Muscle:Serratus Anterior

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Serratus anterior is a large, fan-shaped muscle of the lateral thorax that wraps around the rib cage from the anterolateral chest to the deep surface of the scapula. Its trigger points (TrPs) are one of the most clinically under-examined sources of chest, back, and arm pain in practice, and one of the most consequential: the interscapular pain they produce is so persistently intense, so refractory to positional relief, and so rarely linked to this muscle that patients routinely end up with a diagnosis of enigmatic or behavioural pain and the end of their normal way of life. Left unidentified, serratus anterior TrPs play the same initiating role in a unilateral interscapular pain syndrome that subscapularis plays in frozen shoulder — the syndrome will not resolve until this muscle's TrP component is identified and inactivated.

Serratus anterior TrPs also produce a respiratory symptom complex — dyspnea, air hunger, inability to finish a sentence without pausing to breathe — that is commonly referred for cardiopulmonary workup. Relief is immediate and complete once TrPs are inactivated.

Anatomy

The serratus anterior is composed of three structurally and functionally distinct groups of fibres, all running from the anterolateral ribs to the costal (deep anterior) surface of the scapula's vertebral border.

Superior group (first serration):

  • Attaches anteriorly to rib 1 (and sometimes rib 2); connects posteriorly to the superior angle of the scapula
  • Fibres run nearly parallel to the underlying ribs

Middle group (second and third serrations):

  • Attaches anteriorly to approximately half the length of ribs 2 and 3; attaches posteriorly to the length of the vertebral border of the scapula
  • Forms a flat sheet of nearly parallel fibres at approximately 45° to the ribs

Inferior group (ribs 4–8 or 9):

  • The strongest part of the muscle; five or six serrations converging in a quarter-circle fan posteriorly on the inferior angle of the scapula
  • The lowest digitations interdigitate anteriorly with the costal attachments of the external oblique muscle
  • Central TrPs develop near the midpoint of each digitation near the midaxillary line

Innervation: Long thoracic nerve of Bell, from anterior rami of C5, C6, C7, and sometimes C8. The upper portion derives mainly from C5; the middle from C5 and C6; the lower portion mainly from C6 and C7. The long thoracic nerve lies superficial to the muscle in the line of the anterior axillary fold, anterior to the usual TrP location.

Primary functions:

  • Rotates the scapula to turn the glenoid fossa upward — essential for arm elevation above 30°; the inferior triangular group is the key functional unit for this rotation
  • Abducts and protracts the scapula — stabilises the scapula against the thoracic wall during forward-pushing efforts
  • Elevates the scapula (middle portion)
  • Holds the medial border of the scapula firmly against the thorax — prevents winging
  • Accessory muscle of inspiration in demanding or abnormal situations

Functional synergists: Trapezius (upward rotation), pectoralis minor and upper pectoralis major (protraction), levator scapulae (elevation).

Functional antagonists for abduction: Horizontal fibres of latissimus dorsi, rhomboids, middle trapezius.

Referred Pain Patterns

Essential zones (must be present):

  • Anterolateral chest at midchest level — the primary and most consistently present zone
  • Separate posterior area medial to the inferior angle of the scapula — the interscapular component

Spillover zones (may be present):

  • Down the medial aspect of the arm
  • Extending to the palm and ring finger

Clinically important notes on the interscapular pattern:

  • The interscapular pain is particularly annoying and distressing — persistently intense, refractory to positional relief
  • Few practitioners examine the serratus anterior based on this complaint, because the TrP source is on the anterior chest wall and the pain is posterior
  • This mismatch between TrP location and pain location is the primary reason the diagnosis is missed
  • In a unilateral interscapular pain syndrome involving the rhomboids, middle trapezius, and paraspinal muscles, the serratus anterior is frequently the key unidentified perpetuating TrP — analogous to subscapularis in frozen shoulder

Respiratory symptom pattern:

  • Chest pain at rest in severe cases
  • "Stitch in the side" precipitated by deep breathing during running — distinguished from external oblique TrPs (stitch slightly lower) and diaphragm TrPs by the chest location
  • "Short of breath" or "can't take a deep breath, it hurts"
  • Inability to finish an ordinary sentence without pausing to breathe — particularly bothersome when talking on the telephone
  • Rapid shallow respirations with a feeling of air hunger — these revert to normal depth immediately when all active serratus anterior TrPs are inactivated

Cardiac association: Serratus anterior TrPs can contribute to the pain associated with myocardial infarction; the pain has been relieved by inactivating pectoral and serratus anterior TrPs on the left side. Left-sided serratus anterior pain must never be attributed to a myofascial source until cardiac disease has been excluded.

