Infraspinatus

Origin, Insertion, Action, Innervation

Origin: Infraspinous fossa of the scapula (the large concavity below the spine of the scapula)
Insertion: Middle facet of the greater tuberosity of the humerus, posterior shoulder joint capsule
Action:
Primary: Lateral (external) rotation of the shoulder
Horizontal abduction (minor)
Compression of the humeral head into the glenoid (dynamic stabilization)
Inferior depression of the humeral head during abduction (prevents superior migration)
Innervation: Suprascapular nerve (C5–C6)

Palpation Guide

Client position: Prone or seated.
Landmark sequence:

Locate the spine of the scapula. The infraspinatus fills the entire infraspinous fossa — the large area below the scapular spine.
Place your hand flat on the posterior scapula below the spine. The infraspinatus is directly under your hand, covered only by skin and superficial fascia in most of this area (the trapezius covers only the medial portion).
The muscle converges laterally toward the greater tuberosity. The tendon passes posterior to the glenohumeral joint before inserting on the middle facet.
Palpate the lateral border of the scapula to distinguish infraspinatus (superior and posterior) from teres minor (inferior) and teres major (most inferior).

Tissue feel: A broad, flat muscle filling the infraspinous fossa. In a healthy state, it is pliable and elastic. In clients with shoulder dysfunction, it is often hypertonic with multiple taut bands running from the medial scapular border toward the greater tuberosity. TrPs are common and often exquisitely tender.
Confirmation test: Ask the client to laterally rotate the shoulder against resistance (arm at side, elbow at 90°, forearm rotating outward). You will feel the infraspinatus contract under your hand in the infraspinous fossa.
Common errors:
Confusing infraspinatus with teres minor — teres minor lies along the lateral border of the scapula, inferior to infraspinatus. The two are synergists for lateral rotation but are distinct muscles.
Treating only the medial portion and missing the lateral portion near the greater tuberosity — the musculotendinous junction and the lateral muscle belly are common TrP sites.
Not distinguishing infraspinatus from the overlying posterior deltoid laterally — the deltoid covers the lateral portion of the infraspinatus near the shoulder joint.

Trigger Point Referral

Common TrP locations: The primary TrP is in the medial portion of the muscle belly, approximately 2–3 cm below the scapular spine and medial to the lateral border. Secondary TrPs occur along the musculotendinous junction near the greater tuberosity.
Referral pattern: Refers deep aching to the anterior shoulder (anterior deltoid area), down the anterolateral arm (biceps region), and into the forearm. May refer to the lateral and posterior shoulder as well. The pattern extends further distally than most students expect.
Clinical significance: The anterior arm referral mimics C5–C6 radiculopathy. This is the most common muscle source of "I can't reach behind my back" — the infraspinatus restricts internal rotation when it is shortened, and the TrP refers pain whenever the client attempts the motion. If a client cannot reach behind their back and the pain is in the anterior arm, check the infraspinatus first.

Trigger point referral diagram — coming soon

Image coming soon. For visual reference, see [Infraspinatus at TriggerPoints.net](http://www.triggerpoints.net/muscle/infraspinatus).

Clinical Notes

Innervation significance:

Shares the suprascapular nerve (C5–C6) with the supraspinatus. The nerve passes through the suprascapular notch (where it can be compressed) and then through the spinoglenoid notch (where it can be compressed by a ganglion cyst). Entrapment at the spinoglenoid notch selectively denervates the infraspinatus while sparing the supraspinatus.

Common conditions:

Infraspinatus weakness from suprascapular nerve entrapment at the spinoglenoid notch produces isolated lateral rotation weakness. This is important in overhead athletes (volleyball players, baseball pitchers) where repetitive use predisposes to ganglion formation at the notch.
Key muscle in conditions/rotator-cuff-tendinopathy — the infraspinatus tendon can be involved in impingement and degenerative change, particularly in overhead athletes.
Infraspinatus TrPs are a major contributor to the pain pattern in conditions/frozen-shoulder — the restricted internal rotation (hand-behind-back motion) is partly due to infraspinatus shortening.
Essential for dynamic stability in conditions/shoulder-instability — the infraspinatus and teres minor provide the primary posterior force to resist anterior humeral head translation.

