A student runs a long-distance race and towards the end feels a painful sensation in their leg muscles, forcing them to slow down. Explain why this pain occurs during vigorous exercise.

During vigorous exercise the demand for ATP in the muscle fibres exceeds the amount that can be supplied by aerobic respiration because the blood flow cannot deliver oxygen fast enough. Consequently the muscles resort to anaerobic glycolysis, which does not require oxygen. In this pathway glucose is converted to pyruvate and then to lactic acid, producing only 2 ATP per glucose molecule. The accumulation of lactic acid lowers the pH inside the muscle cells, which interferes with the enzymes involved in ATP production and causes a sensation of fatigue and cramp. The pain is therefore a result of lactic acid build‑up during anaerobic respiration.