Muscle:Deltoid

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Deltoid is a large, superficial muscle of the shoulder with three anatomically and functionally distinct parts — anterior, middle, and posterior — each producing a different local referred pain pattern, a different range of motion restriction, and a different set of associated satellite muscles. It is one of the muscles that commonly develops myofascial trigger points (TrPs), and its TrPs are routinely misdiagnosed as rotator cuff tears, subdeltoid bursitis, impingement syndrome, glenohumeral arthritis, acromioclavicular joint pathology, bicipital tendinitis, or C5 radiculopathy.

The deltoid's most clinically important feature for differential diagnosis is that its referred pain stays local — spreading within and around the affected part of the muscle rather than referring to a distance as most muscles do. This absence of distant referral, combined with the presence of palpable taut bands and local twitch responses, is what distinguishes deltoid TrP pain from structural joint pathology, which can produce a remarkably similar pain distribution.

One rarely finds TrP involvement of the deltoid muscle alone. In the vast majority of cases, active TrPs in the supraspinatus, infraspinatus, or scalenes are the key TrPs driving satellite TrPs in the deltoid — and until these are addressed, deltoid TrPs will recur.

Anatomy

Proximal attachments (by division):

  • Anterior — lateral one-third of the clavicle
  • Middle — acromion (multipennate fibre arrangement with 4 proximal tendons extending downward from the acromion and 3 interdigitating tendons extending upward from the deltoid prominence)
  • Posterior — lateral portion of the spine of the scapula

Distal attachment (all parts): Deltoid prominence on the lateral aspect of the humerus at approximately midshaft. This point usually appears as a dimple in the skin at the base of the "V" formed by the belly of the muscle — a useful surface landmark.

The anterior and posterior parts have a fusiform fibre arrangement — long parallel bundles running directly from origin to insertion. This arrangement provides speed at the expense of strength, and produces a single transverse band of motor endplates (and therefore TrPs) across the midportion of these parts.

The middle part is multipennate — fibres slant obliquely between the interlacing proximal and distal tendons. This arrangement provides greater force through a shorter distance, and distributes motor endplates (and therefore potential TrP locations) throughout much of the muscle bulk. This explains why middle deltoid TrPs can be found almost anywhere in the muscle, while anterior and posterior deltoid TrPs are located close to midmuscle.

Innervation: Axillary nerve (branch of the posterior cord); C5 and C6 spinal roots.

Primary actions by division:

  • Anterior — arm flexion; horizontal adduction across the chest; assists abduction
  • Middle — arm abduction (prime mover; primary function); assists flexion above 60°; helps secure the humeral head in the glenoid fossa in collaboration with the supraspinatus
  • Posterior — arm extension (essential for reaching behind the body to the gluteal area); assists abduction

The anterior and posterior parts are antagonists during flexion and extension. They can therefore develop active TrPs together as antagonistic overload. The middle part is more resistant to sustained-contraction fatigue than the anterior and posterior parts, probably because of its multipennate structure and high proportion of type 1 fibres (60% of deltoid fibres are slow-twitch fatigue-resistant).

Referred Pain Patterns

The deltoid is distinctive in that its TrPs do not refer pain to a distance — they spread locally within and around the affected division. This is a cardinal feature for differential diagnosis.

Anterior deltoid

TrPs in the anterior part refer pain to the anterior and middle deltoid region. Pain concentrates at and around the site of the TrP. No referral below the elbow.

Posterior deltoid

TrPs in the posterior part refer pain concentrated over the posterior shoulder, sometimes spilling into the adjacent posterior arm. Atrophy of the posterior deltoid is easily palpated and is common in many physical labour careers, and all non-physical labour careers and contributes to pronation of the shoulders.

Middle deltoid

TrPs in the middle part produce pain centred in that region with spillover to adjacent areas. Because TrPs can occur throughout the multipennate middle portion, the pain distribution may be more diffuse than in the anterior or posterior parts.

