Another process that can supply ATP relatively quickly is anaerobic glycolysis, which involves the partial breakdown of sugars stored in muscle. You can think of these as the fives in your wallet. The process is quick but rather inefficient, and can get bogged down by the formation of metabolic by-products. The classic by-product is lactic acid, which accumulates in the muscles of sprinters and is part of the reason why they eventually slow down during a race.
