It's the age-old question asked by parents: Why do kids never seem to get tired? Finally, science has an answer. According to a new study, most children have a metabolic rate similar to that of a well-trained endurance athlete, and they can recover from fatigue even faster.

For the study, published in Frontiers in Physiology, researchers compared the metabolic and fatigue rates of three groups of participants: 8- to 10-year-old boys, unathletic adults, and nationally-ranked endurance athletes who compete in triathlons, marathons and cycling. Researchers compared the energy output and recovery rates following exercise by monitoring heart rates, oxygen levels, and lactate-removal levels to see how quickly the participants recovered. Children outperformed adults in all of the tests.

Step for step, researchers found that the kids burned more energy because they had to take more steps to cover a certain distance than their adult peers. They also tended to use less efficient movements to complete each task. But the study found that the children overcame these obstacles with seemingly fatigue-resistant muscles that allowed them to recover quickly from high-intensity exercise.

That's right, fatigue-resistant muscles.

These results come as no surprise to any parent who has ever collapsed on the couch after trying to keep up with kids at the playground. There's a reason why parents often joke about bottling their kids' energy and selling it to make millions. And now, it seems, science finally concurs.

Why do kids never seem to get tired?
A child's metabolism is similar to that of a well-trained endurance athlete.