Updated: Feb 20, 2022
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Why is it important to talk about breathing? We need to continuously breathe to stay alive, even when we are sleeping. This requires the breathing muscles to work continuously. There are many muscles that are involved with our breathing, but I am going to talk specifically about the diaphragm which is arguably the most important respiratory muscle in our body. It is due to the unique function of the diaphragm that our breathing continuously affects our posture, stability, autonomic nervous system, and even myofascial tone.
Let's start by briefly looking at the anatomy of the diaphragm; it is a huge muscle. As you can see, it covers the inner part of the entire lower ribcage. It also attaches to our lumbar spine and sternum. The central tendon is huge and connects the left and right sides of the diaphragm together. This allows it to flatten and descend as it contracts.
In the picture below, you can see that the contraction of the diaphragm causes it to flatten and descend, which increases the volume of the chest cavity. This allows air to come into our lungs. Relaxation of the diaphragm makes it go back to its dome shape, which decreases the volume of the chest cavity thereby allowing us to breathe out: this is only a basic function of the diaphragm. For the purpose of physical training, it is important to remember that inhalation requires the contraction of the diaphragm, which causes it to descend, whereas exhalation requires relaxation or even stretching of the diaphragm.
Our diaphragm is anatomically designed to provide both respiratory function and stability of our thorax and lumbar spine. Dr. Paul Hodges and his research team have found that the diaphragm indeed contributes to our whole body stability during voluntary movement. They also found that there is a trade-off between the respiratory and stability functions of the diaphragm. This indicates that when the diaphragm is recruited too much for the stability function, the efficiency of our breathing can be compromised. Positioning and contractility of the diaphragm hugely affect the balance between stability and respiratory function of the diaphragm. Pavel Kolar and his research group reported that people with chronic low back pain tend to show abnormal positioning and contractility of the diaphragm.
Other researchers from the Postural Restoration Institute have found that positioning and contractility of the diaphragm affect the tone of other postural muscles in the body through their fascial connection. A very common example of this is the hypertonicity of the diaphragm that leads to overactivation of hip flexors, which in turn can anteriorly tilt our pelvis. This means that optimal control of the balance between stability and respiratory function is very important for our postural health. For this reason, it is important to know how we can optimize the coordination between the diaphragm and other important muscles that provide stability to our spine, thorax, and pelvis. I am going to refer to the spine, thorax, and pelvis as our core from now on just for simplicity.
To understand how the diaphragm is coordinated with other stability muscles in our core, we need to first understand how the core stability is achieved. The most widely accepted and evidence-based mechanism for core stability is through distributed activation of the whole abdominal walls that create intra-abdominal pressure. Let me expand a bit more on what that means. Intra-abdominal pressure is defined as the steady-state pressure that is concealed within our abdominal cavity that is formed by the interaction between our organs and abdominal walls. Well, how do our organs and abdominal walls interact to create the pressure inside our abdomen? I am going to borrow the diagram below made by Dr. Morris Gasparin to explain this. I mentioned briefly in the anatomy section that the contraction of the diaphragm flattens and descends the diaphragm. This action of the diaphragm pushes the internal organs in the abdominal cavity downward. The linings of our organs are full of water. So as you can see on the righthand side of the diagram, this water creates hydrostatic forces that push against the diaphragm, pelvic floor, and abdominal walls. What is not shown in this picture is the arrow pointing upward to the diaphragm and downward to the pelvic floor. Now, this outwardly directed hydrostatic force itself is sometimes referred to as intra-abdominal pressure. However, when it comes to core stability, this is not the end of the story. What is more important is the proactive and reactive control of the abdominal wall, pelvic floor, and connective tissues that counter this intra-abdominal pressure (black arrows). As you can see in the picture on the lefthand side, our abdominal cavity is often compared to an engine cylinder with the diaphragm representing a piston. This is a great analogy if we want to explain how outwardly directed pressure is generated. However, unlike the cylinder, our abdominal wall and pelvic floor are not rigid structures. And that is actually a good thing because we can increase the volume within the abdominal cavity to mitigate the excessive pressure built in our body.
As you can see in this formula, increasing the volume can decrease the internal pressure.
As the diaphragm pushes down into the abdominal organs, abdominal walls expand out and the pelvic floor descends a bit to create some volume within the abdominal cavity. However, we also don't want to expand the volume too much and too fast. The picture below is a bird-eye view of our abdominal cavity, and the blue arrows represent the hydrostatic pressure directed outward. As you can see, the pressure directed towards the spine has an important role in stabilizing the spine especially during the activity that creates extensor stress to the spine such as lifting a heavyweight. For this reason, our abdominal muscles should eccentrically contract as they passively lengthen so that we can maintain the optimal range of internal pressure.
As you can see in the green arrow on the right-hand side in the picture below, such eccentric activation of the abdominal walls exerts inwardly directed force into the abdominal cavity. The same thing happens with respect to pelvic floor muscles. As the pelvic floor descends to create volume, it eccentrically contracts to exert upwardly directed force into the abdominal cavity. So in the context of explaining core stability, intra-abdominal pressure is the net force between outwardly directed hydrostatic force and the inwardly directed force from our muscular activation. As you can imagine, this net force is a changing force. And this makes the story about breathing and core stability more interesting. Sometimes, the term "stability" misrepresents the image of being static, but in reality, "stability" comes from ever-changing intra-abdominal pressure that is kept within the range of a healthy amount of variability. So in the next part, I will talk about how our neuromuscular system controls intra-abdominal pressure.
Henning, S., Mangino, L. C., & Massé, J. (2017). Postural Restoration: A Tri-Planar Asymmetrical Framework for Understanding, Assessing, and Treating Scoliosis and Other Spinal Dysfunctions. Innovations in Spinal Deformities and Postural Disorders, 135.
Kolář, P., Šulc, J., Kynčl, M., Šanda, J., Čakrt, O., Andel, R., ... & Kobesová, A. (2012). Postural function of the diaphragm in persons with and without chronic low back pain. journal of orthopaedic & sports physical therapy, 42(4), 352-362.