Compared with most adults, kids seem to have endless amounts of energy. Scientists may now know why: Children’s bodies resist fatigue and recover from exertion quickly—just like trained athletes.

Tony Blazevich studies biomechanics—how organisms move—at Edith Cowan University in Australia. His team recently asked children, adults, and elite athletes to ride a stationary bike. The researchers measured how the participants’ bodies performed as they exercised.

The team found that kids and athletes used more oxygen and got rid of lactate—a chemical made by fatigued muscles—faster than most adults. Their heart rates also returned to normal more quickly after exercising. Blazevich’s conclusion: “It seems that the muscles of children are the same as [those of] endurance athletes, and certainly much fitter than normal adults.”