Functional shoulder motion is complex and is dependent on active and passive forces









Shoulder Abduction Synergy



Threat to Humeral Motion

Line of Pull of Deltoid

Maintenance of Humeral Head on Glenoid Fossa


In the anatomical position, the line of pull of the deltoid is very vertical.


The vertical line of pull, in the anatomical position would yield a linear upward superior motion of the head of humerus on the glenoid fossa with very little angular motion.


However, given the origin, insertion, and horizontal line of pull of the supraspinatus, in the anatomical position, when it contracts angular motion into some abduction is produced.


However, as abduction proceeds beyond the anatomical position, the horizontal fibers of the supraspinatus are unable to perform angular motion and now, along with the subscapularis, pull the head of the humerus onto the glenoid fossa, acting as a stabilizer.


Once the supraspinatus-initiated abduction occurs, the line of pull of the deltoid becomes oblique and is now able to perform abduction.


Therefore, the supraspinatus and deltoid muscle work in synergy to perform shoulder abduction.


A similar synergistic relationship exists between the pectoralis major and deltoid during shoulder flexion.




The threat of deltoid active insufficiency is a challenge to normal shoulder. The scapula upward rotation associated with scapulohumeral rhythm simultaneously lengthens the deltoid during contraction to limit/delay active insufficiency.









Given the anatomical position of the acromion process superior to the humeral proximal epiphysis and diaphysis, it may pose a threat to normal shoulder movement. This is overcome by the active/passive downward line of pull of the infraspinatus and teres minor during abduction and flexion that pulls humeral head in an inferior direction maintaining the space between the head and acromion.







As the humerus is moved into abduction the fibers of the fibrous joint capsule are stretched causing the capsule to untwist. The untwisting places an external rotation moment on the humerus. The external rotation moves the greater tubercle from under the acromion thereby minimizing the threat of the bony block.