Activation and Perpetuating Factors

Physical overload:

  • Excessively fast or prolonged running — the "stitch in the side" mechanism; particularly the lower digitations
  • Push-ups and body-lift exercises — sustained loading in the shortened position
  • Lifting heavy weights overhead
  • Severe coughing due to respiratory disease — one of the most common mechanisms
  • Chinning on a bar

Torsional stress — the characteristic activation mechanism:

  • Abrupt forceful turning of a steering wheel without power steering (particularly to avoid an accident) — the thorax rotates vigorously while the upper limb is in a fixed position
  • EMG monitoring confirms serratus anterior activity during contralateral rotation of the top of the steering wheel in almost all cases
  • Any activity combining vigorous thoracic rotation with a fixed upper limb

Psychogenic factor:

  • High levels of anxiety appear to increase the likelihood of serratus anterior TrPs — the only muscle in T&S for which this association is specifically noted

Emphysema paradox: Patients with emphysema are not especially prone to developing serratus anterior TrPs, possibly because the overexpanded barrel chest keeps the muscle in a stretched position, preventing the sustained shortening that promotes TrP development.

Clinical Examination

Postural Assessment

Observe from behind:

  • Abduction and rotation of the scapula by tense serratus anterior fibres produces prominence of the superior border and spine of the scapula on the affected side — the scapula stands out from behind
  • From the front, the patient has a unilateral round-shouldered posture (distinguished from pectoralis major involvement, which is usually bilateral)
  • Some patients show winging of the scapula — paradoxically caused by TrP inhibition of the serratus anterior and facilitation of its antagonists, not by taut band tension

Respiratory Assessment

This is an essential examination step in any patient presenting with chest pain, dyspnea, or unilateral interscapular pain.

Observe thoracic movement during respiration:

  • Active TrPs inhibit expansion of the lower chest
  • On inspiration, the patient can expand the upper thoracic cage but lower chest circumference expansion is markedly restricted
  • After inactivation: a smaller minimum and a larger maximum lower chest circumference; marked increase in tidal volume; immediate relief of respiratory pain and dyspnea
  • Before treatment: patient likely overuses accessory muscles of respiration in the neck, and makes poor use of the diaphragm — this represents reflex inhibitory influences on respiration

Scapular Range of Motion

The serratus anterior can be tested directly for restricted range of motion using the same position as for spray and stretch (patient sidelying, arm drawn backward):

  • Move the patient's elbow posteriorly and lower it toward the table, monitoring scapular position by palpation
  • Scapular adduction range may be limited by TrPs
  • Pain at the end of available movement — in contrast to the greater and pain-free range on the contralateral, uninvolved side

Scapulohumeral rhythm: Stand behind the patient and observe scapulohumeral rhythm during arm flexion and abduction. Range of elevation may be within normal limits, but scapulohumeral rhythm and muscle balance can be disrupted by serratus anterior TrPs.

Trigger Point Examination

Patient position: Semisupine — turned halfway toward the opposite side with the ipsilateral arm partly extended. This position rotates the anterior axillary fold landmark into alignment with the midaxillary line, and allows the muscle to be palpated against the ribs.

TrP location: Usually in the subcutaneous portion of the muscle at the midaxillary line at approximately the level of the nipple, over the fifth or sixth rib. Occasionally higher or lower.

Technique: Flat palpation against the ribs reveals a tender nodule in a palpable band within the muscle, just under the skin. Pressure on the nodule reproduces the patient's pain complaint. Snapping palpation at the point of exquisite tenderness can induce a local twitch response in the palpable taut band.