Key differentiating feature

No distant referral from any part of the deltoid. If the patient reports pain below the elbow, down the arm to the hand, or in the neck, the primary source lies elsewhere — most likely in the infraspinatus (radial forearm and hand), supraspinatus, subscapularis (wrist band), or scalenes (radial forearm and hand).

Activation and Perpetuating Factors

Direct trauma

Few other muscles are as exposed to direct forceful impacts against underlying bone. Impact activation is a characteristic mode for the deltoid:

  • Hit by a tennis or golf ball directly on the muscle
  • Falling directly on the shoulder
  • Repeated recoil of a gun against the anterior deltoid when shooting
  • Sudden overload reaching for a bannister or railing to arrest a near-fall

Overexertion

  • Prolonged holding of a power tool at shoulder height
  • Episodic overexertion — unaccustomed deep-sea fishing, excessive overhead skiing poles (posterior deltoid)
  • Sustained repetitive work at shoulder height — sorting mail into shoulder-height boxes, assembly work

Iatrogenic activation

Intramuscular injection of locally irritant solutions (B vitamins, penicillin, tetanus toxoid, diphtheria or influenza vaccine) into a latent TrP activates it and causes a persistently painful shoulder. This is one of the more common causes of deltoid TrP activation in clinical practice. Prevention strategies are listed in the Treatment section below.

Satellite activation from key TrPs

The deltoid lies within the essential referred pain zones of both the infraspinatus and supraspinatus muscles. It almost never escapes satellite TrP development when either of these scapular muscles harbours active TrPs. Key TrPs in the scalenes or supraspinatus can induce satellite TrPs in the deltoid. Experimentally, motor unit activity (referred spasm) was recorded in the anterior deltoid in response to pressure on an active infraspinatus TrP causing referred pain over the front of the shoulder — with simultaneous electrical silence in the biceps and triceps brachii.

Clinical Examination

Range of Motion

Patients with multiple deltoid TrPs may show serious impairment of abduction strength, or total inability to reach 90° of abduction. The specific range of motion restriction depends on which division is involved:

Division Restricted movement Clinical test
Anterior Back-rub Test (see Fig. 29.3 in Travell & Simons) — arm behind back at lumbar level Painful restriction on performing the Back-rub Test; difficulty raising the arm to horizontal for bringing the hand to the mouth
Posterior Mouth Wrap-around Test (see Fig. 18.2 in Travell & Simons) — arm reaches over the head but not behind it Arm can reach over the head, but not behind it, because of pain induced by forceful contraction of the posterior deltoid fibres in the shortened position
Middle Abduction to 90° Pain on abduction; may limit to less than 90° with severe involvement

Provocation Tests

Ask the patient to straighten the elbow and attempt to abduct the arm to 90° in two positions:

  • Thumb up (palm forward): Painful when anterior deltoid TrPs are active — loads the anterior fibres
  • Thumb down (palm backward): Painful when posterior deltoid TrPs are active — loads the posterior fibres

Relevant TrP identification

A systematic approach is required: identify restricted range of motion; take up slack to the point of tension; ask where the patient feels tension or pain; then palpate there for a taut band and TrP. A TrP that limits range of motion is a relevant TrP.

Test the glenohumeral joint for normal joint play. If joint play is restricted, restore it before concluding that the deltoid TrPs alone account for the motion loss.

Distinguishing deltoid TrP tenderness from supraspinatus enthesopathy

Tenderness of supraspinatus enthesopathy at the rotator cuff can be confused with deltoid TrP tenderness. When the arm is passively abducted to 90°, the supraspinatus attachment is protected from palpation beneath the acromion while deltoid TrPs remain tender. With supraspinatus enthesopathy, active abduction to 90° or more usually causes shoulder pain. Deltoid TrP tenderness persists to palpation regardless of arm position.

