Your lungs cannot move on their own. They have no muscle tissue and depend entirely on surrounding muscles to pull air in and push it back out. Every breath you take, from a quiet inhale at your desk to a gasping recovery after a sprint, is powered by coordinated contractions of muscles in your torso and neck. The muscular system doesn’t just “help” the respiratory system; it is the engine that makes breathing physically possible.
The Diaphragm Does Most of the Work
The diaphragm is a dome-shaped sheet of muscle sitting beneath your lungs, separating the chest cavity from the abdomen. During quiet breathing, it accounts for roughly 90% of the air you pull into your lungs with each breath. When the diaphragm contracts, it flattens and pulls downward, expanding the chest cavity. That expansion lowers the air pressure inside your lungs below the pressure of the air outside your body, and air rushes in to fill the gap.
When you exhale during normal, relaxed breathing, the diaphragm simply relaxes. It springs back up into its dome shape, the chest cavity shrinks, pressure inside the lungs rises above atmospheric pressure, and air flows out. This passive recoil is why quiet exhalation takes almost no muscular effort at all.
The diaphragm is controlled by the phrenic nerve, which originates from nerve roots in the neck (C3 through C5) and travels all the way down through the chest to reach the muscle. This nerve carries both the motor signals that trigger contraction and sensory information back from the diaphragm’s central tendon. If the phrenic nerve is damaged on both sides, the diaphragm becomes paralyzed and a person loses the ability to breathe independently.
Intercostal Muscles Expand and Compress the Rib Cage
Between each pair of ribs sit two thin layers of muscle called the intercostals. The external intercostals, the outer layer, are primarily active during inhalation. They pull the ribs upward and outward, widening the chest cavity to complement the diaphragm’s downward pull. Together, these two actions increase the volume of the chest from two directions at once.
The internal intercostals, the inner layer, do the opposite. They draw the ribs downward and inward, compressing the chest cavity and forcing air out. During calm breathing, you rarely need them because the elastic recoil of the lungs handles exhalation on its own. But during a cough, a sigh, blowing out a candle, or heavy exercise, the internal intercostals actively squeeze the rib cage to push air out faster and more forcefully.
Abdominal Muscles Power Forced Exhalation
When you need to exhale harder than passive recoil allows, four abdominal muscles step in: the rectus abdominis (the “six-pack” muscle), the external and internal obliques on each side, and the transverse abdominis that wraps around the trunk like a corset. These muscles compress the abdominal wall and its contents, driving up the pressure inside the abdomen. That elevated pressure pushes the relaxed diaphragm upward into the chest, rapidly shrinking the space available for the lungs and forcing air out.
This mechanism is what makes coughing, sneezing, and laughing possible. It’s also critical for singers, wind instrument players, and anyone performing an activity that demands controlled, powerful airflow. Without strong abdominal muscles, clearing mucus from the airways becomes difficult, which is one reason people with weakened core muscles are more vulnerable to respiratory infections.
Accessory Neck Muscles Kick In During Heavy Breathing
During quiet breathing, muscles in the neck stay mostly silent. But when your body demands more air, such as during intense exercise, respiratory distress, or breathing against resistance, a second tier of muscles gets recruited. The scalene muscles, which run along the sides of the neck, and the sternocleidomastoid, the prominent muscle running from behind the ear to the collarbone, both pull the upper rib cage and sternum upward. This lifts and further expands the chest cavity beyond what the diaphragm and intercostals can achieve alone.
The sternocleidomastoid typically doesn’t activate until breathing effort becomes substantial. Research in The Journal of Physiology found it only kicks in after about 70% of inspiratory capacity when breathing is driven by elevated carbon dioxide levels, or at roughly 35% of maximum inspiratory pressure during a static effort. The scalene muscles contribute about one-third of the air volume generated by all non-diaphragm inspiratory muscles during quiet breathing, and that share rises to about half during maximum ventilation.
How This System Responds During Exercise
During intense physical activity, your breathing rate and depth both increase dramatically. The diaphragm contracts harder and faster, the intercostals work more vigorously, the abdominals actively assist every exhalation, and the neck muscles engage to maximize each breath. All of this requires energy and blood flow directed to the respiratory muscles themselves.
This creates an interesting competition inside your body. When respiratory muscles fatigue during prolonged hard exercise, they trigger a reflex that constricts blood vessels supplying your working limbs. The result is less blood flow to your legs or arms, which accelerates fatigue in those muscles. In other words, tired breathing muscles can directly limit how hard and how long you can exercise, not because your lungs run out of capacity, but because the muscles driving them steal resources from the rest of your body.
Strengthening Respiratory Muscles
Because breathing depends on muscle, those muscles can be trained like any other. Respiratory muscle strength training typically targets the diaphragm and external intercostals. The most common approach uses a handheld device that forces you to inhale against resistance, much like lifting a weight but for your breathing muscles. Training protocols generally involve sessions two to three times daily, three to five days per week, over a five- to six-week period, with more than ten repetitions per session.
Strengthening programs use loads set at 55% to 80% of a person’s maximum inspiratory pressure for building strength, or 30% to 40% for building endurance with more repetitions. These programs are individualized and can be done at home once someone learns the technique. They’re used both by athletes looking to improve performance and by people with conditions that weaken respiratory muscles.
What Happens When Respiratory Muscles Weaken
Neuromuscular diseases provide a stark illustration of how dependent breathing is on muscle function. In conditions like ALS (amyotrophic lateral sclerosis) and Duchenne muscular dystrophy, progressive muscle weakness eventually affects the diaphragm, intercostals, and abdominal muscles. As these muscles weaken, the ability to take full breaths declines, and coughing becomes ineffective. Poor cough strength means mucus and aspirated material can’t be cleared from the airways, increasing the risk of pneumonia.
Respiratory failure is the most common cause of death in neuromuscular diseases. As weakness progresses, it first affects breathing during sleep, then eventually compromises daytime breathing as well. Muscles involved in swallowing and speaking can also weaken, compounding the problem by allowing food or liquid to enter the airways. Non-invasive ventilation, a mask-based breathing support system, has been shown to improve survival by about seven months in ALS patients whose swallowing muscles still function reasonably well. Regular monitoring of respiratory muscle strength in these conditions helps clinicians intervene before a crisis develops.
Even outside of disease, the relationship is clear: your respiratory system is only as strong as the muscles that operate it. Every breath is a mechanical event, driven by muscle contractions that change the shape and volume of your chest cavity, creating the pressure differences that move air in and out of your lungs.