Carbohydrates are important fuel molecules, but they play many other biochemical roles, including protection against high-impact forces. The cartilage of a runner’s foot cushions the impact of each step she takes. A key component of cartilage are molecules called glycosaminoglycans, large polymers made up of many repeats of dimers such as the pair shown at the right.

[Untitled x-ray/Nick Veasey/Getty Images.]