What Are Accessory Breathing Muscles?

Primary vs Accessory Respiration

Breathing at rest is, under normal conditions, accomplished almost entirely by the diaphragm — the dome-shaped musculotendinous structure separating the thoracic and abdominal cavities — with a modest contribution from the external intercostals elevating the ribs. The diaphragm is by far the most powerful breathing muscle, generating approximately 70–80% of tidal volume in healthy diaphragmatic breathers. During exercise, deep breathing, respiratory illness, or any condition in which ventilatory demand rises, additional muscles are recruited to augment thoracic expansion: these are the accessory breathing muscles. They are called accessory not because they are unimportant, but because their primary anatomical role is postural and movement-related rather than respiratory — they assist breathing as a secondary function.

The distinction matters clinically because the accessory muscles are not designed for the repetitive endurance load of continuous tidal breathing. At 12–16 breaths per minute, 20,000 or more cycles per day, continuous accessory muscle recruitment at rest produces the muscular fatigue, trigger point formation, and myofascial shortening that underlie many chronic cervical and upper thoracic pain presentations.

Anatomy of the Accessory Breathing Muscles

The principal accessory breathing muscles — the muscles most commonly implicated in dysfunctional breathing patterns — are the following. The scalenes (anterior, middle, and posterior) originate on the transverse processes of C2–C7 and insert on the first and second ribs. Their respiratory function is to elevate the first and second ribs during forced or accessory breathing, expanding the upper thorax. They are also cervical flexors, lateral flexors, and rotators, and they form the lateral wall of the posterior cervical triangle through which the brachial plexus and subclavian vessels pass — making their chronic shortening directly relevant to thoracic outlet presentations.

The sternocleidomastoid (SCM) originates on the mastoid process and superior nuchal line, and inserts on the manubrium and medial clavicle. During forced inspiration (and in upper chest breathers at rest), it elevates the sternum, expanding the upper thoracic cage. Its chronic overactivation produces the anterior neck tension, restricted cervical rotation, and referred pain to the temple and behind the eye that clinicians frequently encounter in patients who present describing tension headaches and neck stiffness.

The upper trapezius and levator scapulae are recruited in upper chest breathing through their action on the shoulder girdle and cervical spine — elevating the scapulae and cervical transverse processes creates more thoracic room during expansion. Their involvement explains why anxious or stressed patients frequently carry their shoulders elevated and forward, and why myofascial trigger points in these muscles are so consistently found in breathing dysfunction presentations. The pectoralis minor, attaching from the third, fourth, and fifth ribs to the coracoid process, assists in elevating the anterior ribs and drawing them anteriorly. Its shortening — a consistent finding in upper chest breathers — produces the characteristic rounded shoulder and anteriorly tilted scapular posture that compromises subacromial space and scapulothoracic mechanics.

Clinical Features of Accessory Muscle Overuse

The clinical presentation of accessory breathing muscle overuse is distinctive once recognised. Patients describe persistent, bilateral upper trapezius tension that returns within days of massage. Suboccipital aching, temporal headache, and restricted cervical rotation are common. Anterior neck tightness and a visible anterior scalene bulk on inspection are found. Deep palpation of the anterior scalene and SCM reproduces familiar head, neck, or arm symptoms. Shoulder mobility is frequently restricted due to pectoralis minor shortening and scapular malposition. Importantly, the breathing pattern at rest shows upper chest dominance — visible expansion in the upper thorax, minimal abdominal rise, and often a resting respiratory rate above 14 breaths per minute.

Treatment

Treatment must address both the overloaded musculature and the underlying breathing pattern. Manual release of the scalenes, SCM, pectoralis minor, suboccipitals, and upper trapezius — through direct compression, myofascial release, and dry needling — reduces the acute muscular load. Thoracic joint mobilisation restores the thoracic extension and rib mobility that allows the diaphragm to function without compensation. Diaphragmatic retraining is then the essential step: without restoring the respiratory primary mover, the accessory muscles will re-load within days.