Trigger Point Examination

The deltoid is superficial, which simplifies detection. The relaxed muscle is examined by snapping palpation across the TrPs with the arm positioned in approximately 30° of abduction. Palpation at 90° of abduction makes taut bands and twitch responses less evident or undetectable.

  • Anterior deltoid TrPs: Located near the midportion of the anterior part. Often close to the anterior border where the cephalic vein lies subcutaneously between the deltoid and pectoralis major — this landmark helps identify which muscle harbours the TrP when their adjacent fibres make distinction difficult.
  • Middle deltoid TrPs: Can be found almost anywhere in the muscle because of the multipennate arrangement; taut bands are shorter than in the other parts; TrPs are more scattered.
  • Posterior deltoid TrPs: Located along the posterior margin of the muscle, slightly more distally than the anterior TrPs. Longer fibres along the posterior margin may require bimanual palpation to localise.

Active deltoid TrPs produce readily visible or palpable local twitch responses and usually produce transient local aching and nearby referred pain when impaled by a needle.

Impingement mechanism: anterior deltoid + supraspinatus enthesopathy

When a patient reports an acutely painful "catch" at approximately 15° of elevation in the anterolateral direction, this can result from the combination of: (a) severe supraspinatus enthesopathy at the attachment (secondary to supraspinatus TrPs), and (b) a taut band and TrP in the anterior deltoid. The increased anterior deltoid TrP tension compresses the tender enthesopathic region against the acromion, producing an "impingement" syndrome that is greatly relieved by releasing the anterior deltoid TrPs. Full function returns once the enthesopathy clears following supraspinatus TrP inactivation.

Differential Diagnosis

Cardinal differentiating features of deltoid TrP pain

Feature Present / Absent / Variable
Local pain within and around the affected deltoid division Must be present — the defining feature of deltoid referral
No distant referral (no pain below the elbow, no hand pain) Must be present — if present, the primary source lies elsewhere (infraspinatus, subscapularis, supraspinatus, scalenes)
Palpable taut bands with spot tenderness Must be present — the physical sign that distinguishes deltoid TrPs from structural diagnoses
Local twitch response on snapping palpation Present — strongly confirmatory; absent in bursitis, arthritis, tendinopathy
History of direct impact trauma, overexertion, or recent intramuscular injection at the shoulder Highly suggestive when present
Active TrPs in infraspinatus or supraspinatus Commonly present — almost never find isolated deltoid TrPs; look for key TrP sources

