The muscles are biological machines which convert chemical energy into force and mechanical work. The energy for |contraction| comes from the mechanical reaction between the food we eat and the oxygen we breath. Almost all the sugars in food are converted into glucose, the fuel of muscles. Muscles, therefore, need a good ~blood~ supply to bring nutrients and oxygen and to remove chemical waste products. The actual chemical process involves the breakdown of glucose to carbon dioxide and water. This process releases energy which is used by the muscle |proteins| to cause |contraction|. This process is known as aerobic glycolysis and involves first the production of a 3-carbon substance, pyruvate, which when combined with oxygen, is broken down further to water and carbon dioxide by a cycle of ~enzymes~ in the |mitochondria|, the Krebs cycle. This chemical reaction requires a great supply of oxygen, which is often not available during intense |exercise|. During |exercise|, a muscle requires about fifty times more oxygen per minute than at rest. To overcome this low level of oxygen the muscles are able to convert the glucose into a substance called lactic acid, without the use of oxygen, which still gives the necessary release of energy. For a short period of time, this process, called anaerobic glycosis, is a highly efficient means of harvesting energy. Unfortunately, an accumulation of lactic acid from intense |exercise| causes the energy processes within a cell to cease, which eventually fatigues the muscles and causes cramps. Removal of lactic acid requires oxygen. To acquire the large amount of oxygen required, the body causes panting after |exercise|. The lactic acid is removed as normal circulation is re-established. Muscles are also able to store glucose. It is stored in the form of glycogen (a carbohydrate) granules. This store is used during |exercise|.
