The diaphragm is the main muscle of the respiratory system. It’s a dome-shaped sheet of muscle and tendon that sits at the base of your chest cavity, separating your chest from your abdomen. Every breath you take, from a quiet inhale while sleeping to a deep gasp during exercise, starts with this single muscle contracting and pulling downward.
Where the Diaphragm Sits and What It Looks Like
The diaphragm forms a curved dome, with its top surface creating the floor of the chest cavity and its bottom surface forming the roof of the abdominal cavity. Muscle fibers radiate outward from its edges, where they attach to the lower ribs, the breastbone, and the spine. These fibers all converge into a flat, strong central tendon at the crest of the dome, giving the muscle its distinctive parachute-like shape.
During quiet breathing, the diaphragm moves about 1 to 2 centimeters with each breath. During a deep breath, that movement increases dramatically, with ultrasound studies measuring average excursions of roughly 5 to 5.5 centimeters and maximum excursions reaching nearly 9 centimeters. That downward piston motion is what drives air into your lungs.
How the Diaphragm Creates Each Breath
When you inhale, the diaphragm contracts and flattens, pushing the organs in your abdomen downward. This expands the chest cavity and creates a drop in pressure inside the lungs relative to the air outside your body. Air naturally flows from high pressure to low pressure, so it rushes in through your nose or mouth to fill the space.
The diaphragm also expands the lower rib cage from the inside. As it contracts, it increases the pressure in the abdomen, and that pressure pushes outward against the lower ribs in the zone where the diaphragm is directly pressed against the inner chest wall. This is why you can feel your lower ribs flare slightly outward during a deep breath.
Exhaling during normal breathing is mostly passive. The diaphragm simply relaxes back into its dome shape, the elastic tissue of the lungs recoils, and air is pushed out without much muscular effort.
The Nerve That Controls It All
The diaphragm is controlled by the phrenic nerve, which originates from the third through fifth vertebrae of the neck (C3 through C5). This nerve travels a surprisingly long path, running down through the neck, past the heart and lungs, before reaching the diaphragm. It provides complete motor control of the muscle and also carries sensation from the central tendon.
A classic medical mnemonic captures the importance of this nerve: “C3, 4, 5 keeps the diaphragm alive.” Damage to the phrenic nerve at any point along its path can weaken or paralyze one or both sides of the diaphragm. Spinal cord injuries above C3 can stop diaphragm function entirely, which is why high cervical spine injuries often require mechanical ventilation.
Muscles That Assist the Diaphragm
While the diaphragm does the heavy lifting, it doesn’t work alone. Three layers of intercostal muscles sit between your ribs and play supporting roles. The external intercostals help with inhalation by pulling the ribs upward and outward, expanding the rib cage. The internal intercostals and the innermost intercostals assist with exhalation by pulling the ribs downward and inward, compressing the chest.
During exercise or respiratory distress, your body recruits additional accessory muscles. The scalene muscles in the neck, the sternocleidomastoid muscles that run from behind your ear to your collarbone, and other neck muscles all pitch in to lift the upper rib cage higher and faster. If you’ve ever seen someone breathing hard after a sprint, with their shoulders visibly rising and falling, that’s the accessory muscles at work. Visible use of these muscles at rest is actually a clinical sign that someone is struggling to breathe, because healthy quiet breathing should rely almost entirely on the diaphragm.
What Happens When the Diaphragm Fails
Diaphragm paralysis can affect one side (unilateral) or both sides (bilateral), and the difference matters enormously. Unilateral paralysis is more common and sometimes goes unnoticed, though it typically cuts forced vital capacity, the total amount of air you can exhale after a full breath, by about 50%. That decline worsens by another 25% when lying down, which is why people with diaphragm weakness often feel more short of breath in bed than when standing.
The causes range widely. Surgical trauma accounts for a significant share, with up to 20% of cardiac bypass cases resulting in temporary diaphragm weakness. Compression from tumors can damage the phrenic nerve, occurring in roughly 5% of lung cancer cases. Neurological conditions like multiple sclerosis and diabetic neuropathy can also impair nerve function. Viral infections including herpes zoster and Zika have been linked to diaphragm paralysis, as have bacterial infections like Lyme disease. Nearly 20% of cases have no identifiable cause.
A standard chest X-ray can identify unilateral paralysis about 90% of the time by showing one side of the diaphragm sitting higher than expected. A “sniff test” under fluoroscopy confirms it: when a patient sniffs sharply, the paralyzed side moves upward into the chest instead of downward, the opposite of what it should do.
Benefits of Diaphragmatic Breathing
Because the diaphragm directly influences both the sympathetic and parasympathetic nervous systems, deliberately engaging it through slow, deep breathing has measurable health effects. Diaphragmatic breathing activates the parasympathetic branch, the body’s “rest and digest” mode, which is why it’s commonly recommended for stress and anxiety management. In one study, anxiety scores dropped by more than 70% over eight weeks of regular diaphragmatic breathing practice.
The benefits extend well beyond stress relief. In people with COPD, four weeks of supervised diaphragmatic breathing training improved six-minute walk distance by about 35 meters, a meaningful gain for someone with limited exercise capacity. A single session of slow diaphragmatic breathing has been shown to lower systolic blood pressure by roughly 4 mmHg. Studies have also found improvements in quality of life for people with asthma, reduced acid reflux medication use in people with GERD, and decreased frequency and severity of migraines when diaphragmatic breathing was combined with biofeedback.
Practicing diaphragmatic breathing is straightforward: place one hand on your chest and one on your belly, then breathe so that only the hand on your belly rises. Your chest should stay relatively still. This ensures you’re engaging the diaphragm rather than relying on the shallower chest muscles that tend to dominate when you’re stressed or sedentary.