Answer: During heavy exercise the muscles respire anaerobically and lactic acid collects in them.
When we exercise vigorously, the supply of oxygen to the muscles is less than the amount needed. The muscle cells then break down glucose without oxygen (anaerobic respiration) into lactic acid, which gives less energy. The accumulated lactic acid causes pain, stiffness and a feeling of tiredness, and an oxygen debt is created which is repaid by deep breathing after the exercise stops.