A remarkable 47-year Swedish study has found that fitness, muscle endurance and physical performance generally begin declining around age 35 — even among people who keep exercising.

Researchers at Karolinska Institutet tracked 427 randomly selected men and women born in 1958 as part of the Swedish Physical Activity and Fitness study (SPAF), assessing them repeatedly from age 16 through age 63.

Aerobic fitness and muscular endurance peaked between ages 26 and 36, with explosive muscle power peaking even earlier. By age 63, average performance had fallen by roughly 30 to 48 percent from peak levels, with the same overall decline rates in men and women.

Unlike most earlier research on physical aging, which compared people of different ages at a single point in time, the Karolinska project repeatedly examined the same individuals as they grew older.

That distinction matters because physical abilities vary considerably from person to person, so following individuals over decades let researchers observe how fitness changed within the same population rather than across generations.

ScienceDaily reports that the decline was initially modest but became progressively faster with age, and that the pattern showed up in aerobic fitness, muscular endurance, and muscle power — the ability to produce force quickly for jumping, climbing stairs and staying mobile.

The most striking finding is a reversal of expectations: regular physical activity did not entirely prevent age-related decline, even among people who stayed active. Yet exercise clearly changed the outcome.

According to ScienceDaily, participants who became physically active during adulthood improved their physical capacity by approximately 5 to 10 percent, and those active during adolescence maintained better fitness, endurance and muscle power throughout adult life.

'It is never too late to start moving,' said lead author Maria Westerståhl, noting the study shows activity can slow the decline in performance even if it cannot completely stop it.

A separate 2026 follow-up in the same cohort, reported by ScienceDaily, found better aerobic fitness at ages 34 and 52 predicted less arterial stiffness at 63, independent of obesity, smoking and blood pressure.

If the goal is preserving the capacities the Swedish study says fade with age, evidence from two 2026 studies suggests how to train.

ScienceDaily reports that older adults who cycled over nine years still lost an average 19.7 percent of peak aerobic capacity, yet nearly all performed at or above the 90th percentile for their age — while men aged 60 to 82 who took up weekly resistance training over four years improved upper and lower body strength, with added aerobic work boosting fitness too.

Meanwhile, bioengineer.org reports a Sri Lankan comparison of 126 adults found yoga practitioners outperformed aerobic exercisers on lower-limb strength and flexibility — averaging 44.81 cm versus 30.97 cm on the sit-and-reach test — while aerobic exercisers held a single-leg stance roughly twice as long.

Karolinska researchers plan another assessment round when participants reach 68, probing the biological mechanisms behind the age-35 turning point.