(1) During the run, the energy demand is high, but the oxygen supply to produce energy is limited.
Therefore, anaerobic respiration takes place in the muscle cells to fulfill the energy demand.
(2) An athlete needs to supply extra energy to his muscle cells which have worked hard during running.
(3) For this, he breathes faster and deeper so that more oxygen is supplied to the cells.
This speeds up the breakdown of food and as a result, more energy is released.