--- title: "External Intercostal Muscles | Anatomy & Physiology I" description: "External intercostal muscles are skeletal muscles between the ribs that lift the rib cage during inhalation in Anatomy and Physiology I." canonical: "https://fiveable.me/anatomy-physiology/key-terms/external-intercostal-muscles" type: "key-term" subject: "Anatomy and Physiology I" unit: "Unit 22" --- # External Intercostal Muscles | Anatomy & Physiology I ## Definition External intercostal muscles are the skeletal muscles between the ribs that lift the rib cage during inhalation. In Anatomy and Physiology I, they show how the thoracic cage expands to pull air into the lungs. ## What It Is External intercostal muscles are the muscles between adjacent ribs that contract to raise the rib cage during inhalation. In Anatomy and Physiology I, they are one of the main ways the thoracic cage changes shape so air can move into the lungs. They sit in each intercostal space, the gaps between the ribs. When these muscles shorten, the ribs move up and outward. That movement increases the volume of the thoracic cavity, which drops pressure inside the chest compared with the outside air. Air then flows into the lungs because gases move from higher pressure to lower pressure. Their fiber direction matters. External intercostals run diagonally downward and forward, like putting your hands in your pockets. That orientation helps them lift the ribs instead of pulling them inward. If you are looking at a rib cage diagram, this is the muscle layer that is more active during quiet inspiration. They work with the respiratory diaphragm, which flattens at the same time to enlarge the thoracic cavity even more. The diaphragm does more of the work in relaxed breathing, while the external intercostals help stabilize and elevate the ribs so the chest wall expands smoothly. You usually do not think about these muscles until they are impaired or overworked. If breathing gets harder, accessory respiratory muscles may join in to help, but the external intercostals are still part of the normal mechanics that make each breath possible. In the thoracic cage, they are a moving part of a protective frame, not just a passive structure between bones. ## Why It Matters External intercostal muscles connect the anatomy of the thoracic cage to the physics of breathing. They show how movement of the ribs changes thoracic volume, which changes air pressure and drives inhalation. That cause and effect is a big idea in respiratory physiology. This term also helps you separate structure from function. The ribs and costal cartilages form the cage, but the intercostal muscles make that cage expandable. Without that muscle action, the chest wall would not lift efficiently, and lung expansion would be limited. It also gives you a clean comparison point with other respiratory structures. The respiratory diaphragm is the main muscle of quiet breathing, while the external intercostals add rib elevation. When breathing demand increases, accessory respiratory muscles can join in, but the external intercostals are part of the baseline breathing pattern you should recognize first. In lab work, diagrams, and model identification, this term helps you read the thoracic wall correctly. If you can spot where the muscles sit, what direction the fibers run, and what happens to the ribs during inspiration, you can explain the mechanics instead of just naming a part. ## Connections ### Intercostal Spaces External intercostal muscles occupy the intercostal spaces, which are the gaps between the ribs. Knowing the spaces helps you place the muscles on a rib cage model or diagram. It also explains why the muscles can move each rib relative to the one above it, instead of acting on the whole thorax as one solid piece. ### Respiratory Diaphragm The diaphragm and external intercostals work together during inhalation, but they do not do the same job. The diaphragm increases vertical space by flattening, while the external intercostals raise the ribs to widen the chest. If you mix them up, breathing mechanics questions get confusing fast. ### Accessory Respiratory Muscles Accessory respiratory muscles step in when breathing becomes harder, such as during exercise or respiratory distress. External intercostals are not usually counted as accessory muscles in quiet breathing, because they are part of the normal inspiratory pattern. Comparing the two helps you separate everyday breathing from extra-effort breathing. ### [Costovertebral Joint](/anatomy-physiology/key-terms/costovertebral-joint) The costovertebral joints connect the ribs to the vertebrae, creating the pivot points that let rib movement happen. External intercostal contraction only changes chest volume because the ribs can move at these joints. If the joints are stiff, the muscles cannot lift the rib cage as effectively. ## On the AP Exam A quiz question might ask you to identify which muscle group elevates the ribs during inhalation, or label the muscles on a thoracic cage diagram. In a lab practical, you may need to point to the intercostal spaces and explain how rib elevation increases thoracic volume. In an essay or short-answer response, connect the muscle contraction to pressure