What Are the Accessory Muscles of Respiration?

The accessory muscles of respiration are muscles throughout the neck, chest, back, and abdomen that assist breathing when the diaphragm and the primary intercostal muscles cannot meet the body’s demand for air on their own. During normal, quiet breathing at rest, the diaphragm does most of the work. But during exercise, respiratory illness, forced coughing, or even singing, the body recruits a wider roster of muscles to generate the extra pressure needed to move air in and out of the lungs efficiently.¹ Understanding which muscles these are, what triggers them, and when their visible engagement signals a problem turns out to be more interesting than the textbook list suggests.

Why the Diaphragm Needs Help

The diaphragm is a dome-shaped sheet of muscle sitting beneath the lungs. When it contracts, it flattens downward, expanding the chest cavity and pulling air in. At the same time, the external intercostal muscles between the ribs lift and spread the rib cage slightly. For a person sitting quietly and reading this article, that duo handles the entire job. Exhalation at rest is mostly passive: the diaphragm and chest wall simply relax, and the natural elastic recoil of the lungs pushes air out.

The system runs into limits when ventilation demands increase. Sprinting, blowing out birthday candles, coughing hard, fighting pneumonia, or breathing against an obstructed airway all require more pressure than the diaphragm and intercostals can comfortably generate alone. That is when the accessory muscles kick in. They are not a separate system; they are skeletal muscles with other primary jobs (turning your head, stabilizing your spine, flexing your trunk) that the brain’s respiratory centers can conscript when breathing effort ramps up.

The Inspiratory Accessory Muscles

The accessory muscles that help you breathe in are concentrated in the neck and upper chest. The two most clinically recognized are the sternocleidomastoid (SCM) and the scalenes.

The SCM is the thick muscle running from behind each ear down to the collarbone and breastbone. Its day job is turning and tilting the head. During labored inspiration, the SCM pulls the sternum and clavicles upward, expanding the upper rib cage. Research on patients with chronic obstructive pulmonary disease (COPD) has found that the thickening of the SCM during inspiration strongly correlates with exercise tolerance, more so than lung function measures like airflow limitation alone.² In other words, people whose SCM muscles are better at pitching in during breathing tend to tolerate physical activity better when their lungs are compromised.

The scalenes are a group of three muscles on each side of the neck, running from the cervical spine down to the first and second ribs. They lift the upper ribs during inspiration. In people with intact diaphragm function, scalene activity during quiet breathing is minimal or absent. But in individuals with spinal cord injuries who have lost diaphragm control, the scalenes often fire during every breath, becoming a primary engine rather than an accessory one.³

The trapezius, the large diamond-shaped muscle spanning the back of the neck and upper back, also contributes. When researchers increased the resistance subjects had to breathe against while lying on their backs, the trapezius showed a greater rise in electrical activity than either the SCM or scalenes.⁴ This makes sense mechanically: the trapezius can elevate the shoulders and upper rib cage, creating more room for the lungs to expand. Its role is especially relevant when a person is lying flat, since gravity works against the rib cage in that position.

Other muscles that can assist inspiration include the pectoralis minor, which pulls the ribs upward when the shoulder is braced, and the latissimus dorsi, which in COPD patients actually shows structural changes. Fibers of the latissimus dorsi increase in diameter in proportion to the severity of airflow obstruction, suggesting the muscle physically adapts to its new respiratory workload over time.⁵

The Expiratory Accessory Muscles

While normal exhalation at rest is passive, forced exhalation absolutely is not. Coughing, sneezing, blowing, singing a sustained note, and breathing out hard during exercise all require active muscular compression of the abdominal and thoracic cavities. The main expiratory accessory muscles are in the abdominal wall.

During forced expiration, the external oblique and internal oblique muscles are the stars. At maximum expiratory effort, both obliques produce significantly more electrical activity than the rectus abdominis (the “six-pack” muscle), making the obliques the dominant force generators for pushing air out of the lungs.⁶ This hierarchy holds even at relatively low levels of effort, not just during all-out exhalation.

The transversus abdominis, the deepest abdominal muscle layer, also contributes. During graded expiratory loading, where subjects breathe out against progressively higher resistance, the transversus fires first and strongest, followed by the internal oblique, with the rectus abdominis and external oblique joining in roughly equal measure only at higher loads.⁷ During a cough, however, all four abdominal muscles fire simultaneously and with far greater force than during any level of simple resistance breathing. Gastric pressures during a cough can be at least eight times higher than during the highest expiratory loading, which explains why a violent coughing fit can leave your abs sore the next day.⁷

The internal intercostal muscles (distinct from the external intercostals that assist inspiration) also help compress the rib cage during active exhalation. Together, this team of abdominal and intercostal muscles works to increase intra-abdominal and intrathoracic pressure, pushing the diaphragm upward and squeezing the lungs to expel air faster and more completely.

