The big idea: Antagonistic muscles move parts inside the body too. The external and internal intercostals move the ribcage in opposite directions.
The points to remember
- Two layers of muscle lie between each pair of ribs; their fibres run at an angle to each other.
- External intercostals contract: the ribcage moves up and out (breathing in).
- Internal intercostals contract: the ribcage moves down and in (forced breathing out).
- Each layer stretches the other; the stretch stores potential energy in titin.
| External intercostals | Internal intercostals | |
|---|---|---|
| Fibres | Slope down and forwards | Slope down and backwards |
| When they contract | Breathing in | Forced breathing out |
| Ribcage moves | Up and out | Down and in |
| Meanwhile the other layer | Internal stretched | External stretched |
Remember it as: Fibres cross, so the pulls oppose.
Real example: coughing. When you cough or blow out candles, your internal intercostals contract hard to pull the ribs down and force air out fast.
Trap: In quiet breathing out, the internal intercostals hardly work: the ribcage falls back as the external intercostals relax. They pull only in forced breathing out.
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How it is asked: Expect 'explain how the intercostal muscles act antagonistically', often next to a question on ventilation of the lungs.
IB-style questionExplain[3 marks]
Explain how the external and internal intercostal muscles act as an antagonistic pair.
Model answer plan
See the answer broken into clear points, with guidance on how each point earns marks.