The thoracic diaphragm is the main breathing muscle of the body. It is a double doming structure that divides the thoracic cavity from the abdominal cavity. It attaches to the lower circumference of the ribcage. And its fibers run from out to in like the spokes of a wheel to attach to the central tendon in the middle of the body just forward of the spine. The diaphragm also has two long kite tails called crura that run along the front of the bodies of the vertebrae. While most anatomists state that the crura go to T12, Bonnie Bainbridge Cohen believes that the fibers actually go all the way down to the tip of the coccyx offering support to the front of the spine. Her argument is that sitting in chairs allows the lower part of the muscle to atrophy among those of us in the west. Regardless of whether the muscle goes to the tail or not, the fibers interdigitate with those of the anterior longitudinal ligament and there has been increasing research to suggest a contractile component to the function of connective tissue. So it is not unreasonable to suggest that the contractile support of the crura of the diaphragm can be felt to support the spine all the way to the tail.
During breathing, the fibers of the diaphragm contract to pull the central tendon down an inch or two (though the felt sense is much bigger). This action changes the pressure in the two cavities. It creates a negative pressure in the lungs by increasing the lung volume, allowing air to rush into the lungs. The abdominal cavity is a closed fluid system. As the diaphragm draws down, it displaces the belly organs slightly down into the pelvis and out into the belly. This is the action known as the ‘belly breath’ in yoga. This can be felt in nearly any pose, but especially in a reclining pose where the abdominal muscles are not needed to support the spine.
There are many cool things about the thoracic diaphragm. One of them is that the fascia that surrounds the top of the diaphragm interdigitates with the fascia that comprises the pericardium, the sack that surrounds the heart. So that as we breathe, the heart is softly rocked and cradled by the breath. The fascia on the lower part of the diaphragm is continuous with the fascia of the peritoneal cavity which contains most of the abdominal organs. All of our organs are massaged by the movement of the breath – one of the benefits of pranayama practice.
Another very cool thing that I recently learned about the diaphragm from anatomist Dr. Willard at an osteopathic conference has to do with its role in the immune system. Occasionally, the abdominal cavity becomes congested with excess fluid. Both the lower and upper surfaces of the diaphragm have pore-like structures called stromata, but the lower half has more. And within the structure of the diaphragm is a network of lymphatic vessels that lead from the stromata to the cysterna chyli, the main lymph channel. What does this mean? It means that the thoracic diaphragm pumps fluid from the abdominal cavity and returns it to the lymph. For yogis, this gives us another reason to do two things.. our pranayama practice and inversions, as inversions use gravity to bring fluid closer to the diaphragm. These can be active or passive. Two of my favorite restorative inversions are legs up the wall (with pelvis higher than head) and extended bridge.
Sometimes it feels that the thoracic diaphragm is locked down. One technique for releasing the diaphragm is to add a little bit of compression. Lying in sivasana with a 5-10 pound sandbag resting on the base of the ribcage is one option. The other option is crocodile pose. For crocodile, come onto your belly with the legs as wide as your mat, toes turned outwards. Stack the forearms on top of each other so that you touch your elbows with your fingers and rest your head on your arms. This will put a little more weight on the base of your ribcage. Both of these positions require the diaphragm to work a little bit harder against the weight, but it does so without strain. After 5-15 minutes, remove the sandbag or release your crocodile pose and notice the change in your breath and the fullness of the movement of the diaphragm.
Many pranayama practices also tone the diaphragm, but we’ll save those for another day.