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Like most people, you have probably heard that muscle strength, absent some weightlifting, starts to decline in middle age. And you probably expect things like your balance, coordination and flexibility to naturally take a bit of a downturn .... someday. But new research from Duke University shows that these fitness-related changes begin much earlier than many people expect — often when they’re still in their 50s.

To assess age-related changes in people’s fitness abilities, researchers at Duke’s Center for the Study of Aging and Human Development had 775 participants from their 30s to their 90s perform tests designed to measure things like strength, endurance, balance and walking speed.

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At all ages, the men generally performed better than the women, but the age at which physical declines became truly apparent was consistent for both genders — the 50s. That’s when both sexes began to have trouble rising from and sitting in a chair repeatedly for 30 seconds (an indicator of declines in lower body strength) or standing on one leg for up to 60 seconds (a measure of balance).
Additionally, people in their 60s and 70s showed a marked slowing of gait speed (based on distance covered per second of a four-meter walk) and a drop in aerobic endurance (based on a six-minute walk test). By contrast, those in their 80s and 90s had dramatic declines in their balance, gait speed, lower body strength and aerobic endurance.
“People were very surprised by these changes because most of these tests aren’t typically done if you go to the doctor,” says study coauthor Katherine Hall, an assistant professor in medicine at Duke University School of Medicine's Division of Geriatrics. “Some of this is inevitable — our bodies are machines, and if you put 60 or 70 years of wear on any machine, it’s going to show some decline.” Even so, the rate or severity of these declines varies significantly from one person to another based on how active — or sedentary — you are.
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“What’s really happening is many of these changes start to occur earlier in life but don’t manifest themselves and become problematic until later in age,” explains Cedric X. Bryant, chief science officer at the American Council on Exercise. There's also a domino effect involved: Declines in muscle strength and bone mass start to occur in the 30s, he notes, and “losses of lower body strength and balance will eventually impact walking speed.”