The only undesirable complication of excessive skeletal muscle contractions or exercise is that it results in the accumulation of lactic acid in muscles. Normally glucose is converted to carbon dioxide and water in the mitochondria using oxygen molecules (see page 19). When the muscles are very active, these mitochondria cannot work fast enough to meet the demand for energy molecules, and so the muscle tissue releases energy molecules by anaerobic (without oxygen) conversion of glucose into lactic acid and energy molecules. If the demand for energy molecules persists and the mitochondria cannot meet the demand because of inadequate oxygen supply the level of lactic acid rises. This results in changing the chemistry of muscle fibres and they stop contracting till the demand for oxygen supply is met and the mitochondria are able to convert lactic acid quickly into carbon dioxide and water. This by-product of incomplete combustion of glucose, lactic acid, is generally harmful to the body especially the heart tissue. Besides producing cramps, stiffness and aches in muscles, it slows down muscle function in general, leading to fatigue. The lactic acid level of blood is frequently checked in sportsmen during training to assess the state of muscle functioning and fitness.