Why Childhood Exercise May Make Movement Cheaper for Life: Lessons from Guinea Fowl

Is being active as an adult partly decided by how active you were as a child? A new study in birds suggests that the answer may be written, at least in part, into how efficiently the body learns to move while it is still growing.

Researchers at Penn State report that guinea fowl raised with a small weight on one leg became adults that could carry extra load without spending any additional energy. Birds raised without the weight paid a steep metabolic price when the same load was added. The work comes from the lab of kinesiologist Jonas Rubenson and was published in Proceedings of the Royal Society B.

Why the energy cost of moving matters

Moving costs energy, and animals have limited energy to spend. A long-standing hypothesis holds that animals which move cheaply have more left over for finding food, finding mates or escaping danger, so efficient movement may be favored by evolution. The same idea has been applied to people: if movement feels effortful, adults tend to avoid it.

What was not well established is whether the energy cost of moving is fixed or whether it can be shaped by experience during development. According to the Penn State team, this is the first study to directly show that developmental experience influences how efficiently adults use energy while moving.

The experiment

The researchers chose guinea fowl, close relatives of chickens that, like humans, walk on two legs. Twelve birds were followed from two weeks of age to 16 weeks, when they are close to full size.

·         Six birds wore a lead band on one leg throughout development. The band weighed 4% of the bird’s body weight.

·         Six birds wore no extra weight.

·         All twelve were raised and exercised in the same way, and all were trained to walk on a treadmill from nine weeks of age.

At 16 weeks, a flow-through metabolic chamber was placed over the treadmill. It tracked each bird’s oxygen consumption and its output of carbon dioxide and water vapor, which is considered the gold-standard way to measure how much energy an animal is using. Each bird was measured standing still, walking unweighted, and walking with the leg band.

What they found

Standing still, all the birds used the same amount of energy. The difference appeared during walking with a load:

·         Birds that had never carried weight used 23% more energy when walking with the band than when walking without it.

·         Birds that had carried weight while growing used no more energy with the band than without it. In fact, they used slightly less, which surprised the team.

·         On average, the chronically loaded birds carrying the band spent no more energy than typical birds walking with nothing extra.

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"The birds became so efficient that they were not using any more energy to carry the weight than they needed to walk without the weight."

Jonas Rubenson, Penn State

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In other words, the energy cost of movement appears to be more plastic than previously thought: the growing body seems to tune itself to the mechanical demands placed on it.What it could mean for children

Rubenson and colleagues suggest the findings may extend to humans. If adults who were inactive as children need to spend more energy for the same movement, exercise would feel harder to them, and they might be less likely to stay active. Regular activity in childhood and adolescence, the authors propose, could instead make exercise easier to sustain throughout life. The researchers point to the fact that children in the United States are less active than previous generations.

Reading the result with some caution

This is an elegant experiment, but it is worth keeping its limits in view:

·         Small sample. Twelve birds, six per group, is a modest number, even for a controlled lab study.

·         One species, one intervention. A lead band on a single leg is not the same as play, sport or varied childhood activity.

·         The human link is a hypothesis. The authors themselves say the result probably applies to humans, but the study did not test people. Showing that childhood activity lowers the energy cost of movement in humans, and that this in turn changes adult activity levels, would need human data.

Still, the study offers a clear mechanism to test: that what a growing body is asked to do shapes how cheaply it can do it later. If that holds in children, the case for active play becomes not only about fitness today, but about making movement easier for the decades ahead.

Sources

·         Press release: Bird bulking: Exercise in youth decreases energy used for moving as an adult, Penn State University (A. Wagner, August 24, 2026)

·         Original paper: Proceedings of the Royal Society B: Biological Sciences, DOI 10.1098/rspb.2026.0199

Source: Why Childhood Exercise May Make Movement Cheaper for Life: Lessons from Guinea Fowl