A long-term study in Sweden spanning nearly half a century has shown that human physical fitness can begin to decline around age 35. However, the study also offers a positive message: physical exercise still provides significant benefits, even if you start later in life.
This research project, conducted by the Karolinska Institute over 47 years, focused on tracking changes in aerobic fitness, muscle strength, and physical endurance over time. The results showed that physical performance tends to peak around the mid-30s, then declines, but physical activity can help slow this decline.
The study was part of the Swedish Physical Activity and Fitness (SPAF) program, in which scientists tracked hundreds of randomly selected men and women, aged 16 to 63. The work, published in the scientific journal Journal of Cachexia, Sarcopenia and Muscle , provides a detailed look at how human physical capacity develops and changes over decades.
Previously, much of our understanding of the relationship between age and fitness was based on cross-sectional studies, that is, comparing different age groups at the same point in time. In contrast, the SPAF study is unique because it repeatedly measured the fitness and strength of the same participants over nearly 50 years across Sweden, providing a more accurate reflection of physical aging over time.
Exercise always brings benefits.
Data collected shows that a decline in fitness and strength can begin in the mid-30s, regardless of individual training levels. From this point on, physical capacity continues to decline gradually, and the rate of decline often accelerates as people enter old age. However, researchers also found very encouraging evidence: adults who started exercising more actively were still able to improve their physical performance by 5 to 10%. This demonstrates that improving fitness is still entirely possible even after peak performance.
Maria Westerståhl, a lecturer in the Department of Laboratory Medicine and the lead author of the study, said:
It's never too late to start exercising. Our research shows that physical activity can slow performance decline, though it can't stop it entirely. In the future, we will continue to investigate the mechanisms by which most people peak fitness around age 35, as well as why exercise can slow decline but not stop it completely.
The research is ongoing. It is expected that next year, when the participants turn 68, they will be reassessed. Scientists hope to link changes in physical capacity to lifestyle, health status, and underlying biological mechanisms.