Important technical note: When the operator extends the arm backward to adduct the scapula for palpation, the midaxillary line projected down the chest aligns with the anterior axillary fold — this landmark alignment helps locate the muscle.

Articular dysfunction caveat: Abnormal tension of the serratus anterior alone can make it appear as if articular dysfunction is present when it is not — the apparent articular dysfunction is simply the result of the increased muscle tension caused by myofascial TrPs. Inactivation of the myofascial TrPs alleviates whatever apparent articular dysfunction is present.

Nerve Entrapment

No reports were found of nerve entrapment by the serratus anterior muscle itself.

However, two of the three cervical roots (C5, C6, C7) that form the long thoracic nerve pass through the scalenus medius muscle before reaching the nerve. TrP activity in the scalenus medius can entrap these roots, compromising the nerve supply to the serratus anterior. This is therefore an indirect entrapment — at the scalene level, not the serratus anterior level.

Clinical implication: Serratus anterior weakness from long thoracic nerve dysfunction may originate in scalene TrPs. Winging of the scapula due to a neuropathic process of the long thoracic nerve may not be painful if predominantly motor nerve fibres are involved — a neurogenic cause must be carefully considered regardless of whether pain is present. A C7 root lesion can also cause winging of the scapula (distinct from serratus anterior TrP winging).

Differential Diagnosis

Condition Must-have distinguishing features Notes
Costochondritis Localised tenderness directly over the costochondral junctions; pain reproduced by direct pressure on the joint; no TrP tenderness in the midaxillary line Serratus anterior TrPs may coexist with or mimic costochondritis; examine the muscle before accepting this diagnosis
Intercostal nerve entrapment Dermatomal band of pain or dysesthesia following the intercostal space; hyperesthesia or hypoesthesia in the intercostal distribution Serratus anterior TrP pain does not follow a strict intercostal dermatome; TrP tenderness in the midaxillary line is the key distinguishing finding
C7–C8 radiculopathy Dermatomal sensory deficit; motor weakness in the myotome; reflex changes; neuroimaging correlation Serratus anterior weakness can result from scalenus medius entrapment of the cervical roots forming the long thoracic nerve — electrodiagnostics required to distinguish
Herpes zoster Dermatomal vesicular eruption; burning quality; allodynia; prodromal phase may precede vesicles by days Examine the skin carefully in any patient with unilateral anterior chest pain; zoster may precede the rash by several days
Broken rib / stress fracture Point tenderness over a specific rib; history of trauma or repetitive stress; confirmed by imaging; a stress fracture of the first rib has been attributed to serratus anterior tension Serratus anterior TrP chest pain does not localise to a single rib point; no bony tenderness
Pleurisy / cardiac pain Left-sided serratus anterior TrP pain can contribute to or mimic cardiac pain; serratus anterior TrPs have contributed to pain of myocardial infarction Cardiac and pulmonary disease must always be excluded before attributing left-sided chest pain to myofascial TrPs — see Trigger Point
Long thoracic nerve palsy (neuropathic winging) Winging present without pain or with only mild pain; predominantly motor nerve involvement; electrodiagnostic evidence of denervation TrP-caused winging is accompanied by other TrP signs (tenderness, taut bands, referred pain) and resolves when TrPs are inactivated; neuropathic winging persists

Back pain component: When serratus anterior TrPs produce interscapular pain, this requires active consideration of TrPs in the middle trapezius, rhomboids, and paraspinal muscles as co-contributors. Midthoracic articular dysfunction can produce similar symptoms. The back pain will not fully resolve until the serratus anterior TrP component is identified and treated.

Treatment

Trigger Point Release

Spray and stretch (Fig. 46.4 in T&S):

If the serratus anterior is primarily weak, TrPs are most effectively treated first by trigger point pressure release or postisometric relaxation, not by stretch techniques.

