What is the 2 substances that muscles use when they respire?

Muscles use two substances when they respire: oxygen and glucose. Oxygen is a gas that is inhaled from the air, while glucose is a sugar that is obtained from the breakdown of food. These two substances are essential for the process of cellular respiration, which is how cells in the body convert food into energy. During cellular respiration, oxygen is used to break down glucose into carbon dioxide and water, and this process releases energy that can be used by the cells to power their activities.