Children are more physically fit, by some measures, than nationally competitive runners and cyclists.

Parents have long noted that children do not tire as quickly as adults — an observation that has been confirmed by scientists. To compare the two groups, Sébastien Ratel at the University of Clermont Auvergne in Clermont-Ferrand, France, and his colleagues tested the fitness of two groups of adults — university students and elite endurance athletes — and compared them with 8–12-year-old boys who did not regularly engage in strenuous physical training.

After intense workouts on a stationary bike, the boys’ heart rates returned to normal faster than those of both groups of adults. The children also eliminated lactate — a metabolic by-product that contributes to muscle fatigue — from their blood faster than the adults did.

Although the number of children enrolled in the study was small, the researchers say their results show that children’s muscle endurance is naturally high but falls on the path to adulthood.