For spray and stretch:

  1. Patient lies on the uninvolved side with the back toward the operator and the uppermost arm drawn backward — the weight of the arm initiates passive stretch of the serratus anterior
  2. Operator applies vapocoolant anteroposteriorly over the muscle before and during release
  3. The patient assumes the position in Figure 46.4A; the clinician monitors progress and assists in taking up slack as the patient's arm weight assists scapular adduction; the pelvis is blocked from rotation by the clinician's hip
  4. Full adduction position (Fig. 46.4B): the operator's hand holds the shoulder to guide the scapula back; the patient takes a deep breath and momentarily holds it to enlarge the lower rib cage (this further stretches the muscle); vapocoolant is applied in slow parallel sweeps from the TrP area backward along the fibre direction, then over the posterior pain reference zone, then over the anterior pain reference zone
  5. Sweeps are extended down the arm to the palm in patients with arm referral

PIR augmentation:

  • Patient in Figure 46.4B position, clinician's hand stabilising the scapula
  • Patient reaches anteriorly (toward the ceiling) attempting to abduct the scapula — contract phase
  • Patient relaxes, slowly breathes out while allowing gravity to assist arm drop and scapular adduction — lengthening the serratus anterior
  • Gentle voluntary effort to assist the stretch adds reciprocal inhibition

TrP pressure release: Highly effective as the initial treatment, especially when the muscle is primarily weak. Also used to clean up residual TrPs following spray and stretch or injection. See Apropos Treatment.

Trigger Point Injection

See Concept:Trigger_Point_Injection.

  • Patient lying on the contralateral side (same as spray and stretch)
  • TrP located by flat palpation and pinned against a rib between the fingers of one hand
  • Needle directed toward the rib at a shallow angle nearly tangential with the chest wall — the TrP lies in the thin layer of muscle between the rib and the skin
  • The pain reaction on needle contact is often less intense than that from TrPs in many other muscles
  • Replace immediately any needle that has developed a burr on its tip from contact with bone
  • Since the long thoracic nerve supplies exclusively the serratus anterior, some degree of temporary anesthesia of this motor nerve is to be expected when injecting a local anaesthetic — the patient will not notice temporary weakness of part of the serratus anterior in the absence of any skin sensory change

Myofascial Release

Myofascial release of the pectoral, thoracic, and lumbodorsal fascial structures is an additional beneficial technique for this region.

Patient Education

Patients must avoid or modify the activities that activated the TrPs. Key instructions:

  • Coughing: Learn to clear the throat rather than cough; reduce cough reflex stimulation
  • Breathing: Use synchronised (not paradoxical) abdominal and chest breathing — see Perpetuating Factors
  • Exercise: Avoid push-ups, heavy overhead lifting, and hanging from or chinning on a bar until TrPs are fully inactivated
  • Sleep: Patients with very irritable TrPs cannot sleep on the affected side (direct pressure on TrPs). They also cannot sleep on the other side if the arm of the affected side falls forward onto the bed, placing the muscle in the shortened cramped position — remedy with a pillow to support the arm and keep the scapula from falling forward (see Fig. 22.6 in T&S)

Serratus Anterior Self-stretch Exercise (Fig. 46.6 in T&S):

  • Patient seated, stabilises the scapula of the involved side by placing the ipsilateral arm behind the chair back
  • Takes a deep breath, then exhales slowly while turning the thorax toward the contralateral side (rotating the front of the chest away from the involved side)
  • Some patients include PIR — contracting first, then relaxing into the stretch on exhalation

In-doorway Stretch Exercise: Lower and middle hand positions (Fig. 42.9 in T&S) can also be used for the home programme.

Satellite Trigger Points

Serratus anterior as the key TrP in a unilateral interscapular syndrome:

In a multiple-TrP, predominantly unilateral interscapular pain syndrome, the serratus anterior is often the key unidentified TrP maintaining the whole complex. Associated muscles frequently involved include:

Muscles that may become overloaded and develop associated TrPs due to serratus anterior dysfunction:

  • Latissimus Dorsi — antagonist for abduction; may become overloaded
  • Scalene — neck accessory muscles of inspiration; surprisingly, may develop TrPs secondary to serratus anterior involvement (both are accessory inspiratory muscles)
  • Sternocleidomastoid — accessory muscle of inspiration; may develop associated TrPs

Serratus posterior superior: Serratus posterior superior may also be involved in the interscapular component of the pain syndrome.

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 46.
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