Usain Bolt sprints to a win in the 200-meter finals at the Olympics in London in 2012. Glucose metabolism can generate the ATP to power muscle contraction. During a sprint, when the ATP needs outpace oxygen delivery, as would be the case for Bolt, glucose is metabolized to lactate. When oxygen delivery is adequate, glucose is metabolized more efficiently to carbon dioxide and water.

[Christophe Karaba/epa/Corbis.]