01/04/2026
Supraspinatus and Infraspinatus Muscles
The supraspinatus and infraspinatus muscles are the primary components of the superior and posterosuperior rotator cuff. They provide essential dynamic stability to the glenohumeral joint, pulling the humeral head into the glenoid cavity during arm movement.
1. The Supraspinatus Muscle
The supraspinatus is the most commonly injured muscle in the rotator cuff. It is divided into a large anterior part and a smaller posterior part.
Origin: Supraspinous fossa of the scapula and the superior surface of the scapular spine.
Insertion: The superiormost impression (facet) of the greater tuberosity of the humerus.
Function: Responsible for approximately 60% of arm elevation and abduction. It is particularly crucial for initiating the first 15° of abduction before the deltoid takes over.
2. The Infraspinatus Muscle
The infraspinatus is a powerful external rotator that often blends with the teres minor.
Origin: Infraspinous fossa and the inferior surface of the scapular spine.
Insertion: The middle and highest facets of the greater tuberosity. Note that the anterior part of this insertion actually sits lateral to the supraspinatus insertion.
3. Innervation and Clinical Anatomy
Both muscles are supplied by the suprascapular nerve (C5–C6).
The Pathway: The nerve passes through the suprascapular notch (beneath the superior transverse scapular ligament). It then winds around the lateral border of the scapular spine (spino-glenoid notch) to reach the infraspinatus.
Nerve Entrapment: Compression of the nerve at the suprascapular notch affects both muscles. However, compression further down at the spino-glenoid notch (often by a paralabral ganglion) will cause isolated infraspinatus atrophy.
4. Clinical Considerations
Muscle Atrophy: Infraspinatus atrophy is a very frequent finding in chronic rotator cuff tears. It is a key prognostic indicator; severe fatty atrophy often suggests that a surgical repair may be less successful.
Rotator Cuff Tears: Because the supraspinatus and infraspinatus tendons blend with the underlying joint capsule, a full-thickness tear of the tendon usually results in a direct communication between the joint space and the subacromial bursa.
Clinical Pearl
"When testing for suprascapular nerve entrapment, look at the shoulder blades from behind. If both the top (supraspinatus) and bottom (infraspinatus) of the scapula look hollow, the nerve is likely pinched at the suprascapular notch. If only the bottom looks hollow, look for a cyst or compression at the spino-glenoid notch."
Function: Acts as the primary external rotator of the arm (alongside the teres minor).
