Muscles can only do work when they contract and contracting muscles require a great deal of energy.
This energy is obtained through the process of cellular respiration. During this process, carbon dioxide is released by the muscle cells as a waste product. Carbon dioxide is very toxic to the human body and must be excreted quickly. When carbon dioxide is added to water it forms a weak acid called carbonic acid. Carbonic acid lowers the pH of the solution making it more acidic.