All cells in the body continuously burn energy following glycolysis (Embden-Meyerhof) and the citric acid (Krebs) cycle. The citric acid cycle uses oxygen to efficiently break glucose into two, 3-carbon sugars and then completely oxidize the molecules into CO2 and H2O. It is why you have to pee-pee and exhale. It also takes more time to get energy and sometimes the body demands more energy than can be delivered. Fuel is demanded of the blood stream, glucose enters the cells and glycogen, a temporary storage molecule (fat is more permanent storage) is broken-down into glucose, replenishing the blood stream. Muscles that store lots of glycogen and have lots of mitochondria (in which the Krebs cycle mostly runs) is the dark meat while the muscles with less glycogen and less mitochondria are light meat. But the citric acid cycle can't keep-up, so the Embden-Meyerhof pathway cranks-up--glycolysis --> glyco - lysis --> sugar - breaking.

Embden-Meyerhof is very fast but has unfortunate bi-products and is much less energy efficient. The final products of Embden-Meyerhof is ethanol which in most animals is converted to lactic acid, which is safer for the tissue.

Embden-Meyerhof predominates the white muscle tissue, krebs predominates the dark muscle. White muscle is also called "fast-twitch" and dark is "slow-twitch." Sprinters are fast-twitch dominant, marathoners are slow-twitch dominant. Sprinters quickly build-up lactic acid and cramp-up if they continue. Marathoners maintain a steady pace that carefully avoids Embden-Meyerhof until approaching the finish line. The differences are mostly genetic, though through exercise a fast-twitch muscle will create more mitochondria, which quickly go away if exercise is not continued.

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