What you'll typically find:

Infraspinatus is hypertonic and harbors TrPs in the majority of clients with shoulder complaints. It is one of the most reliably symptomatic muscles in clinical practice. Palpation of the infraspinous fossa frequently reproduces the client's shoulder and arm pain.
The "reach behind the back" restriction is the clinical hallmark. If the client cannot reach behind their back to fasten a bra or reach a back pocket, infraspinatus and subscapularis are the first muscles to assess.
In overhead athletes, the infraspinatus is eccentrically loaded during the deceleration phase of throwing — chronic overload produces tendinopathy and TrPs.

Treatment effects:

Responds extremely well to sustained compression and longitudinal stripping in the infraspinous fossa. Hold TrPs for 30–90 seconds. The client may report anterior arm referral during compression — this confirms you are on the right point.
Cross-fiber friction at the musculotendinous junction is effective for chronic tendon involvement.
Post-treatment, internal rotation range typically improves immediately. The "hand behind back" motion often increases by several vertebral levels after a single treatment.

Cautions:

The suprascapular nerve passes through the spinoglenoid notch at the lateral base of the scapular spine. Avoid aggressive deep pressure directly over the notch area.
In clients with suspected rotator cuff tears, test strength before and after treatment. If lateral rotation weakness is present and does not improve with TrP treatment, the weakness may be structural (tear) rather than inhibition from TrPs.

Postural significance:

In upper crossed syndrome with protracted, internally rotated shoulders, the infraspinatus is both lengthened (by the internal rotation position) and eccentrically loaded (working to resist further internal rotation). This combination of lengthening and loading produces the high TrP prevalence. Correcting the internal rotation posture by strengthening lateral rotators and stretching medial rotators reduces infraspinatus load.

Clinical pearl:

If a client reports that sleeping on the affected shoulder increases their anterior arm pain, the infraspinatus is likely involved. Side-lying on the shoulder compresses the infraspinatus between the body weight and the scapula, activating TrPs that refer to the anterior arm. Recommending the client sleep on the opposite side often reduces morning pain dramatically.

Assessment

Manual muscle testing:

Lateral (external) rotation: Client seated or prone with arm at side, elbow at 90°. Ask the client to rotate the forearm outward against your resistance. Compare bilaterally. Weakness suggests infraspinatus or teres minor pathology, or suprascapular nerve compromise.

Stretch test:

Internal rotation (hand behind back): Client seated. Ask the client to reach behind the back as if to touch the opposite scapula. Measure the highest vertebral level the thumb can reach. Compare bilaterally. Restriction in internal rotation is the hallmark of infraspinatus shortening.

Related special orthopedic tests:

Infraspinatus strength test — resisted lateral rotation with the arm at the side
Hornblower's sign — inability to laterally rotate with arm at 90° abduction suggests infraspinatus/teres minor tear
Neer and Hawkins-Kennedy — subacromial impingement tests (infraspinatus tendon may be involved)

Muscle Groups

Rotator cuff (anatomical):

Lateral (external) rotators of the shoulder (functional):

Infraspinatus (this article) — primary lateral rotator
anatomy/muscles/teres-minor — assists lateral rotation
anatomy/muscles/deltoid — posterior fibers (minor)

Suprascapular nerve group (innervation):

anatomy/muscles/supraspinatus
Infraspinatus (this article)

Related Muscles

Synergists for lateral rotation:

anatomy/muscles/teres-minor — lies immediately inferior along the lateral scapular border; shares lateral rotation function

Antagonists:

anatomy/muscles/subscapularis — primary medial (internal) rotator; the infraspinatus-subscapularis balance is critical for glenohumeral stability
anatomy/muscles/pectoralis-major — medial rotator; chronic shortening overloads the infraspinatus
anatomy/muscles/teres-major — medial rotator
Latissimus dorsi — medial rotator

Same innervation (suprascapular nerve):

anatomy/muscles/supraspinatus — shares C5–C6 suprascapular nerve; both affected in proximal suprascapular entrapment

Key Takeaways

The anterior arm referral is the most commonly misdiagnosed muscular pattern in the shoulder — it mimics C5–C6 radiculopathy.
"I can't reach behind my back" is the clinical hallmark — infraspinatus shortening restricts internal rotation, and its TrPs refer pain during the motion.
Side-sleeping on the affected shoulder activates infraspinatus TrPs — changing sleep position often reduces morning pain.
Entrapment at the spinoglenoid notch selectively denervates infraspinatus while sparing supraspinatus — look for isolated lateral rotation weakness in overhead athletes.