Comparison with the most important differential diagnoses

Condition Key differentiating features from deltoid TrPs
Rotator cuff tear Full-thickness tear: drop-arm sign; inability to sustain abduction against gravity; marked supraspinatus weakness on empty-can test; MRI/ultrasound confirmation. Deltoid TrPs produce pain-inhibited weakness, not structural loss of force transmission.
Subdeltoid (subacromial) bursitis When attention is directed to the subacromial referred pain and tenderness zone and deltoid TrPs are overlooked, "subdeltoid bursitis" is often diagnosed, a normal bursa is injected, and the clinical result is poor. Distinguish by palpating the deltoid for taut bands and TrPs before injecting any bursa. If TrPs are found, inactivate them and reassess before proceeding to a bursal injection.
Impingement syndrome May be produced or perpetuated by the anterior deltoid TrP + supraspinatus enthesopathy mechanism (see above). Release the anterior deltoid TrPs and inactivate supraspinatus TrPs before attributing symptoms to structural impingement alone. Positive Neer's and Hawkins–Kennedy impingement tests can occur with the above mechanism.
Glenohumeral arthritis Deltoid TrP pain can mimic pain arising in the glenohumeral joint. A misdiagnosis leads to erroneous joint injection. Note: anterior deltoid TrPs may be inadvertently needled during the anterior approach to the joint, producing apparent pain relief that incorrectly reinforces the joint diagnosis. Inactivate deltoid TrPs and observe the response before injecting the joint.
Acromioclavicular joint pathology The acromioclavicular joint underlies the proximal attachment of the anterior deltoid. AC sprain/subluxation/dislocation mimics anterior deltoid TrP pain, and vice versa. Distinguish: AC pathology produces localised tenderness directly over the joint, pain on passive mobilisation that rotates or elevates the scapula, and increasing motion loss with subluxation/dislocation. Bilateral weight-bearing X-ray (comparing with weight held bilaterally) helps confirm AC joint depression and forward clavicular displacement. Both the joint and the deltoid TrPs may need treatment simultaneously.
Bicipital tendinitis Tenderness specifically at the bicipital groove; positive Speed's test and Yergason's test; pain reproduced by resisted elbow flexion and supination. Deltoid TrP tenderness is in the muscle belly, not the bicipital groove.
C5 radiculopathy Dermatomal sensory deficit (lateral upper arm, deltoid patch); biceps reflex depression or absence; motor weakness in C5 myotome (shoulder abduction and external rotation, elbow flexion); neuroimaging confirms disc or foraminal pathology. Deltoid TrPs produce no reflex change, no dermatomal sensory deficit, and no true myotomal weakness — only pain-inhibited weakness that resolves with TrP inactivation.
Infraspinatus TrPs (satellite relationship) Infraspinatus TrPs commonly drive satellite TrPs in the deltoid. Distinguish by their essential referral zone: infraspinatus refers to the anterior shoulder and extends to the radial forearm and hand. If distant referral is present, the primary source is infraspinatus, not deltoid. Inactivate infraspinatus first.
Supraspinatus TrPs (satellite relationship) Supraspinatus TrPs drive satellite TrPs in the middle and anterior deltoid. If deltoid TrP inactivation restores abduction only to about 90° (not beyond), active supraspinatus TrPs should be sought and eliminated. This usually restores full overhead range of motion.

Entrapment

Entrapment of the axillary nerve due to TrP activity in the deltoid has not been observed.

Treatment

Trigger Point Release

Trigger point pressure release: Applied with the deltoid relaxed in a position of ease (supported at approximately 45° of abduction). Gentle pressure directly against the underlying bony humerus is effective for all three divisions and avoids excessive stretch.

Spray and stretch — anterior deltoid:

Patient seated, leaning back in a relaxed position. Arm lengthened by horizontal abduction (horizontal extension) and lateral rotation. Vapocoolant applied distally along the muscle fibres then over the referred pain zone. Patient exhales slowly while the operator takes up slack. Elbow is slightly flexed to avoid excess stretch on the long head of the biceps.

Spray and stretch — posterior deltoid:

Arm medially rotated and moved horizontally across the chest (horizontal adduction). Spray sweeps directed over posterior deltoid fibres distally, covering the muscle and referred pain zone. This position also stretches the supraspinatus and infraspinatus — include both in the spray pattern if they are tender or if full range is not achieved.

Spray and stretch — middle deltoid:

Two positions, used sequentially:
  • Anterior arm position: Arm flexed and moved into horizontal adduction (also stretches posterior deltoid)
  • Posterior arm position: Arm extended behind the back and adducted as far as possible (also may inactivate anterior deltoid TrPs)
Since all parts are stretched to some degree in these positions, the spray pattern should cover the entire muscle in both positions.

Following any deltoid stretch procedure, the patient moves the arm slowly through three cycles of full active range of motion.

Postisometric relaxation and reciprocal inhibition can be used separately or in conjunction with spray and stretch, and are particularly effective augmentations for this muscle.

Trigger Point Injection

All three parts are readily accessible with flat palpation. TrPs are localised between the fingers and injected. Active deltoid TrPs give readily visible or palpable local twitch responses, usually with transient local aching and nearby referred pain on needle penetration.

Anterior deltoid: Patient supine. TrPs near the midportion of the muscle, close to the anterior border where the cephalic vein lies subcutaneously. Avoid the vein by placing one palpating finger on it, entering the skin close to it, and directing the needle away from the vein into the TrP.

