Fitness and strength start slipping around age 35, and continues to worsen progressively with age regardless of how much people trained earlier in life, a long-running Swedish study conducted by Karolinska Institutet has revealed.
The study followed adults for nearly five decades to uncover when physical decline truly begins.
The encouraging twist: adults who began exercising later still improved their physical capacity by up to 10 per cent. It's a powerful reminder that staying active matters, even if you start late.
The study followed people for 47 years to examine how fitness, strength, and muscle endurance evolve during adulthood.
These findings, published in the Journal of Cachexia, Sarcopenia and Muscle, emphasise that while the biological clock starts ticking in our mid-thirties, staying active at any age can significantly alter the trajectory of physical aging.
The research is part of the Swedish Physical Activity and Fitness study (SPAF), which tracked several hundred randomly selected men and women between the ages of 16 and 63.
The study offers rare long-term insight into how physical capacity changes over decades rather than snapshots at a single point in time.
Most earlier research in this area relied on cross-sectional data, comparing different age groups rather than following the same individuals.
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In contrast, the SPAF study repeatedly measured fitness and strength in the same participants across Sweden for nearly half a century, making it one of the most comprehensive efforts of its kind.
"It is never too late to start moving. Our study shows that physical activity can slow the decline in performance, even if it cannot completely stop it. Now we will look for the mechanisms behind why everyone reaches their peak performance at age 35 and why physical activity can slow performance loss but not completely halt it," says Maria Westerstahl, lecturer at the Department of Laboratory Medicine and lead author of the study.
The research is ongoing. Next year, the participants will be examined again when they reach age 68. The team hopes to better understand how changes in physical performance are connected to lifestyle choices, overall health, and underlying biological processes.
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