Infraspinatus

The infraspinatus is a flat, triangular muscle located on the posterior surface of the scapula, just inferior to the spine of the scapula. It is part of the rotator cuff, a group of muscles that stabilizes the shoulder joint and controls fine motor movements of the upper extremity.

The muscle covers most of the infraspinous fossa and lies between the teres minor muscle inferiorly and the supraspinatus muscle superiorly. It is partially covered by the deltoid and trapezius muscles in its superolateral portion, and the tendon blends with the joint capsule of the glenohumeral joint.

Origin

• The entire medial and central part of the infraspinous fossa, the flat depression located inferior to the spine of the scapula.

• The surrounding fascial tissue covering the infraspinous fossa.

• Some fibers may also originate from the medial border of the scapula and the deep fascia.

The origin is broad and flat, providing a strong base for force production. The fibers gradually converge into a narrower tendon that runs laterally toward the humerus.

Insertion

• The tendon inserts on the middle facet of the greater tubercle of the humerus, on the lateral aspect of the upper arm bone.

• A substantial portion of the tendon blends with the joint capsule of the glenohumeral joint.

This integration into the capsule allows the infraspinatus not only to rotate the arm, but also to help keep the humeral head centered in the glenoid during movement.

Innervation

• The suprascapular nerve, originating from spinal roots C5 and C6, arising from the superior trunk of the brachial plexus.

• The suprascapular nerve passes through the suprascapular notch beneath the superior transverse scapular ligament and is therefore susceptible to compression during repetitive use or anatomical variations.

Injury to this nerve, such as in suprascapular neuropathy, can lead to isolated weakness of both the infraspinatus and supraspinatus and typically results in reduced strength in external rotation.

Blood supply

• The suprascapular artery, usually a branch of the thyrocervical trunk arising from the subclavian artery.

• The circumflex scapular artery, a branch of the subscapular artery that contributes to the anastomotic network in the region.

• Minor contributions may also come from the dorsal scapular artery.

The blood supply is rich and supported by collateral circulation in the scapular region. This reduces the risk of ischemia but is clinically relevant in cases of trauma or compression.

Shape and muscle structure

The fibers of the infraspinatus are arranged in a fan-shaped pattern, originating broadly from the infraspinous fossa and converging laterally toward the insertion on the humerus. This structure provides a large moment arm for external rotation, particularly when the arm is in a neutral or slightly abducted position.

The muscle is enclosed in a strong fascia that separates it from the teres minor inferiorly and the supraspinatus superiorly. Inferiorly, it partially blends with the teres minor but has a deeper position and a broader distribution of muscle fibers.

Function and movement

Primary function

The primary function of the infraspinatus is external rotation of the humerus at the shoulder joint. This occurs most prominently when the arm is hanging at the side or is slightly abducted. The movement is essential in daily activities such as reaching behind the body, throwing, or opening a door.

During external rotation, the infraspinatus pulls the greater tubercle posteriorly and laterally, rotating the humeral head outward. This action occurs in cooperation with the teres minor, but the infraspinatus is the stronger of the two and bears most of the load during pure external rotation.

Secondary functions

In addition to producing movement, the infraspinatus plays a key role in shoulder stability as part of the rotator cuff.

The muscle draws the humeral head toward the glenoid fossa and centers it during movement. It counteracts superior translation of the humeral head when the deltoid elevates the arm during flexion or abduction. This reduces the risk of impingement and overload of subacromial structures.

Dynamic and static stability

In static positions, the infraspinatus holds the humeral head in place posteriorly. During dynamic movements, such as throwing or rapid arm elevation, it regulates motion to maintain centering of the glenohumeral joint. This mechanism is referred to as concavity compression.

Synergy and interaction with other muscles

• The supraspinatus and infraspinatus work together in the initial phase of shoulder abduction, where the former initiates elevation and the latter provides stabilization.

• The teres minor assists the infraspinatus during external rotation, particularly in the neutral arm position.

