A newly identified muscle protein may help explain why people who stay active as they age often remain stronger and healthier for longer, according to new research.

Scientists found that a protein called NOX4 naturally declines with age and inactivity. As levels dropped, researchers observed signs of frailty, muscle loss, insulin resistance and liver disease in mice.

The findings were published in the journal Science Advances.

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Researchers believe NOX4 helps muscles repair themselves and adapt to the physical demands of exercise.

When NOX4 was removed from the muscles of mice, the animals became weaker, lost muscle mass and developed health problems commonly associated with aging.

The researchers also found that exercise helped restore NOX4 levels in older mice.

Josephine Hunt, an educational leader, former group fitness instructor and founder of The Resilience Revolution based in New Jersey, who was not involved in the study, told Fox News Digital that the findings help explain why exercise benefits so many aspects of health.

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"Movement is medicine," Hunt said.

"The emerging NOX4 research is exciting because it helps explain something exercise scientists have observed for decades. Physical activity does far more than strengthen muscles."

Hunt said many people view exercise as a way to improve appearance or fitness, but its effects reach much deeper.

"Exercise appears to activate biological signaling pathways that help the body adapt, repair and become more resilient over time," she said.

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She said one of the study's biggest takeaways is that physical activity helps the body maintain its ability to recover from challenges.

"Exercise does not simply help us look younger or stay physically fit," Hunt said. "It appears to help the body maintain its ability to adapt, repair and respond to stress."

Hunt added that healthy aging is about more than simply living longer.

"Healthy aging is not just about adding years to life," she said. "It is about preserving strength, function, independence, cognitive health and overall quality of life."

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Researchers stressed that additional studies are needed, but the findings may help explain why regular physical activity remains one of the most effective tools for maintaining health as people age.

The study was conducted in mice, meaning the findings do not necessarily translate directly to humans.

While the team also examined muscle samples from younger and older men and found similar declines in NOX4, additional research is needed to better understand the protein's role in human aging.