The diaphragm is the primary respiratory muscle that is located between the abdomen and thoracic cavity. Its, location, just under the lungs, and its unique dome structure allows it to play this vital role in respiration. We rarely think about how our diaphragm is functioning, but hopefully I will change that.
For the rest of us with a fully functioning diaphragm, we may still not know how to use our diaphragm properly, especially during strength training and endurance exercise. First, lets look at typical labored breathing (which uses accessory muscles as much or more than the diaphragm), and a more efficient technique called diaphragmatic breathing.
Typical, labored breathing, which is common with moderate or hard exercise, involves a combination of diaphragm and accessory muscle breathing. While diaphragmatic breathing emphasizes an almost exclusive use of the diaphragm and little use of accessory muscles of the rib cage and neck. Diaphragmatic breathing is advantageous and should be used for two reasons: it is more efficient and it provides better stabilization for your spine and torso.
Efficiency: The diaphragm's shape and location make it the most efficient muscle to draw air into the lungs. The accessory muscles of the torso and neck can expand the rib cage to draw air in, but this action is not very efficient. With endurance exercise, respiratory inefficiency can be significant, and increase energy expenditure/decrease efficiency.
Stabilization: The diaphragm's location and shape allow it to also play a pivotal role in lumbar spine stabilization. Think of the abdomen as a box, with the abdominal wall compromising the front, the oblique muscles compromising the sides, the lower back muscles compromising the back, the pelvic floor muscles compromising the bottom, and the diaphragm compromising the top. With a co-contraction of these muscles, and inhalation by contraction of the diaphragm, intra-abdominal pressure increases. This muscular tension and pressure provides extra stabilization of the lumbar spine.
You can learn to use your diaphragm more efficiently during breathing by copying the drill I was doing in the video. Stand in front of a mirror and take some deep breaths with one hand on your chest and one hand on your abdomen. Watch to see which hand moves. Practice breathing down, into your abdomen with each breath. As you get better, you will see less movement with the hand on your chest and more expansion of your abdomen. After you can regularly inhale and only make your abdomen expand, you can practice abdominal bracing and diaphragmatic breathing. The ultimate challenge is to be able to both, breath deeply to exchange oxygen and carbon dioxide, and develop intra-abdominal pressure to strongly stabilize your spine.