The intercostal muscles are a group of muscles located in the spaces between the ribs, forming a dynamic part of the chest wall. Their name literally means “between the ribs.” These muscles play a central role in life-sustaining respiration by working in precise coordination with the diaphragm to change the volume of the thoracic cavity. This mechanism is fundamental, allowing air to move in and out of the lungs. Without their coordinated action, the ability to breathe, especially during physical activity, would be severely compromised.
Understanding the Layers of Intercostal Muscles
The intercostal muscles are organized into three distinct layers, each with a specific location and fiber orientation within the eleven intercostal spaces between the twelve ribs. This multi-layered arrangement provides both strength and the ability for complex, coordinated movement.
The most superficial layer is the external intercostal muscles, whose fibers run obliquely downward and forward, similar to placing hands in pockets. They extend from the spine to the costochondral junctions near the front of the chest. Deep to this layer are the internal intercostal muscles, whose fibers run obliquely downward and backward, perpendicular to the external layer. This opposing fiber direction allows them to perform antagonistic functions during breathing.
The deepest layer is the innermost intercostal muscles, which have a fiber direction similar to the internal layer. They are separated from the internal layer by the neurovascular bundle (intercostal vein, artery, and nerve). These innermost muscles are most prominent in the lateral portions of the chest wall.
How Intercostal Muscles Drive Quiet Breathing
Quiet, resting breathing is a mechanical cycle driven primarily by the diaphragm, but assisted by the external intercostal muscles. Quiet inhalation is an active process that begins with the contraction of the diaphragm, which moves downward to increase the chest cavity volume. Simultaneously, the external intercostals contract, pulling the ribs upward and outward.
This movement is often described as the “bucket handle” motion, where the ribs rotate to increase the volume of the thorax. The expansion of the chest wall creates a negative pressure within the lungs, drawing air in. The external intercostals play a role in lifting and stabilizing the rib cage for this expansion.
Quiet exhalation, in contrast, is a largely passive process that does not require muscle contraction. Once the diaphragm and external intercostal muscles relax, the elasticity of the lungs and the rib cage itself—known as elastic recoil—causes the thoracic volume to decrease. This reduction in volume increases the pressure inside the lungs, pushing the air out. The internal and innermost intercostal muscles remain inactive during this resting phase.
Accessory Roles in Deep Breathing and Chest Stability
The internal and innermost intercostal muscles become active during periods of increased respiratory demand, transitioning to forced breathing. Forced exhalation, such as blowing out a candle or exercising intensely, requires the active contraction of the internal and innermost intercostals. These muscles pull the ribs downward and inward, decreasing the volume of the chest cavity beyond what passive recoil alone could achieve.
Forced inhalation also recruits additional muscles. The external intercostals contract more vigorously to maximize the upward and outward lift of the ribs. Accessory muscles like the scalenes and sternocleidomastoid also assist in elevating the rib cage further to achieve maximum lung capacity.
Beyond moving air, all three layers provide dynamic stabilization to the chest wall. They maintain the rigidity of the rib spaces, preventing them from being sucked inward during large changes in intrathoracic pressure. This stabilizing action is also performed reflexively during sudden movements, like coughing or sneezing, ensuring the rib cage protects the heart and lungs.
Common Sources of Intercostal Muscle Pain
One of the most frequent causes of pain in the intercostal region is an intercostal muscle strain, which occurs when the muscle fibers are overstretched or torn. This injury is common after sudden, forceful movements, such as heavy lifting with a twisting motion, or following a prolonged, intense bout of coughing or sneezing. The resulting pain is typically sharp, localized tenderness that worsens noticeably with deep breaths, coughing, or any movement that stretches the rib cage.
Costochondritis
Pain in this area may also stem from inflammation of the cartilage that connects the ribs to the breastbone, a condition known as costochondritis. This cause of chest wall pain can often be mistaken for muscle strain because the tenderness is located near the rib joints. The pain is specifically reproduced by pressing on the inflamed cartilage. Unlike a muscle strain, costochondritis pain may feel more like a dull ache or pressure and can affect multiple adjacent rib junctions.
Intercostal Neuralgia
A third distinct source of pain is intercostal neuralgia, which involves irritation or compression of the intercostal nerves that run along the rib spaces. This neuropathic pain is often described as a sharp, shooting, burning, or electric-shock sensation that follows the path of the nerve in a band-like pattern around the chest or back. Intercostal neuralgia can be triggered by trauma, prior surgery, or conditions like shingles, which cause inflammation of the nerve itself.
It is important to seek medical attention if chest pain is severe or accompanied by difficulty breathing, as it may indicate a more serious underlying issue.