Why does exercise seem effortless for some people while others find the same activity exhausting? New research suggests that part of the explanation may lie in our early experiences of physical activity.

Scientists at Pennsylvania State University have discovered that physical demands during development can produce lasting changes in movement efficiency.

Their experiments revealed that animals accustomed to carrying additional weight while growing up could perform the same task as adults without using extra energy.

Published in Proceedings of the Royal Society B in October 2026, the findings raise intriguing questions about the long-term benefits of an active childhood.

What The Researchers Discovered

The study followed 12 young guinea fowl, birds that walk on two legs and share certain movement characteristics with humans. Researchers divided the birds into two groups. Six wore small weights attached to one leg during development, while the others grew without additional loads. Each weight represented approximately 4% of the bird's body mass.

Both groups experienced otherwise identical living conditions and exercise routines.

When the birds reached 16 weeks of age, scientists measured their energy consumption while walking on a treadmill.

The results revealed a remarkable difference between the groups.

Early Activity Changes Energy Use

Birds that had never carried additional weight required 23% more energy when walking with the attached load. However, those accustomed to carrying weights throughout development showed no comparable increase.

Their movements had become so efficient that carrying the additional weight required slightly less energy than walking without it.

Even more surprisingly, these trained birds used approximately the same amount of energy as untrained birds walking without any extra weight.

The findings demonstrate how physical demands during growth can influence the energy required for movement later in life.

How Scientists Measured The Difference

Rather than relying on observations of walking speed or distance, the researchers measured metabolism directly. The birds walked on a treadmill equipped with a specialised chamber that recorded oxygen consumption and the gases released during breathing. Researchers compared energy use while standing, walking normally and walking with additional weight.

No meaningful differences appeared while the birds were standing still.

The advantages became apparent during movement, suggesting that the adaptation was specifically related to walking efficiency.

Could The Same Effect Occur In Humans?

The findings raise an important possibility for human health. If similar adaptations occur in people, childhood activity might influence how demanding exercise feels during adulthood. Researchers suggest that more efficient movement could make physical activity easier to maintain throughout life. However, the experiment involved only 12 birds, and the same effect has not yet been demonstrated in humans.

The study also investigated controlled limb loading rather than ordinary childhood activities such as running, cycling or playing outdoors.

Further research is needed before drawing conclusions about human exercise habits.

Why Childhood Movement Matters

Researchers believe their discovery could contribute to understanding why some adults find physical activity more demanding than others.

If movement requires less energy, individuals may be more willing to remain active, potentially supporting cardiovascular health and general wellbeing.

The findings also highlight how developing bodies can adapt to repeated physical demands.

Nevertheless, there is no evidence from this study that children should exercise with added weights to achieve similar benefits.

Age-appropriate activities, including walking, outdoor games, swimming and cycling, remain practical ways to encourage movement.

The research offers a fascinating new perspective on physical development: experiences during childhood may influence how efficiently the body moves long after growing up.