Physical capacity starts to slip earlier than most of us assume. A Swedish study that tracked the same 427 people from age 16 to 63 found that fitness and muscle endurance generally began to decline around the mid-30s, even in people who stayed active.[1] The encouraging half of the finding is that exercise still changed the trajectory.
What the study did
Researchers at Karolinska Institutet followed a random sample of men and women born in 1958 as part of the Swedish Physical Activity and Fitness study. Participants were tested repeatedly over 47 years. That design matters. Most earlier work compared different age groups at one point in time, which can blur differences between generations. Here, the same people were measured as they aged.
The tests covered three things: aerobic fitness (how well your body uses oxygen), muscular endurance (how long muscles keep working), and muscle power (how quickly you can produce force, which matters for climbing stairs or catching yourself on a stumble).
What it found
- Aerobic fitness and muscular endurance generally peaked between ages 26 and 36. Explosive muscle power tended to peak even earlier.
- Decline began around the mid-30s, was modest at first, and sped up with age.
- By age 63, average performance had fallen roughly 30 to 48 percent from peak levels.
- The pattern was similar in men and women.
- Adults who became physically active gained about 5 to 10 percent in physical capacity. People who were active as teenagers tended to keep better fitness, endurance, and power into adulthood.
Exercise did not stop the decline completely, and the authors say they plan to study why. As lead author Maria Westerståhl put it, activity can slow the decline in performance even if it cannot fully stop it.
How to read the numbers
These are population averages. A peak in the mid-30s does not mean you become weaker at 35, and people can keep improving their own fitness after the average starts to fall. The study also does not prove what causes the decline. Changes in muscle cells, the nervous system, and energy production in mitochondria are all suspects, and the researchers are still looking.
What to do with this at 40 and beyond
- Start now. The adults who began exercising gained ground, and a later start still counted.
- Train power as well as endurance. Include some quick, forceful movements such as stair climbing, brisk step-ups, or jumps your joints tolerate.
- Keep strength work in the week, not just walking. Strength training is one of the most practical tools for holding onto muscle as you age.
- Check with your clinician before a big jump in intensity, especially if you have joint pain, heart concerns, or have been inactive for years.
References
[1] Westerståhl M, Jörnåker G, Jansson D, Aasa U, Ingre C, Pourhamidi K, et al. Rise and Fall of Physical Capacity in a General Population: A 47-Year Longitudinal Study. Journal of Cachexia, Sarcopenia and Muscle, 2025;16:e70134. Population: 427 randomly selected Swedish men and women born in 1958, assessed from age 16 to 63. DOI: https://doi.org/10.1002/jcsm.70134. Finding as used: aerobic fitness and muscular endurance peaked at ages 26 to 36 and declined from the mid-30s; performance fell roughly 30 to 48 percent by age 63; similar in men and women; becoming active in adulthood improved capacity by about 5 to 10 percent. Reported by ScienceDaily, October 15, 2026. ↑︎
Kurt Greiner
Kurt is a digital strategist and IT professional blending emerging technology with practical application to help businesses and individuals streamline their digital presence. His current work focuses on the intersection of intentional living and technological resilience, exploring how individuals can leverage modern tools to navigate the second half of life with purpose.
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