Anatomy of the Rotator Cuff
Supraspinatus, Infraspinatus, Teres Minor, and Subscapularis
Jun 13, 2009
The rotator cuff is a group of four distinct muscles that wrap around the head of the humerus and attach it to the scapula. As a group, they are responsible for stabilising the shoulder joint; individually, they serve to rotate the shoulder inwards or outwards, or abduct the shoulder.
The rotator cuff muscles can be easily remembered using the acronym SITS.
Supraspinatus
Though it is part of the rotator cuff, supraspinatus does not rotate the shoulder joint. It lies in the fossa at the top of the scapula and attaches to the head of the humerus, where it pulls the shoulder joint into abduction – that is, raising the arm out to the side.
Though the larger deltoid muscle also abducts the shoulder, the deltoid tends to pull the humerus upwards, where it can butt against the acromion process. Supraspinatus, on the other hand, rolls the humeral head down into the hollow of the glenoid fossa as it lifts the arm. When it is working correctly, therefore, it creates a feeling of space and freedom in the shoulder.
- Origin: supraspinous fossa of the scapula
- Insertion: superior facet of greater tubercle of the humerus
- Action: shoulder abduction and stabilisation
- Innervation: suprascapular nerve
Infraspinatus
This muscle originates in the broad, shallow fossa inferior to the scapular spine and inserts on the humeral head. It externally rotates the shoulder joint – twirling the humerus outwards in a tennis backhand. If the humerus is fixed in a set position, infraspinatus also helps to pull the inferior angle of the scapula outward, toward the side of the body.
- Origin: infraspinous fossa of the scapula
- Insertion: middle facet of greater tubercle of the humerus
- Action: shoulder external rotation and stabilisation
- Innervation: suprascapular nerve
Teres Minor
Teres minor is paired with infraspinatus as an external rotator. It sits at the bottom outside corner of the scapula.
- Origin: lateral border of the scapula
- Insertion: inferior facet of greater tubercle of the humerus
- Action: shoulder external rotation and stabilisation
- Innervation: axillary nerve
Subscapularis
The deepest muscle of the rotator cuff, subscapularis sits sandwiched between the scapula and the ribcage. It turns the shoulder joint inwards, in the opposite movement to infraspinatus and teres minor. Unlike other internal rotators, like the pectorals, subscapularis draws the humerus snugly back into the glenoid fossa, allowing freedom of movement.
- Origin: subscapular fossa of the scapula
- Insertion: lesser tubercle of the humerus
- Action: shoulder internal rotation and stabilisation
- Innervation: upper and lower subscapular nerves
The Rotator Cuff Muscles are Vital for Shoulder Mechanics
Despite their relatively small size and specialised actions, the rotator cuff muscles are essential players in almost every shoulder movement. Rotator cuff injuries can be devastating to the function of the entire shoulder girdle. Balanced strength and flexibility in each of these four muscles, therefore, is vital.
See also: Bony Anatomy of the Shoulder Girdle
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