• The posterior fibers of the deltoid and the teres major act as important synergists in coordinated shoulder movement.

• During throwing activities, the infraspinatus works in combination with scapular stabilizers such as the trapezius and serratus anterior to ensure proper positioning of the entire shoulder complex.

Functional significance in activity

In sports that require precise and powerful external rotation, such as handball, tennis, and baseball, the infraspinatus is highly activated. It is also crucial during controlled deceleration following overhead movements, representing eccentric muscle activity.

Examples include:

• When the throwing arm is decelerated after ball release.

• When the arm is moved backward to reach behind the body.

• During postural correction of internally rotated shoulders.

In these situations, the infraspinatus prevents the humeral head from translating anteriorly or superiorly, which could otherwise provoke irritation of bursae or tendons beneath the acromion.

Weakness or injury – consequences

• Weakness or rupture of the infraspinatus results in reduced strength in external rotation.

• This may lead to compensatory overactivity of the teres minor, which cannot fully compensate for the loss.

• Shoulder instability and an increased risk of impingement are common consequences.

• Clinically, this is often observed as a positive Hornblower’s sign and difficulty placing the hand behind the head.

Clinical relevance and examination

Susceptibility to injury during throwing and overuse

The infraspinatus is one of the muscles most frequently affected by overuse in individuals participating in overhead sports. It is heavily involved during both the acceleration and deceleration phases of throwing. The eccentric phase, when the arm decelerates after ball release, places particularly high demands on the infraspinatus.

As a result, the following patient groups are commonly affected:

• Handball and baseball players

• Swimmers and tennis players

• Individuals working extensively with the arms above shoulder level, such as painters and electricians

Common clinical conditions

• TendinopathyLoad-related pain and degenerative changes within the tendon.

• Partial or complete rupturesMost common in older individuals or following a fall onto an outstretched arm.

• Suprascapular nerve neuropathyMay lead to atrophy and weakness of the infraspinatus without significant pain.

Clinical signs and symptoms

• Pain during resisted external rotation

• Pain radiating posteriorly over the shoulder and into the upper arm

• Reduced strength in external rotation

• Tenderness on palpation over the infraspinous fossa

• Possible visible atrophy, presenting as a sunken fossa

Special tests for the infraspinatus

These tests assess both strength and pain during active or resisted external rotation. No single test is definitive, but in combination they provide a reliable assessment of function.

Infraspinatus resisted external rotation test

Execution:The arm is held at the side with the elbow flexed to 90 degrees. The patient performs external rotation against resistance.

Positive finding:Pain or weakness indicates possible infraspinatus involvement.

Drop sign

Execution:The shoulder is positioned in 90 degrees of abduction and maximal external rotation. The patient is asked to maintain the position.

Positive finding:Inability to hold the position, with uncontrolled internal rotation, suggests a full-thickness infraspinatus rupture.

Hornblower’s sign (modified)

Although primarily associated with the teres minor, weakness of the infraspinatus may contribute to a positive result.

Execution:The arm is positioned in 90 degrees of abduction and elbow flexion. The patient attempts to maintain external rotation against resistance.

Imaging studies

• UltrasoundUseful for evaluating tendon structure and identifying tears.

• MRIGold standard for assessing rotator cuff integrity and muscle atrophy.

• EMGIndicated when nerve injury is suspected, particularly involving the suprascapular nerve.

Treatment strategies

Management of infraspinatus pathology depends on the severity of injury and degree of functional impairment. Most non-ruptured conditions respond well to conservative treatment.