change and air movement, not just the name of the muscle. If a question describes difficult inspiration, think about the chest wall and whether the external intercostals, diaphragm, or accessory respiratory muscles are involved. The strongest answers trace the sequence: muscle contraction, rib movement, larger thoracic cavity, lower pressure, air flows in. ## External Intercostal Muscles vs Internal Intercostal Muscles These are easy to mix up because both sit between the ribs, but they do opposite jobs in normal breathing mechanics. External intercostals help raise the ribs during inhalation, while internal intercostals are more associated with pulling the ribs down during forced exhalation. If you remember the direction of rib movement, the pair becomes much easier to separate. ## Key Takeaways - External intercostal muscles are the skeletal muscles between the ribs that help expand the thoracic cavity during inhalation. - Their contraction lifts the ribs up and outward, which increases chest volume and lowers pressure so air moves into the lungs. - They work with the respiratory diaphragm, especially during quiet breathing, to make inhalation smooth and efficient. - They sit in the intercostal spaces and are part of the thoracic wall, so they show up in both anatomy diagrams and breathing mechanics questions. - If breathing becomes difficult, these muscles may be less effective or get help from accessory respiratory muscles. ## FAQs ### What are external intercostal muscles in Anatomy and Physiology I? They are the muscles between the ribs that contract to lift the rib cage during inhalation. In Anatomy and Physiology I, they are a core part of how the thoracic cavity expands so air can enter the lungs. They are skeletal muscles, so they work by contraction, not by passively stretching. ### What do external intercostal muscles do during breathing? They pull the ribs upward and outward during inhalation. That increases thoracic volume and lowers pressure inside the chest, which draws air into the lungs. They are especially easy to see in diagrams that show how the rib cage moves during inspiration. ### How are external intercostal muscles different from internal intercostal muscles? External intercostals help with inhalation by elevating the ribs, while internal intercostals are more associated with depressing the ribs during forced exhalation. Both are between the ribs, which is why they get confused. The main clue is the direction of rib movement. ### Where are the external intercostal muscles located? They are located in the intercostal spaces, the spaces between adjacent ribs. On a rib cage model or diagram, they form part of the muscular wall of the thorax. Their placement lets them move one rib relative to the next rib during breathing. ## Related Study Guides - [22.6 Modifications in Respiratory Functions ](/anatomy-physiology/unit-22/6-modifications-respiratory-functions/study-guide/ZKRTzSeJUXl1eU12) - [7.4 The Thoracic Cage ](/anatomy-physiology/unit-7/4-thoracic-cage/study-guide/apeZdUFsBBgpboDm) ## About This Document Canonical Fiveable pages are available as Markdown at the same path plus `.md`. - [llms.txt](https://fiveable.me/llms.txt): index of Fiveable's sections and URL patterns - [llms-full.txt](https://fiveable.me/llms-full.txt): complete subject and unit listing - [MCP server](https://fiveable.me/mcp): call Fiveable as tools instead of fetching pages (`https://fiveable.me/api/mcp`) - [MCP server for AP teachers](https://fiveable.me/mcp/teachers): a teacher's classes, assignments and AP-rubric grading (`https://fiveable.me/api/mcp/teacher`) ## Structured Data ```json {"@context":"https://schema.org","@graph":[{"@type":"LearningResource","@id":"https://fiveable.me/anatomy-physiology/key-terms/external-intercostal-muscles#resource","name":"External Intercostal Muscles | Anatomy & Physiology I","url":"https://fiveable.me/anatomy-physiology/key-terms/external-intercostal-muscles","learningResourceType":"Concept explainer","educationalLevel":"AP® / High School","about":{"@id":"https://fiveable.me/anatomy-physiology/key-terms/external-intercostal-muscles#term"},"audience":{"@type":"EducationalAudience","educationalRole":"student"},"dateModified":"2026-07-03T02:20:38.182Z","isPartOf":{"@type":"Collection","name":"Anatomy and Physiology I Key Terms","url":"https://fiveable.me/anatomy-physiology/key-terms"},"author":{"@type":"Organization","@id":"https://fiveable.me/editorial#content-team","name":"Fiveable Content Team","url":"https://fiveable.me/editorial","parentOrganization":{"@id":"https://fiveable.me/#organization"},"member":[{"@type":"Person","name":"Amanda DoAmaral","sameAs":["https://www.linkedin.com/in/adoamaral"]},{"@type":"Person","name":"Becky Bahr","sameAs":["https://www.linkedin.com/in/becky-bahr-b52743132"]}]},"publisher":{"@type":"Organization","name":"Fiveable","url":"https://fiveable.me"}},{"@type":"DefinedTerm","@id":"https://fiveable.me/anatomy-physiology/key-terms/external-intercostal-muscles#term","name":"External Intercostal Muscles","description":"External intercostal muscles are the skeletal muscles between the ribs that lift the rib cage during inhalation. 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