A long-distance runner completes a marathon. Towards the end of the race, the runner's muscles begin to ache and feel heavy. After the race, the runner breathes rapidly and deeply for several minutes before their breathing returns to normal. Explain why the runner's muscles ache during the race and why the runner continues to breathe rapidly after the race has finished.

During the marathon the runner’s muscles are working hard and the demand for oxygen rises. At the end of the race the oxygen supply is insufficient, so the muscles cannot get enough oxygen to meet the demand.

When oxygen is limited the muscles switch to anaerobic respiration in addition to aerobic respiration.

Anaerobic respiration produces lactic acid, which accumulates in the muscle cells and causes the aching, heavy feeling and fatigue.

After the race the runner still has an oxygen debt – the amount of oxygen that would have been required if the muscles had been supplied with enough oxygen during the race.

The body continues to breathe rapidly to provide the extra oxygen needed to repay the debt and to oxidise the lactic acid, which restores normal muscle function.