Conservative management

• Activity modification and unloading

• Gradual strengtheningWith emphasis on eccentric external rotation and scapular stability

• Manual therapyIncluding stretching, joint mobilization, and trigger point treatment

• Pharmacological supportNSAIDs or corticosteroid injections in cases of marked inflammation, used with caution

Management of rupture

• Partial ruptureExercise-based rehabilitation with follow-up, often yielding good outcomes

• Complete ruptureMay require surgical repair, particularly in younger or highly active patients

Postoperative rehabilitation

• Early immobilization

• Gradual loading progressionPassive to active to resisted movement

• Focus on humeral head centering and external rotation strength

• Return to sport typically after 4 to 6 months

Prevention

Preventive strategies focus on:

• Regular strengthening of the rotator cuff

• Adequate scapular control

• Limiting repetitive overload

• Early intervention in the presence of pain or fatigue

Exercises for the infraspinatus

Why specific exercises are important

The infraspinatus is the primary muscle responsible for active external rotation of the shoulder and plays a key role in dynamic stabilization of the glenohumeral joint. In cases of pain, overuse, or injury, targeted and progressive activation is essential.

Training focus should include:

• Isolated activation for selective strengthening

• Eccentric control, particularly in overuse conditions

• Functional integration related to sport, work, and daily activities

Isometric external rotation at the side

Purpose:Gentle activation without joint movement, useful in early rehabilitation.

Execution:

• Stand with the upper arm against the body and the elbow flexed to 90 degrees.

• Press the hand outward against a wall or the therapist’s hand without allowing arm movement.

• Hold for 5 to 10 seconds, 5 to 10 repetitions.

Evidence:Well tolerated in painful conditions with minimal joint loading.

External rotation with resistance band – neutral position

Purpose:Isolated strengthening of the infraspinatus and teres minor.

Execution:

• Anchor a resistance band at elbow height.

• Keep the elbow close to the body at 90 degrees and rotate the forearm outward.

• Avoid shoulder elevation.

• Perform 3 sets of 10 to 15 repetitions.

Tip:Use a mirror to ensure correct movement pattern.

Side-lying external rotation

Purpose:One of the most selective exercises for the infraspinatus based on EMG studies.

Execution:

• Lie on the side with the training arm on top.

• Hold a light dumbbell and externally rotate the arm with the elbow flexed to 90 degrees.

• Keep the upper arm stable against the body.

• Perform 3 sets of 10 to 12 repetitions.

Progression:Increase load gradually or modify tempo with eccentric emphasis.

Prone horizontal abduction with external rotation

Purpose:Combined activation of the infraspinatus, posterior deltoid, and scapular stabilizers.

Execution:

• Lie prone with the arm hanging off the edge of a bench.

• Rotate the arm externally with the thumb pointing upward and lift the arm horizontally away from the body.

• Hold for 2 to 3 seconds at the top, then lower slowly.

• Perform 3 sets of 10 repetitions.

Variation:Add light resistance using a dumbbell or hand weight.

Bilateral external rotation with resistance band

Purpose:Functional shoulder activation and scapular control.

Execution:

• Hold a resistance band with both hands in front of the chest, thumbs pointing upward.

• Pull the arms outward while keeping the elbows flexed to 90 degrees, bringing the shoulder blades together.

• Hold for 2 seconds, then return in a controlled manner.

• Perform 3 sets of 12 repetitions.

Goal:Develop strength and coordination in the posterior rotator cuff.

Eccentric external rotation with manual resistance

Purpose:Particularly effective in cases of tendinopathy.

Execution:

• The therapist applies resistance while the patient slowly controls the return from external rotation.

• Use a 3 to 5 second eccentric phase, 6 to 8 repetitions.

Benefit:Controlled loading with targeted focus on tendon tolerance.

Sport-specific transfer exercises

Examples:

• Throwing movements with resistance band loading

• Plank variations with rotational components

• Wall slides combined with external rotation

Sources

1. Moore, K. L., Dalley, A. F., & Agur, A. M. R. (2014). Clinically Oriented Anatomy (7th ed.).

2. Palastanga, N., & Soames, R. (2012). Anatomy and Human Movement (6th ed.).

3. Standring, S. (2016). Gray’s Anatomy (41st ed.).

4. Singh, V. (2010). Anatomy of Upper Limb and Thorax.