The diaphragm is a large, flat muscle that rests under the lungs and goes across the entire chest cavity. It has two big jobs. First, the diaphragm is the major muscle of breathing. When taking a breath in, the diaphragm moves down to allow more air in the lungs. When breathing out, the diaphragm moves back to its resting position.
The diaphragm can also be used to increase pressure inside the abdomen. This is an important function when performing tasks that increase the load on the low back.
Anytime the low back or sacroiliac joints are injured, nearby muscles such as the diaphragm can be affected. When the stomach muscles and diaphragm stop working together, sacroiliac joint pain can increase. Some people find their pain is controlled by learning to coordinate these muscles while doing activities. Physical therapists design specific programs of exercise to address these types of symptoms.