The diaphragm is the primary muscle responsible for human respiration, a large, dome-shaped sheet of muscle and tendon separating the chest cavity from the abdomen. While its main function involves contracting and flattening to draw air into the lungs, the diaphragm is not a single, uniform muscle. It consists of two distinct functional components: the costal diaphragm, which attaches to the ribs, and the crural diaphragm. This crural portion is a specialized part of the muscle complex, serving an important function beyond breathing mechanics.
Anatomical Placement and Structure
The crural diaphragm is formed by two muscular extensions, known as the right and left crura, which anchor the diaphragm posteriorly to the lumbar spine. The right crus is typically longer, originating from the first three lumbar vertebrae (L1 to L3) and their intervertebral discs. The left crus is slightly smaller, arising from the first two lumbar vertebrae (L1 and L2).
These fibrous and muscular bands ascend toward the center of the body, where the muscular fibers converge into the central tendon of the diaphragm. Crucially, some fibers from the right crus loop around the esophageal hiatus, the opening through which the esophagus passes from the chest to the abdomen. The crural diaphragm forms a muscular ring around the lower end of the esophagus at this point.
The Crural Diaphragm and Esophageal Health
The most significant non-respiratory function of the crural diaphragm is its role in preventing the backflow of stomach contents, particularly acid, into the esophagus. It acts as an “external sphincter” that works in coordination with the lower esophageal sphincter (LES), which is an “internal” ring of smooth muscle.
During periods of increased abdominal pressure, such as coughing, straining, or heavy lifting, the crural diaphragm contracts reflexively. This contraction squeezes the esophagus at the hiatus, significantly augmenting the pressure barrier created by the LES. The dual-sphincter mechanism is necessary because the force generated by strain can create a pressure gradient that pushes stomach acid upward. When functioning correctly, the crural diaphragm prevents this pressure from overcoming the LES and causing reflux.
The intrinsic smooth muscle of the LES provides a resting pressure tone. The skeletal muscle of the crural diaphragm provides a dynamic, inspiratory-related pressure increase. This coordinated action ensures that the junction remains sealed during both quiet conditions and physical exertion.
Common Clinical Issues Involving Dysfunction
When the crural diaphragm is weakened or its opening is too wide, it can lead to a failure of the anti-reflux barrier, contributing to Gastroesophageal Reflux Disease (GERD). This dysfunction is often associated with a hiatal hernia, which is the displacement of a portion of the stomach up through the esophageal hiatus and into the chest cavity.
In a sliding hiatal hernia, the most common type, the stomach and the lower esophageal sphincter shift upward together. This occurs when the crural diaphragm’s opening is lax and the ligaments holding the esophagus in place have stretched. A separation of two centimeters or more between the crural diaphragm and the LES is often used as a diagnostic indicator. This anatomical separation means the two sphincters can no longer work in a coordinated manner, severely impairing the reflux prevention mechanism.
The resulting lack of coordination means the crural diaphragm’s inspiratory squeeze no longer reinforces the LES, particularly during sudden increases in intra-abdominal pressure. This structural failure is a major factor in the chronic nature of GERD in many patients. Dysfunction of the crural muscle can also occur in patients with chronic lung diseases, which may explain the higher prevalence of GERD in those populations.
Strengthening and Functional Improvement Techniques
Since the crural diaphragm is a skeletal muscle, its function can potentially be improved through targeted exercise. Techniques focused on diaphragmatic breathing, often called “belly breathing,” aim to increase the muscle tone of the entire diaphragm complex. Consistent practice of these exercises can help strengthen the muscle fibers, which may enhance the squeeze it provides to the esophagus.
A common technique involves lying down or sitting comfortably and placing one hand on the chest and the other on the abdomen. The individual inhales slowly through the nose, focusing on pushing the abdomen out so the hand on the belly rises, while the hand on the chest remains relatively still. The exhalation is slow and controlled, often through pursed lips, as the abdominal muscles pull inward. Regular diaphragmatic breathing training may help to restore the competence of the esophagogastric junction and reduce the symptoms of acid reflux in selected patients.