The diaphragm, the breathing muscle

Where it is and what it looks like

Cleveland Clinic describes it as a thin, dome-shaped muscle below your lungs and heart. It separates your chest from your abdomen and attaches to your sternum, your lower rib cage and your spine.

A 2013 anatomy review by Bordoni and Zanier fills in the detail. It's very thin, 2 to 4 millimeters. It has a tendon at the center, which doesn't contract, and a rim of muscle around it. That rim starts in three places: behind the tip of the sternum, on the last six ribs, and on pillars that run down to the lumbar vertebrae.

It isn't a sealed wall. The aorta, the esophagus and the inferior vena cava pass through it, among other structures.

What it does when you breathe in and out

When it contracts, the diaphragm moves down toward the abdomen. Your chest gains room and air comes in. At the same time, the muscles between your ribs move them up and out. That's how the OpenStax physiology textbook explains it.

A normal exhale is a different matter: it's passive. The diaphragm relaxes, the elastic tissue of the lungs recoils, and air leaves without any energy spent. Only when you breathe out hard, to blow or to sing a long phrase, do your abdominal muscles start pushing. And when you breathe in hard, muscles in your neck help.

That's where the thing you see in Belly breathing comes from. As it drops, the diaphragm pushes the contents of your abdomen forward, and your belly rises. You don't breathe with your belly: it shows that the diaphragm moved.

Who gives the order

The diaphragm is moved by the phrenic nerve, which leaves the neck from the third through fifth cervical roots and runs down through the chest to the muscle.

The rhythm comes from higher up. Centers in the medulla and the pons, at the base of the brain, generate each breath without your deciding to, asleep or awake. The cerebral cortex can step in for a while: that's why you can hold your breath, stretch out an exhale or follow a count.

That double status makes it unusual. It's a muscle you can direct whenever you like and one you never have to think about.

It does more than breathe

• It helps you go to the bathroom. By raising pressure inside the abdomen, it helps with urination and bowel movements, according to Cleveland Clinic.
• It guards the top of the stomach. By pressing on the esophagus, it helps prevent acid reflux.
• It moves with the pelvic floor. Bordoni and Zanier describe how, when the diaphragm descends on an inhale, the pelvic floor lowers too, and how the pair steadies the trunk during breathing and coughing.
• It plays a part in posture. The same review calls it important for posture and for proper organ function.

The diaphragm in slow breathing

When you breathe slowly, each breath is bigger, and the diaphragm travels farther. A 2017 review of slow breathing cites an MRI study of healthy people that found exactly that: greater diaphragm excursion during slow breathing.

The same review reports that breathing with the diaphragm makes slow breathing easier, and that people who breathe this way show more efficient return of blood to the heart, most of all at a slow pace. It presents these as findings from specific studies, not as a general rule.

One limit is worth keeping in mind. Studies nearly always mix two things: breathing with the diaphragm and breathing slowly. Nobody knows how much each contributes.

How to feel it

Lie on your back with your knees bent and rest a hand on your belly, just below your ribs. Breathe in through your nose and let that hand rise. Breathe out and let it fall by itself. Don't push your belly out with your muscles: it rises because air is coming in.

Belly breathing on this site puts a count to it: 4 seconds in, 6 seconds out and 2 seconds of rest, for five minutes. Three-part breath (dirga) carries the movement from the belly to the ribs and chest. Qigong abdominal breathing makes it slower and even, 6 and 6. And Pursed-lip breathing stretches out the exhale, which is the passive part.

When to see someone

A short bout of hiccups is common. Cleveland Clinic notes that hiccups that don't go away, or that come back often, can be a sign of a diaphragm problem and are worth a visit. So is any other symptom that makes you suspect something is off with your diaphragm.

Chest pain or pressure, or shortness of breath, calls for immediate medical help. And if you've had recent abdominal or chest surgery, or you have a lung or heart condition, check with your care provider before you practice.