor. 2
Posterior stabilization and joint compression: Located posteriorly in the rotator cuff, the infraspinatus helps maintain proper humeral head positioning within the glenoid fossa and prevents anterior translation during arm movements. 7
Anatomical Position and Overlap
The infraspinatus is positioned posteriorly in the rotator cuff complex, alongside the teres minor. 4, 7
The infraspinatus tendon attaches to the entire middle facet of the greater tuberosity, overlapping the posterior half of the supraspinatus tendon (approximately 9.8 mm of overlap). 5
The anatomic sulcus is located slightly posterior to the posterior margin of the supraspinatus tendon (approximately 4.3 mm), not at the supraspinatus-infraspinatus interval. 5
Clinical Significance in Movement Patterns
During throwing motions, the infraspinatus experiences significant eccentric stress during the deceleration phase as it controls internal rotation, making it vulnerable to undersurface tears from repetitive overuse. 7
The infraspinatus activates simultaneously with the supraspinatus, deltoid, and upper trapezius prior to movement onset, with no significant differences in initial activation timing across different planes of movement or loading conditions. 1
Integrated Function and Clinical Pitfalls
Both tendons work synergistically: While the supraspinatus has a more effective moment arm for abduction, the infraspinatus generates substantially greater force (two to three times more) during scapular plane elevation, highlighting their complementary roles. 2
Compensation capacity: The deltoid can compensate for loss of supraspinatus function by increasing force by one-third of the lost supraspinatus force, with only a 6% decrease in glenohumeral elevation when supraspinatus force is eliminated. 3
Scapular dyskinesis contribution: Poor scapular coordination during arm elevation disrupts the normal function of both the supraspinatus and infraspinatus, increasing impingement risk and reducing force production efficiency. 2
Scapular retraction enhances function: Compared with scapular protraction, retraction increases subacromial space width and enhances supraspinatus force production during humeral elevation, reducing impingement risk. 2