Intercostal muscles act antagonistically during ventilation, with external intercostals lifting ribs for inspiration and internal intercostals assisting forced expiration movements.
External and internal intercostal muscles form antagonistic layers with different fibre orientations, so their contraction moves the ribcage in opposite directions.
External intercostals contract to lift ribs up and out during inspiration, increasing thoracic volume. Internal intercostals contract during forced expiration to pull ribs down and in, decreasing thoracic volume.
When one intercostal layer contracts, the opposing layer is stretched. Stretching its sarcomeres stores elastic potential energy in titin; titin recoil assists return as the active layer relaxes.
Forced expiration: internal intercostals contract → ribs move down/in → thoracic volume falls → pressure rises above atmospheric pressure → air flows out; the external layer is stretched.
Quiet expiration mainly uses relaxation and elastic recoil; strong internal-intercostal contraction is associated with forced expiration. The muscles change pressure indirectly by moving the ribs.
A strong answer links movement benefit, muscle contraction mechanism, force transfer, joint range, and locomotion adaptations. For contraction, use calcium-troponin-tropomyosin and ATP-driven myosin cross-bridge cycling. For movement, use antagonistic muscles, tendons, ligaments, skeletons as levers, and joint type. For locomotion, link body form to survival or swimming advantage. Locomotion can improve survival and reproductive success.