Intercostal Muscles Definition

The intercostal muscles are a group of muscles found between the ribs which are responsible for helping form and maintain the cavity produced by the ribs, and assisting with expansion and contraction during breathing. The intercostal muscles consist of 11 muscle trios in humans, one innermost intercostal muscle towards the chest cavity, one between the ribs, and one external muscle between each of the 12 ribs. The first and last ribs only have intercostal muscles on one side.

Function of the Intercostal Muscles

In humans, the intercostal muscles play a large part in breathing. During inhalation, the diaphragm is relaxed, allowing the lungs to expand. The innermost intercostal muscles relax, while the external intercostal muscles contract, causing the chest cavity to expand. This expansion allows the lungs to fill with air, due to the negative pressure created by the extra space. Air fills the lungs, gases are exchanged, and it is time to exhale.

Thorax zoom

Thorax zoom

To force the air out, the space must become smaller, to put pressure on the air. To do this, the opposite process of inhalation happens. The diaphragm contracts, applying force from the bottom of the lungs. The innermost intercostal muscles now contract, while the external intercostal muscles relax. This causes the chest cavity to contract as a whole, forcing the air out of your lungs. The internal intercostal muscles, or middle layer of muscle, helps keep the ribs from separating and holds the shape of the chest cavity.

Oftentimes, people will experience pain in their intercostal muscles when exercising heavily. This pain is often caused by lactic acid build-up in the intercostal muscles. The muscles must work at a feverish pace when breathing hard, constantly contracting and releasing as a person breathes harder. Any muscle, when limited in the oxygen it is receiving, cannot undergo respiration as a source of energy, and must resort to lactic acid fermentation. As lactic acid fermentation proceeds to provide the muscles with energy, lactic acid builds up. The burning pain people feel in their ribs when exercising is the acid building up in their muscles. Luckily, lots of practice increases the amount of time a person can go before muscles switch to lactic acid fermentation.

Examples of Intercostal Muscles

Example #1: Intercostal Muscles in Snakes

In different animals, the intercostal muscles can perform different tasks. Snakes use their intercostal muscles in a different way than humans, based on their anatomy. Snakes typically only have one functioning lung, which fills a long part of their body. Essentially, a snake has reduced its functional skeleton to a spine and rib cage. As in humans, the intercostal muscles in a snake play a role in bringing air into the lungs. By a similar process, the innermost and external intercostal muscles alternate their contracting and relaxing to expand and contract the lung.

Bitis gabonica skeleton

Bitis gabonica skeleton

Snakes, having no limbs, must also use their intercostal muscles when making a variety of other movements. While they have layers of muscles on top of their ribs that control their motion, the intercostal muscles have a much larger role to play in maintaining the shape and size of the body cavity. For instance, when a constricting a prey item, a large snake creates an enormous amount of pressure with their muscles. This pressure is exerted not only on the prey item, but also on the snake’s own body. To keep itself from damaging its internal organs, the snake must contract its intercostal muscles and create a strong cage around its lung, heart, and other sensitive organs.

Example #2: Collapsible Rib Cages

Not surprisingly, snakes are not the only animals to have adapted their rib cage to be more flexible than humans. Many rodents have flexible rib cages. While it is a huge disadvantage when you meet a snake, having a flexible rib cage means you can get into tighter spaces. For a mouse or rat, this could mean the difference between survival and death. Whether escaping a predator or squeezing into a store of food, the ability to collapse the rib cage is important for many animals. Not surprising, many animals that hunt rodents also have collapsible rib cages, to chase their prey into tight spaces. These animals include cats, ferrets, and badgers, to name a few.

Related Biology Terms

  • Lactic Acid Fermentation – The process by which the leftovers of glycolysis are used to produce lactic acid, and in doing so, obtain a little ATP.
  • Breathing – The act of bringing air in and out of the lungs.
  • Respiration – The process of converting the products of glycolysis into lots of ATP, water and carbon dioxide.


1. In doing a dissection, you see a muscle on the rib, and think it might be an intercostal muscle. It does not seem to be attached to any ribs, however. Is this an intercostal muscle?
A. No.
B. Yes.
C. Impossible to tell.

Answer to Question #1

2. When a boxer is about to take a punch, he tightens up his chest cavity, forcing most of the air out of his lungs. Which intercostal muscles are at work?
A. Innermost
B. Internal
C. External
D. A Combination of several

Answer to Question #2

3. A rat is typically limited by the size of its head when it comes to figuring which holes it can squeeze through. If a rat got cornered by a cat in a tube that was exactly the size of the rat’s head, could the rat hide forever?
A. Probably, if it could get food.
B. Depends how smart the cat is.
C. No, it could not.

Answer to Question #3

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