Early Exercise May Permanently Change How Much Energy the Body Uses To Move

An active childhood might train the body to make movement easier for life. A new study found that birds raised with extra weight became very efficient walkers. Once grown, they could carry the added load without using more energy than other birds walking without any weight.

This research offers direct proof that physical demands during development can permanently affect how much energy it takes to move. In this animal study, early activity led to more energy-efficient movement later on.

Why Moving Efficiently Matters

Movement uses energy that the body could use for other important things. These include growth, reproduction, staying warm, fighting off sickness, or finding food. Even small improvements in how efficiently an animal moves can impact its health, behavior, and survival.

Jonas Rubenson, a kinesiology professor at Penn State, led the study. He believes that "limb loading" — adding mechanical stress to arms or legs through weight or exercise — might be one way the body learns to reduce this energy cost.

Rubenson noted that this is one of the first studies to show that an animal's early life and environment can affect how much effort movement takes as an adult. The findings challenge the idea that how efficiently an animal moves is mostly fixed. Instead, the body might adjust during growth to handle repeated physical demands.

Growing Up with Extra Weight

The experiment followed twelve guinea fowl from two weeks old until they were nearly fully grown at sixteen weeks. Guinea fowl are similar to chickens, and their two-legged walking makes them good subjects for studying how humans move.

Six of the birds wore lead bands on one leg. These bands weighed 4% of their body mass. The other six birds grew up without any added weight. All birds were raised and exercised the same way, except for the bands. Starting at nine weeks, they were also trained to walk on a treadmill.

At sixteen weeks, researchers measured the birds while they stood still, walked normally, and walked with the leg weight attached. A special chamber recorded how much oxygen they used and how much carbon dioxide and water vapor they produced. This allowed the team to calculate the energy needed for each activity.

All twelve birds used similar amounts of energy while standing. However, walking with the added weight showed a big difference between the two groups.

A Disappearing Energy Penalty

Birds that had never carried the leg weight before needed 23% more energy to walk with it. But the birds that grew up with the weight showed no such penalty.

In fact, the experienced birds used slightly less energy when walking with the band than when walking without it. On average, carrying the load cost them no more energy than what ordinary guinea fowl typically use to walk without any weight.

Rubenson explained that the birds became so efficient they didn't use any more energy to carry the weight than they needed to walk without it. The experiment did not pinpoint exactly how the birds achieved this efficiency. Changes in their anatomy, muscle performance, posture, coordination, or walking style could all play a role. More research is needed to find these adaptations.

Could Childhood Activity Shape Adult Exercise?

These findings might help researchers understand why movement feels easier for some people than for others. One idea is that humans tend to avoid activities they see as physically demanding. If being inactive early in life makes movement harder later, it could create a cycle where exercise feels tougher and is avoided more often.

On the other hand, regular play and exercise during childhood might build lasting physical ability. This could make movement less tiring in adulthood. This possibility is important as children in the United States are less physically active than previous generations, Rubenson noted.

He believes that if people don't need to use as much energy to move as adults, then exercise and its many health benefits will be more accessible. This could impact many aspects of well-being, from heart health to mental health.

Deep Dive & References

Chronic limb loading results in remarkable load carriage economy in growing fowl - Proceedings of the Royal Society B: Biological Sciences, 2026