Upper Airway Muscles You Might Not Expect

Not all accessory respiratory muscles are in the torso. Several small muscles in the face and throat play a supporting role by keeping the airway open so that the bigger muscles can move air efficiently.

The alae nasi, the tiny muscles that flare the nostrils, are a classic example. Their activity reduces nasal resistance by roughly 20 to 23%, which either lowers the effort needed to pull air through the nose or increases the airflow rate for the same effort.⁸ In preterm infants, who breathe obligately through the nose, nasal flaring is a critical mechanism for maintaining adequate ventilation. In adults, nasal flaring during breathing is a red flag for clinicians: it signals that the person is working unusually hard to breathe.

The genioglossus, the main muscle of the tongue, stiffens and pulls the tongue forward during inspiration to prevent it from falling back and blocking the pharynx. During REM sleep, when most skeletal muscles lose tone, the genioglossus still shows phasic bursts of activity timed to inspiration, and its activity actually increases in the second half of a REM period.⁹ When the genioglossus fails to fire adequately during sleep, the tongue collapses backward, and the result is the hallmark event of obstructive sleep apnea.

A comprehensive review of respiratory muscles catalogued muscles all the way from the cranial region down through the cervical and thoracic areas, emphasizing that recognizing these lesser-known contributors can sometimes offer clinicians predictive clues during diagnostic evaluation.¹⁰

When Accessory Muscle Use Becomes a Clinical Sign

In a healthy person at rest, visible use of the accessory muscles of respiration is not normal. When a doctor or paramedic sees the SCM straining, the nostrils flaring, or the intercostal spaces retracting inward with each breath, they recognize increased work of breathing. It is one of the most immediate and reliable bedside signs that something is wrong, whether it is an asthma exacerbation, a COPD flare, pneumonia, or an upper airway obstruction.

In COPD, the relationship between accessory muscles and disease progression is nuanced. Hyperinflation of the lungs pushes the diaphragm flat, which strips it of its mechanical advantage. The flatter the dome, the less downward excursion it can produce, and the less pressure change each contraction generates. The same principle applies to the intercostals and scalenes: as the chest wall expands chronically, these muscles lose leverage too.⁵ The accessory muscles compensate, and over months and years they remodel. The SCM thickens. The latissimus dorsi fibers enlarge. The body adapts, but at a metabolic cost: keeping these muscles active breath after breath is energy-intensive.

When the diaphragm is partially or fully paralyzed, the compensation story is even more dramatic. In animal models of unilateral diaphragm paralysis, the ability to generate pressure across the diaphragm dropped by about 40% immediately after paralysis, but within two weeks the deficit improved to roughly 20%, partly through increased recruitment of other inspiratory muscles.¹¹ People with spinal cord injuries that spare the neck but paralyze the trunk face a permanent version of this: the scalenes and SCM become their primary breathing muscles, while the loss of abdominal muscle control makes coughing and forced exhalation severely impaired.³

Accessory Muscles During Exercise and Heart Failure

Even in healthy people, exercise is the most common trigger for accessory muscle recruitment. As you transition from a walk to a jog to an all-out sprint, the breathing muscles consume an increasing share of your total oxygen budget. In healthy individuals, the respiratory muscles claim a modest slice of oxygen during moderate exertion.

In heart failure, the picture changes. Patients with heart failure with preserved ejection fraction (HFpEF) devote a substantially larger share of their total oxygen consumption to the respiratory muscles during exercise. At a moderate workload, their breathing muscles consumed about 8.3% of total body oxygen, compared to roughly 4.6% in healthy controls.¹² This “respiratory steal” diverts blood and oxygen away from the working limb muscles, which likely contributes to the exercise intolerance that defines the condition. When a larger fraction of each heartbeat’s output goes to keeping the breathing muscles running, there is less left for the legs.

The SCM appears to be particularly relevant to exercise capacity in lung disease. In COPD patients, the correlation between how much the SCM thickened during inspiration and peak exercise performance was strong, explaining more of the variation in exercise tolerance than body mass index or even some lung function measurements.²

How Breathing Changes During Sleep

Sleep alters the accessory muscle landscape in ways that matter for airway stability. During non-REM sleep, muscle tone drops mildly, but the respiratory system stays relatively stable. During REM sleep, most voluntary skeletal muscles become atonic, which is the normal mechanism that prevents you from acting out your dreams.

