A 47-year study found fitness and strength start declining in the mid-30s

A Swedish study that followed 427 randomly selected men and women born in 1958 for 47 years, from age 16 to age 63, found that aerobic fitness and muscular endurance peaked between ages 26 and 36 and began to decline from the mid-30s. By age 63, average performance had fallen roughly 30% to 48% from those peak levels, and the pattern was similar in men and women. Decline was gradual before it sped up, a shape that makes the loss easy to miss in any single year.

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Maria Westerståhl, a lecturer at Karolinska Institutet who led the work, did not frame the result as a verdict. Her message, as quoted in the research summary, was “It is never too late to start moving,” because physical activity can slow the decline even though it cannot completely stop it.

Following the same people from 16 to 63

The Swedish Physical Activity and Fitness study, known as SPAF, was reported in the Journal of Cachexia, Sarcopenia and Muscle. Its distinguishing feature, according to the ScienceDaily summary, is that it measured the same individuals repeatedly rather than comparing young people with old people at a single moment. The participants were assessed on aerobic fitness, muscular endurance and muscle power, giving the researchers three separate curves to compare. The first two peak somewhere between 26 and 36; the third peaks sooner and falls on its own schedule.

ScienceAlert’s report on the study adds that the measurements were taken at ages 16, 27, 34, 52 and 63, and that the researchers said cross-sectional studies appear to have underestimated the age-related decline. The distinction is the study’s central claim: tracking the same 427 people from adolescence to their early 60s shows the shape of the curve, including the point where it bends, in a way that a snapshot of different age groups cannot.

Endurance, power and the mid-30s turning point

Aerobic fitness and muscular endurance generally peaked between 26 and 36. Explosive muscle power peaked even earlier; ScienceAlert reports that it topped out at age 27 in men and 19 in women. In that sense the mid-30s is when decline in the broader measures of fitness becomes visible, while the quickest-fading quality, power, had already begun to slip.

The loss was modest at first and then accelerated with age, so the decade after the peak looks gentle compared with the later ones. ScienceAlert’s account puts endurance and aerobic capacity falling 0.3% to 0.6% per year initially and later by up to 2.5% per year, with muscle power falling 0.2% to 0.5% per year at first and 2% or more later. By 63, the average drop from peak ran from roughly 30% to 48%.

What staying active changed

People who were physically active in adolescence tended to keep better fitness, endurance and power through adulthood, and those who became active later improved their capacity by about 5% to 10%. Exercise, however, slowed the decline without preventing it, even in people who stayed active. The practical reading is that a fitness habit built early pays dividends later, while a habit started in adulthood still moves the numbers upward, if modestly. The mechanisms behind the age-related decline are still unknown; the article lists mitochondrial function, muscle mass and signaling, and nervous-system changes as hypotheses.

The same Swedish cohort has produced a follow-up on blood vessels. Published in Scientific Reports, it found that better aerobic fitness in midlife was associated with less arterial stiffness at age 63, even after adjusting for obesity, smoking and blood pressure. Association is the operative word, because the report does not establish that fitness itself prevented the stiffening.

Cautions about reading the age-35 figure

A population-average peak at around 35 does not mean any one person loses strength at that age; people can keep improving through training. The study is observational and does not test directly whether exercise causes the differences it observed. Other work cited alongside it points the same way for older adults: in a nine-year study published in GeroScience, 82 older people who kept cycling saw their peak aerobic capacity fall an average of 19.7%, and nearly all still performed at or above the 90th percentile for their age.

Evidence on training later in life comes from a small study in The American Journal of Medicine. It followed 49 men aged 60 to 82 with no prior structured exercise for four years: one group did weekly resistance training only, and another combined resistance and aerobic work. Both groups improved upper and lower body strength, and the combined group also improved aerobic fitness. The summary calls the study relatively small, with participants choosing their own programs, so it is suggestive rather than conclusive.

The researchers plan another assessment at age 68, which has not yet been reported. Until then, the 47-year record gives a measured answer to when decline starts and a more hopeful one to whether it can be slowed.

This article was produced with the assistance of AI and reviewed by Morning Overview editors prior to publication.

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