08 Muscle fatigue and cramp
- Fatigue. Repeated and rapid contractions of a muscle (hard exercise) cause fatigue: the blood cannot supply the muscle quickly enough with the oxygen it needs for aerobic respiration, so aerobic energy production falls.
- To keep contracting, the muscle breaks down its glycogen (animal starch) to glucose, which is oxidised anaerobically (without oxygen), forming a small amount of ATP and lactic acid. Lactic acid accumulates and causes fatigue and pain.
- Muscle spasm (cramp). Shortage of ATP prevents the cross-bridges from separating from the actin filaments; the muscle stays contracted and cannot relax, causing a painful spasm. Excessive spasm can tear the muscle and cause bleeding.
- Recovery. At rest the muscle receives enough oxygen for aerobic respiration, which produces a large amount of ATP; the cross-bridges separate; the lactic acid is removed (carried to the liver and oxidised or converted back to glycogen, which is why heavy breathing continues after exercise — the "oxygen debt"). The muscle can then again alternate contraction and relaxation.
✍️ Worked examples
Worked example 10.
- A sprinter finishes a 100 m race and breathes heavily for several minutes.
- Explain: during the race the muscles worked partly anaerobically, building up lactic acid; the extra oxygen intake afterwards repays the oxygen debt, oxidising the lactic acid and restoring ATP and glycogen.