Respiratory muscles are partially exempt from this atonia, but not uniformly. The diaphragm keeps working throughout REM. The genioglossus, the tongue muscle that holds the airway open, shows bursts of phasic activity that actually increase as a REM period progresses, with activity rising by about 64% compared to non-REM levels in the second half of a REM episode. The diaphragm itself shows increased breath count (about 28% more breaths) and breath amplitude (about 30% greater) during late REM.⁹ Meanwhile, the intercostal muscles lose most of their contribution during REM, leaving the diaphragm and upper airway muscles to pick up the slack. This is why REM sleep is the period when obstructive sleep apnea events are most frequent and severe: if the upper airway muscles cannot compensate for the loss of rib-cage muscle tone, the airway collapses.

Can You Train Accessory Respiratory Muscles?

Respiratory muscle training has gained attention in both clinical rehabilitation and athletic performance circles. The basic idea is straightforward: just as you can strengthen your biceps by curling dumbbells, you can strengthen your breathing muscles by breathing against resistance.

Inspiratory muscle warm-up protocols illustrate this. When healthy subjects performed high-intensity inspiratory breathing exercises against resistance, the electrical activity of the SCM and intercostal muscles increased significantly compared to moderate-intensity or placebo conditions. Maximal inspiratory pressure improved by about 6.5% after the high-intensity warm-up, and the gains correlated directly with how much more the SCM and intercostals activated during the protocol.¹³ The correlation between SCM activation and inspiratory strength gain was particularly strong, suggesting that getting these muscles to fire harder translates fairly directly into the ability to generate greater inspiratory force.

For people with COPD or neuromuscular disease, respiratory muscle training can be genuinely life-improving. Stronger accessory muscles can improve cough effectiveness, reduce the sensation of breathlessness, and increase exercise tolerance. Some rehabilitation programs focus specifically on expiratory muscle training to improve cough strength, since the abdominal muscles that generate cough force are the same ones weakened by deconditioning, obesity, or spinal cord injury.

Accessory Muscles and the Voice

Singing, speaking loudly, and sustained vocalization all depend on precise control of airflow, which in turn depends on both inspiratory and expiratory accessory muscles. The relationship is especially important for people with spinal cord injuries, who have lost much of their abdominal muscle control and rely on residual or compensatory muscle function.

In a study of people with spinal cord injuries, different song characteristics activated different accessory muscles. The SCM was most active while singing high-pitched songs, which demand large lung volumes and quick inhalations. The rectus abdominis, by contrast, was most active during slow-tempo songs with simple rhythms, when the challenge shifts to controlling a slow, steady stream of exhaled air.¹⁴ This finding underscores that the accessory muscles are not an all-or-nothing system. They are recruited selectively, in patterns that match the specific mechanical demand of the moment. A fast, deep gasp before a high note uses the neck muscles; a long, controlled exhale on a low sustained note uses the abdominals.

Professional singers and wind instrument players develop this selective recruitment to a remarkable degree through years of practice, even if they could not name the muscles involved. Voice therapists and singing coaches who work with patients recovering from neurological injuries increasingly use targeted exercises that exploit these muscle-specific recruitment patterns, choosing song tempos and pitch ranges to challenge specific parts of the respiratory apparatus.

Posture and Body Position

How you hold your body changes which accessory muscles can contribute and how effectively they work. The tripod position, where a person leans forward with hands braced on the knees, is instinctively adopted by people in respiratory distress. It works because bracing the arms fixes the shoulder girdle in place, which allows the pectoralis minor and SCM to pull more effectively on the rib cage instead of just moving the shoulders.

Lying flat, on the other hand, puts the accessory muscles at a disadvantage. Gravity compresses the chest, abdominal contents push the diaphragm upward, and the rib cage muscles have to work harder. The trapezius appears to compensate more aggressively in the supine position, showing a steeper rise in activity than either the SCM or scalenes as inspiratory resistance increases.⁴ This is one reason why patients with severe COPD or heart failure often cannot tolerate lying flat and prefer to sleep propped up on pillows: the upright position gives their accessory muscles better leverage and lets gravity pull the diaphragm into a more mechanically favorable shape.

Obesity adds another layer. Excess abdominal fat pushes the diaphragm upward and restricts its movement, forcing the accessory muscles to compensate during routine breathing. Over time, this chronic extra workload can lead to respiratory muscle fatigue, which compounds the breathlessness many people with obesity experience during physical activity.