Posterior deltoid: Patient lying on the opposite side. TrPs nearly always in the midbelly region; longer posterior-margin fibres may require bimanual palpation for localisation.

Middle deltoid: Patient partially supine. TrPs more scattered throughout the multipennate muscle; taut bands are shorter.

Following injection: counterpressure for at least 1 minute to ensure haemostasis, then three cycles of active full range of motion for the specific part(s) injected.

Preventing iatrogenic activation

When intramuscular injection of any irritant solution is necessary in the posterior deltoid (the standard injection site for vaccines, B vitamins, etc.):

  1. Palpate the injection site for tender spots (latent TrPs) and avoid injecting into them
  2. Add 2% procaine to the syringe before injection to bring it to a 0.5% procaine solution
  3. If a TrP was inadvertently activated (continuing pain at the injection site), immediately pepper the site with 1 ml of 0.5% procaine solution
  4. Perform routine stretch and spray of the muscle following any intramuscular injection at this site
  5. Alternatively, select the lateral thigh as the injection site

Note on Satellite TrP Relationships

Deltoid as satellite (driven by key TrPs elsewhere):

  • Infraspinatus — most important key TrP; experimentally demonstrated to drive motor unit activity (referred spasm) in the anterior deltoid
  • Supraspinatus — if deltoid TrP inactivation fails to restore abduction beyond 90°, supraspinatus TrPs are the residual limiting factor; inactivating them usually restores full overhead motion
  • Scalenes — reported to drive satellite TrPs in the deltoid

Treatment implication: Any TrPs that refer pain to the deltoid region should be inactivated first. If deltoid TrPs recur after direct treatment, key TrPs in the infraspinatus and supraspinatus have not been fully addressed.

Associated TrPs by division:

  • Anterior deltoid TrPs are associated with TrPs in the clavicular section of pectoralis major, biceps brachii, and the posterior deltoid (antagonist)
  • Posterior deltoid TrPs are associated with TrPs in the proximal long head of triceps brachii, latissimus dorsi, and teres major; posterior deltoid is unlikely to develop TrPs alone unless latent TrPs were activated by a local irritant injection

Patient Education

Activity Modification

  • When lifting heavy objects: rotate the arm so the thumb points in the direction that unloads the affected deltoid division (anterior TrPs — lateral rotation; posterior TrPs — medial rotation)
  • Take precautions on stairs; traverse slowly holding the railing and watching foot placement to prevent a near-fall and the reflex grab that overloads the muscle
  • Shooting enthusiasts: use a shoulder pad to minimise the impact of gun recoil on the anterior deltoid

Home Stretching

For continuing relief, daily passive stretching of the affected part may be necessary:

  • Anterior deltoid self-stretch: Middle and lower hand positions of the In-doorway Stretch Exercise (Fig. 42.10 in Travell & Simons) and the Against-doorjamb Exercise (Fig. 30.7 in Travell & Simons), performed slowly without forcing.
  • Posterior deltoid self-stretch: Arm in the horizontal adduction position (Fig. 28.4B); grasp the elbow of the affected arm with the other hand and pull it across the chest; perform seated under a warm shower with water directed over the muscle.

Satellite Trigger Points

Anterior deltoid:

Posterior deltoid:

Key TrP sources driving deltoid satellites:

  • Infraspinatus — most important key TrP; both sides of deltoid lie in its essential referral zone
  • Supraspinatus — drives middle and anterior deltoid satellites; inactivate to restore abduction beyond 90°
  • Scalenes — reported key TrP source for deltoid satellite activity

References

  • Travell JG, Simons DG. Myofascial Pain and Dysfunction: The Trigger Point Manual, Volume 1: The Upper Half of Body. 2nd ed. Baltimore: Williams & Wilkins; 1999. Chapter 28.
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