If you do the same activity over and over again your muscles can become tired and sore. They may even refuse to move. Why does this happen? When muscles are exercised a lot, they need large amounts of food and oxygen. Blood brings food and oxygen to the muscles. Inside the muscle cells, the food reacts with oxygen. And in the process, energy is released and waste is produced. Carbon dioxide and water are the waste products. The process of releasing energy from food is called respiration.

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Justin Gatlin Photo Courtesy University of Tennessee

Why do we breathe harder while exercising? Harder and deeper breathing brings in more oxygen for your muscles and exhales more carbon dioxide. If breathing hard brings in enough oxygen to meet the needs of working muscles, the respiration is called aerobic (a robe ick). But if muscles use oxygen faster than breathing can supply it, they begin a process called anaerobic respiration (an a robe ick). With anaerobic respiration energy is still released from food, but lactic acid is made as the waste product.

As lactic acid builds up, muscle contraction slows and muscles hurt. By resting, heavy breathing can supply enough oxygen for muscles to return to aerobic respiration. With continued heavy breathing, the lactic acid changes to water and carbon dioxide, and the pain goes away. The muscles have returned to their normal state.

Aerobic Respiration

food + oxygen ---> energy + carbon dioxide + water

Anaerobic Respiration

food ---> less energy + lactic acid