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.
If muscle activity is high, like it is when climbing a step slope, more carbon dioxide will be produced. This excess carbon dioxide must be eliminated quickly or the pH of the blood will become too acidic. An indicator is a chemical that is used to indicate the presence of another substance. Bromothymol blue is an indicator that is blue in a neutral substance such as water. In the presence of an acid, bromothymol blue turns from green to yellow depending on the pH of the solution.