Oxygen Oxygen is required for the release of energy from food molecules (aerobic respiration). Instead of breaking down food molecules completely, muscle cells switch to a form of partial breakdown that does not require oxygen (anaerobic respiration) so that they can continue to generate energy. This partial breakdown produces lactic acid, which results in a sensation of fatigue when it reaches certain levels in the muscles and the blood. Once the vigorous muscle movements cease, the body breaks down the lactic acid, using up extra oxygen to do so. Panting after exercise is an automatic mechanism to 'pay off' the oxy