During physical activity, oxygen is able to diffuse from the capillaries to the muscle cells due to the concentration gradient, of higher oxygen in the bloodstream, as compared to in the muscle cells. At moderate levels of activity, the rate of diffusion of oxygen is enough for the mitochondria to meet the needs of the muscle cells for ATP. As long as aerobic respiration can meet the energy demands of the muscle fiber, it will continue. However, if the activity rate continues to increase in the muscle fiber, and the oxygen can no longer diffuse fast enough into the cell and the mitochondria alone can no longer meet energy needs, anaerobic respiration occurs. The muscles have a reserve called glycogen, which is a storage product of glucose. In situations where the mitochondria alone can't meet the energy needs, glycolysis produces pyruvic acid which can't all be used in aerobic respiration. When it builds up faster than the mitochondria can use it, levels of pyruvic acid rise in the sarcoplasm. Eventually, pyruvic acid is converted to lactic acid and the cell gains 2ATP molecules. This process occurs anaerobically and it adds extra ATP to muscles that are working hard to supplement their energy needs.