Breathing Muscles: The Diaphragm & Intercostals
How the chest moves air, and how breathing supports safe lifting.
Before this: The Chest: An Overview
You will be able to
- Describe the diaphragm's role in breathing.
- Explain what the intercostals do.
- Connect breathing to core bracing and the 'exhale on effort' cue.
⚠ Safety note
Breath-holding under heavy load raises blood pressure. General exercisers should usually exhale on effort; people with cardiovascular or blood-pressure conditions should follow professional guidance.
The diaphragm: your breathing engine
The diaphragm is a dome of muscle under the lungs. When it contracts it flattens and drops, enlarging the chest so air flows in (inhalation). When it relaxes, the chest recoils and air flows out (exhalation). It is controlled by the phrenic nerve.
The intercostals
The small intercostal muscles between the ribs help. The external intercostals raise the ribs to assist breathing in; the internal intercostals help pull them down for a forceful breath out.
Breathing and lifting
The diaphragm also helps stiffen the trunk. Together with the abdominal wall it builds intra-abdominal pressure — the basis of "brace your core." A common, safe cue for general exercisers is to exhale on the effort (e.g. as you press up). Prolonged breath-holding under heavy load (the Valsalva maneuver) sharply raises blood pressure and is not appropriate for everyone.
Sources for this lesson
“The diaphragm is the primary muscle of inspiration; the external and internal intercostals assist inspiration and forced expiration respectively.”
Anatomy and Physiology 2e — OpenStax, Rice University · Ch. 11.4 Axial Muscles of the Abdominal Wall and Thorax
Peer-reviewed textbook · License: CC BY-NC-SA 4.0 · Last checked 2026-07-19
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