I’ve run an ultra marathon and still nothing compares to the exhaustion I feel at the end of a day spent keeping up with my two kids. Now science explains why: Children’s fitness levels are on par with well-trained adult endurance athletes.

According to a 2018 study published in the journal Frontiers in Physiology, the metabolic and fatigue profiles of 8 to 12 year old boys were far superior to those of untrained adults when tested with rigorous physical activity. They could also rebound quicker due to efficient heart-rate recovery and the ability to remove blood lactate. In fact, not even national-level endurance athletes were as good at recovering.

That should provide some comfort to wiped-out parents, the researchers say. “We found the children used more of their aerobic metabolism and were therefore less tired during the high-intensity physical activities,” says the study’s co-author Sébastien Ratel, Associate Professor in Exercise Physiology at the Université Clermont Auvergne in France. “This may explain why children seem to have the ability to play and play and play, long after adults have become tired.”

For the study, the researchers tested the boys, untrained adults and endurance athletes on a series of cycling tasks. They were all assessed for their aerobic and anaerobic efficiency as well as their bodies’ abilities to recover. Not surprisingly, the children outperformed the untrained adults across the board and even rivaled the top-tier athletes.

The researchers say the findings do lend themselves to further research on muscular changes and disease risk as we age. “Our research indicates that aerobic fitness, at least at the muscle level, decreases significantly as children move into adulthood—which is around the time increases in diseases such as diabetes occur,” Ratel says. “At least, our results might provide motivation for practitioners to maintain muscle fitness as children grow up; it seems that being a child might be healthy for us.”

As for the naturally fit kids, does this mean we should see more of them on the starting lines of marathons? Not so much. In fact, the researchers say that because the endurance levels of kids seem to already be so good, it may be best for them to focus most of their sports training on the development of skills, speeds and muscle strength. This, Ratel says, “may help to optimize physical training in children, so that they perform better and enjoy sports more.”

For the rest of us, that means the opportunity to catch our breath from the sidelines every now and then.

[A version of this post was originally published on April 24, 2018.]

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