Infraspinatus muscle: the main lateral rotator of the shoulder

What it is

A rotator cuff muscle on the back of the scapula.

Location

Origin: infraspinous fossa of the scapula. Insertion: middle facet of the greater tubercle of the humerus.

Function

Laterally rotates the arm and stabilizes the shoulder joint.

Body system

Muscular · paired (left and right)

FMA ID

FMA32547 (Foundational Model of Anatomy)

Key facts

• Innervation: suprascapular nerve (C5–C6)
• Blood supply: suprascapular and circumflex scapular arteries

Clinical relevance

Part of the rotator cuff; tears usually start in supraspinatus but can extend into infraspinatus.

Origin: infraspinatus arises mostly from the medial two-thirds of the infraspinous fossa of the scapula. Some fibres also arise from the deep surface of the thick infraspinous fascia that covers the muscle. (Kenhub: Infraspinatus muscle.)

Insertion: its tendon crosses the back of the shoulder joint and inserts on the middle impression (middle facet) of the greater tubercle of the humerus. It also attaches to the posterior part of the shoulder joint capsule. (Kenhub: Infraspinatus muscle.)

The three posterior cuff tendons line up on the greater tubercle: supraspinatus on the superior facet, infraspinatus on the middle facet and teres minor on the inferior facet. (University of Washington Musculoskeletal Atlas: Teres minor.)

The infraspinatus footprint is large. Pandey and Willems note that the infraspinatus insertion covers the largest part of the greater tuberosity, larger than the area covered by the supraspinatus insertion. The same review confirms that supraspinatus, infraspinatus and teres minor insert over the superior, middle and inferior facets of the greater tuberosity respectively. (Pandey V, Willems WJ. Rotator cuff tear: a detailed update. 2015.)

Infraspinatus lies on the dorsal surface of the scapula, deep to the trapezius, parts of the deltoid and latissimus dorsi. It runs almost parallel to teres minor and teres major, separated from them by thick fascia, and its tendon may be separated from the joint capsule by a bursa that can communicate with the joint cavity. (Kenhub: Infraspinatus muscle.)

Teres minor arises from the lateral border of the scapula just below infraspinatus. The two muscles do the same job but have different nerves: infraspinatus is supplied by the suprascapular nerve and teres minor by the axillary nerve. (TeachMeAnatomy: The Intrinsic Muscles of the Shoulder.)

Infraspinatus laterally rotates the arm and helps to hold the humeral head in the glenoid cavity of the scapula. (University of Washington Musculoskeletal Atlas: Infraspinatus.)

Kenhub calls it a powerful lateral rotator of the humerus. Together with teres minor it stabilises the humeral head against the glenoid fossa, so the joint stays centred while other muscles move the arm. (Kenhub: Infraspinatus muscle.)

Infraspinatus also takes part in two force couples. In the transverse plane it and teres minor balance subscapularis at the front of the joint. In the coronal plane it is one of the lower cuff muscles (with subscapularis and teres minor) that balance the upward pull of the deltoid. Burkhart's work, summarised in this review, highlighted the transverse couple as a key factor in whether a shoulder with a massive cuff tear still functions. (Pandey V, Willems WJ. Rotator cuff tear: a detailed update. 2015.)

Teres minor arises from the upper two-thirds of the lateral border of the scapula, with infraspinatus lying above it, and inserts on the greater tubercle. Like infraspinatus it laterally rotates the arm. Its blood comes from the circumflex scapular and posterior circumflex humeral arteries. (Kenhub: Teres minor.)

The University of Washington atlas places the teres minor insertion on the inferior facet of the greater tubercle, just below infraspinatus, and lists its nerve as the axillary nerve (C5 and C6). A lesion of the suprascapular nerve therefore weakens infraspinatus but spares teres minor, and an axillary nerve lesion does the reverse. (University of Washington Musculoskeletal Atlas: Teres minor.)

Infraspinatus is supplied by the suprascapular nerve (C5 and C6) and receives blood from the suprascapular and circumflex scapular arteries. (University of Washington Musculoskeletal Atlas: Infraspinatus.)

The suprascapular nerve comes from the superior trunk of the brachial plexus, passes through the suprascapular notch beneath the superior transverse scapular ligament and supplies supraspinatus. It then passes through the greater scapular (spinoglenoid) notch to reach and supply infraspinatus. (Kenhub: Suprascapular nerve.)

The level at which the suprascapular nerve is compressed decides which muscles weaken. Entrapment at the suprascapular notch weakens both supraspinatus and infraspinatus, while entrapment at the spinoglenoid notch weakens infraspinatus alone. (Joo YB, Lee WY, Chung HJ. Suprascapular nerve entrapment caused by a large hematoma of the scapula: a case report. 2023.)

Recognised causes include ganglion cysts, direct trauma, scapular fractures, vascular malformations and injury during surgery. Paralabral cysts, often linked to SLAP (superior labrum) tears, are a common cause, and repetitive overhead activity is another. (Joo YB, Lee WY, Chung HJ. Suprascapular nerve entrapment caused by a large hematoma of the scapula: a case report. 2023.)

Suprascapular nerve entrapment causes pain at the back of the shoulder and weakness, mainly of abduction and external rotation. It typically follows a traumatic injury or compression of the nerve at the suprascapular notch. (Kenhub: Suprascapular nerve.)

Problems in infraspinatus can show up as pain, guarding or weakness during resisted external rotation of the shoulder. (Cotter EJ et al. Comprehensive Examination of the Athlete's Shoulder. Sports Health. 2018.)

In the external rotation lag sign the examiner bends the elbow to 90 degrees, rotates the arm outward and asks the patient to hold it there. If the patient cannot maintain the position, the test is positive. It is reported to be highly sensitive and specific for combined full-thickness tears of supraspinatus and infraspinatus. (Cotter EJ et al. Comprehensive Examination of the Athlete's Shoulder. Sports Health. 2018.)

Teres minor, the other lateral rotator, is checked separately with the hornblower's sign: with the shoulder abducted to 90 degrees and the elbow bent, the patient rotates the arm outward against resistance, and the test is positive if the arm falls into internal rotation. (Cotter EJ et al. Comprehensive Examination of the Athlete's Shoulder. Sports Health. 2018.)

Infraspinatus alongside supraspinatus, teres minor and subscapularis. Rotator cuff muscles.

The nearest named structures in the 3D model, measured from the centre of each part:

• Scapula · skeletal
• Subscapularis · muscular
• Teres minor · muscular
• Spinal part of deltoid · muscular
• Teres major · muscular
• Circumflex scapular artery · cardiovascular
• Dorsal scapular artery · cardiovascular
• Thoracodorsal artery · cardiovascular
• Subscapular artery · cardiovascular
• Supraspinatus · muscular