A newly identified muscle protein may provide insight into why physically active people often age with greater strength and resilience, according to a new study published in the journal Science Advances.

Researchers found that a protein called NOX4 declines with both age and inactivity, and that lower levels were linked to signs of frailty, muscle loss, insulin resistance, and liver disease in mice.

When the team removed NOX4 from the muscles of mice, the animals became weaker and exhibited health problems commonly associated with aging.

However, when older mice exercised, their NOX4 levels increased again.

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The findings suggest that NOX4 could be part of the reason regular physical activity supports muscle maintenance and overall health later in life.

Marfred Suazo, known online as Fonz the Trainer in New York City, emphasized the importance of building and maintaining muscle for daily function and longevity.

Josephine Hunt, an educational leader and founder of The Resilience Revolution in New Jersey, said the research highlights what many fitness experts have long observed.

“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 told Fox News Digital.

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According to Hunt, exercise triggers deep biological responses that go beyond appearances or fitness goals.

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

Hunt added that one of the most important messages from the study is that staying active supports the body’s ability to recover from physical stress and challenges.

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

She also noted that healthy aging goes beyond longevity. “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.”

Researchers said further studies are needed, especially because the current findings were based primarily on animal models.

While the team also reviewed muscle samples from younger and older men and found similar declines in NOX4, they cautioned that additional research would be required to fully understand the protein’s role in human aging.

Despite those limitations, experts say the study may help explain why regular exercise remains one of the most effective tools for supporting strength and health with age.