GLOSSARY

The muscles of respiration are skeletal muscles that, by changing the volume of the thorax, create the pressure differences needed for air to flow into and out of the lungs; the lungs themselves have no muscle.

Main Inspiratory Muscles

• Diaphragm: the most important respiratory muscle, responsible for about 70–75 percent of tidal volume at rest; on contraction it descends (about 1–2 centimetres at rest and up to 10 centimetres in deep inspiration).
• Diaphragm innervation: the phrenic nerve (C3, C4, C5); mnemonic: “C3, 4, 5 keeps the diaphragm alive”.
• External intercostals.

Expiratory Muscles

• At rest: expiration is passive, driven by the elastic recoil of the lungs and chest wall.
• Active expiration (exercise, coughing): the abdominal muscles (rectus abdominis, internal and external obliques, transversus abdominis) and the internal intercostals.

Features of the Diaphragm

• Fatigue-resistant fibres: about 50 percent type I.
• Non-respiratory role: trunk stability and raising intra-abdominal pressure together with the abdominal and pelvic floor muscles.

Relevance to Sport

• Respiratory muscle fatigue: during prolonged intense exercise (above about 85 percent of VO₂max), diaphragm fatigue may occur; the respiratory muscle metaboreflex, by activating the sympathetic system, reduces limb blood flow and increases leg fatigue.
• Cost of breathing: the respiratory muscles use about 10–15 percent of cardiac output and VO₂ during maximal exercise.
• Practical example: diaphragmatic breathing is used both for relaxation and for lumbar stability in weightlifting (bracing with abdominal breathing).