Between every pair of ribs sits a set of thin muscles, the intercostals, which turn the rib cage from a rigid cage into something that can move with each breath. They are helpers rather than the main engine, since the diaphragm under the lungs does most of the work of quiet breathing. Their job shifts with effort, so they matter most when you breathe hard, cough, or the lungs are struggling. The gap they fill also carries the nerve and blood vessels that make the area between the ribs so sensitive to pain.
The Three Layers Between Your Ribs
The word intercostal simply means between the ribs, and there are 11 such gaps on each side of the chest. Each gap holds three thin layers: external on the outside, internal in the middle, and innermost deepest.1PubMed Central. Respiratory muscle ultrasonography evaluation and its clinical application in stroke patients: A review Each layer starts at one rib and attaches to the top edge of the rib just below.2Kenhub. Intercostal muscles
The layers don’t run the same way. The external fibres slant down and toward the front, while the internal fibres slant down and toward the back, roughly at right angles.3Kenhub. Intercostal muscles The innermost layer runs in the same direction as the internal layer and lines up with a thin sheet of muscle on the inside of the front chest wall.4Kenhub. Intercostal muscles5Kenhub. Innermost intercostal muscles Crossed fibres make the wall strong in several directions, which is why the chest wall resists bulging when you cough.
Because each layer spans from one rib to the next, it can only pull neighbouring ribs toward or away from each other. Done all along the chain, that lifts or lowers the whole cage. A thin plane of connective tissue between the outer and middle layers lets them slide a little over each other as the chest moves.
The layers aren’t identical along the whole length of a gap. The external layer stops short of the breastbone, where a thin sheet of tissue takes over, and the internal layer thins toward the spine, where a similar membrane replaces it. So a strain can feel different at the side, where all layers are thick, than near the front or back.
The outer layer sits just under the bigger chest muscles, such as the pecs. A pulled “rib muscle” usually means a strain in these thin layers, which are only millimetres thick and easy to injure with twisting. Because the muscles sit right between the ribs, an ultrasound probe on the skin can show them directly.6PubMed Central. Respiratory muscle ultrasonography evaluation and its clinical application in stroke patients: A review
What the Intercostals Do When You Breathe
The classic view, dating to 1749, is that fibre direction decides the job: external layer breathes in, internal layer breathes out.7PubMed Central. Respiratory effects of the external and internal intercostal muscles in humans So the external intercostals are mostly active when you inhale, and the internal ones when you exhale.8PubMed. Respiratory function of the rib cage muscles The internal and innermost layers lower the ribs, shrinking the chest and pushing air out during forceful breathing.9Kenhub. Internal intercostal muscles The innermost layer also steadies the ribs during ordinary breathing.10TeachMeAnatomy. Innermost Intercostals – Actions – Innervation – TeachMeAnatomy
The fuller picture is messier. Working on their own in a single gap, both external and internal layers can actually lift the ribs, so fibre direction isn’t the whole story.11PubMed. Mechanics of intercostal space and actions of external and internal intercostal muscles. In real breathing, position on the rib cage matters. The outer layer and the front portion near the breastbone take a breathing-in role, while the side portion of the internal layer takes a breathing-out role.12PubMed. Respiratory action of the intercostal muscles This front portion seems to do the most rib work during quiet breathing.13PubMed. Respiratory function of the rib cage muscles Together with muscles in the neck, it expands the upper chest and stop the diaphragm from sucking it inward.14PubMed. Respiratory function of the rib cage muscles
In one quiet breath, the diaphragm contracts and flattens, which pulls air into the lungs and pushes the belly contents down and outward. At the same moment the front portion and the neck muscles lift and hold the upper ribs. Breathing out at rest needs little effort, because the stretched lungs and chest wall spring back as the muscles relax. The main job of the intercostals here is to stiffen the chest wall so the diaphragm’s pull doesn’t draw it inward.
Most ribs slope downward as they curve forward from the spine, like a bucket handle hanging at its side. When muscles lift a rib, it swings up and outward like a handle being raised, widening the chest from side to side. A larger chest lowers the pressure inside the lungs below the air outside, so air flows in. Pulling the ribs back down shrinks the chest, raises the pressure, and pushes air out.
Active exhaling kicks in with coughing, sneezing, blowing up a balloon, or shouting. The internal and innermost layers then join the belly muscles to squeeze the chest. That’s why a strained rib muscle hurts most when you cough.
The Nerve and Vessels Running Under Each Rib
Along the lower edge of each rib, a vein, an artery and a nerve run together.15TeachMeAnatomy. Thoracic Wall Vasculature – Arterial Supply, Venous Drainage & Intercostal Vessels It sits in a shallow groove on the rib’s underside, between the internal and innermost layers.16Kenhub. Intercostal spaces From top to bottom the order is vein, artery, nerve.17TeachMeAnatomy. Thoracic Wall Vasculature – Arterial Supply, Venous Drainage & Intercostal Vessels That puts the nerve lowest and least sheltered by the rib.
The intercostal nerves come from the spinal cord at the mid-back levels T1 to T11, with a twelfth nerve running below the last rib.18Kenhub. Intercostal nerves They control the between-the-ribs muscles and the muscles of the side and front belly wall.19Kenhub. Intercostal nerves They also carry feeling from the skin, ribs and the lining of the chest and belly.20Kenhub. Intercostal nerves The muscles themselves contain stretch sensors spread through the gap and tension sensors along the rib edges, feeding back how the chest is moving.21PubMed Central. The transduction properties of intercostal muscle mechanoreceptors
Each nerve leaves the spine and wraps around the body along the lower edge of its rib toward the front. Irritating one nerve therefore tends to cause pain in a strip following that rib rather than in a single spot. Shingles on the chest is a well-known example: the virus affects one nerve and produces pain and a rash along one band, usually on one side only. The lower nerves also supply the belly wall, so pain from a low rib level can be felt in the upper abdomen.
The same layout explains where needles go. When doctors drain fluid from around the lung, they place the needle just above the lower rib of a gap, away from the nerve and blood vessels tucked under the rib above.
How the Intercostals Change With Exercise, Age and Lung Disease
During exercise the brain drives the breathing muscles harder.22PubMed Central. The respiratory muscles during exercise The diaphragm mostly moves air fast, while rib cage muscles build the pressure needed to move the chest.23PubMed Central. The respiratory muscles during exercise Breathing out becomes active too, timed with the rib cage muscles breath by breath.24PubMed Central. The respiratory muscles during exercise
When breathing effort rises, the neck muscles may stand out with each breath and the skin between the ribs can be seen pulling inward. In babies and small children, drawing in between the ribs is a sign that breathing is a struggle, because their chest walls are soft and give way easily. A child whose ribs suck inward with each breath, or who seems to be working to breathe, needs urgent medical care.
Age changes the muscle itself. Slow, fatigue-resistant fibres make up about 65% of adult intercostal muscle.25PubMed. Developmental pattern of muscle fiber types in human ventilatory muscles Full-term newborns have about 46% and premature babies about 19%.26PubMed. Developmental pattern of muscle fiber types in human ventilatory muscles Newborn breathing muscles tire more easily, one reason premature babies may need breathing support.27PubMed. Developmental pattern of muscle fiber types in human ventilatory muscles
In severe COPD, the front intercostals near the breastbone shift toward slow fibres, about 73% versus 51% in people without it.28PubMed. Parasternal intercostal muscle remodeling in severe chronic obstructive pulmonary disease This is thought to reflect the muscles being worked harder to help breathing.29PubMed Central. Parasternal intercostal muscle ultrasound in chronic obstructive pulmonary disease correlates with spirometric severity Ultrasound of these front muscles tracks with how severe COPD is on lung tests.30PubMed Central. Parasternal intercostal muscle ultrasound in chronic obstructive pulmonary disease correlates with spirometric severity
In lung disease, doctors may use pulmonary rehabilitation programs and breathing trainer devices to strengthen breathing muscles. Anyone with COPD should ask their doctor before starting device-based breathing training, since the right type and intensity depend on the person.
Intercostal Muscle Strain and Rib Injuries
A strain typically causes sharp pain between the ribs that catches with deep breaths, coughing, or twisting. It often follows lifting, a hard bout of coughing, or sports with forceful twisting such as rowing, golf, or tennis. Pressing along one band between two ribs is often tender, though tenderness alone doesn’t rule out other causes. Mild strains commonly improve over a few weeks with rest from aggravating activity and gentle movement.
Pain can tempt you into shallow breaths, but shallow breathing can let areas of lung collapse and get infected. Good pain control helps people breathe and cough properly.
Some symptoms point away from a muscle. Chest pain with breathlessness, sweating, or pain spreading to the arm or jaw needs emergency care, since it can signal a heart problem. Pain after a fall or crash may mean a rib fracture, which needs checking. Fever or coughing up blood alongside the pain suggests a lung cause rather than a strain and should be checked promptly.
Severe injuries show what these muscles can do. When several ribs break in two places, a loose segment can sink inward as you breathe in. In animal studies, though, the broken ribs still rose with breathing, largely thanks to the front intercostals.31PubMed. Actions of the inspiratory intercostal muscles in flail chest The outer layer became more than three times as active after the injury.32PubMed. Rib cage distortion in a canine model of flail chest
How Doctors Use the Intercostal Space
An intercostal nerve block is a numbing injection beside the nerve under a rib, used for chest and upper belly pain, including breast and chest wall surgery.33NYSORA. Intercostal Nerve Block – Landmarks and Nerve Stimulator Technique Blocks guided by ultrasound or X-ray work better than those done by feel,34PubMed. A Review of Techniques of Intercostal Nerve Blocks because ultrasound lets the doctor see the nerve, vessels, muscle and lung.35PubMed. A Review of Techniques of Intercostal Nerve Blocks
You are usually positioned on your side or front while the skin is cleaned. A thin needle is guided near the lower edge of the rib under imaging, and local anaesthetic is placed beside the nerve. Numbness then spreads through the band of skin and tissue that nerve supplies, which is why one block can cover a strip of the chest. It is reasonable to ask beforehand whether ultrasound guidance will be used.
The lung sits just beneath, so a punctured lung is a rare risk. Bruising can occur because the artery runs alongside. New breathlessness after the procedure should be reported.
Surgeons also borrow the muscle. They can swing a strip of intercostal muscle, still attached to its blood supply, to cover a repaired airway.36PubMed. Intercostal muscle flap reduces the pain of thoracotomy: a prospective randomized trial In one large series of 391 patients, leaks at the repaired airway occurred in under 1%.37PubMed. Intercostal muscle flap to buttress the bronchus at risk and the thoracic esophageal-gastric anastomosis