A 47-year Swedish study found that physical capacity usually peaks well before 40. Aerobic fitness and muscular endurance peaked between ages 26 and 36, while explosive muscle power peaked earlier—at 27 in men and 19 in women—then declined gradually before losses accelerated with age.
What the 47-year study measured
The findings come from the Swedish Physical Activity and Fitness study, published by Maria Westerståhl and colleagues in the Journal of Cachexia, Sarcopenia and Muscle in 2025. Researchers followed 427 people—48% women—all born in 1958. Participants completed repeated objective fitness tests from age 16 in 1974 through age 63 in 2021.
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That long follow-up allowed the researchers to estimate how performance changed within the same birth cohort instead of comparing unrelated groups of different ages. The study examined aerobic capacity, muscular endurance and explosive power. It did not measure every definition of strength, such as a one-repetition maximum barbell lift, so its results describe physical capacity rather than a single universal “strength” score.
When each type of physical capacity peaks
| Capacity | Study measure | Average peak | Estimated early decline | Estimated decline at older ages | Average loss from peak to age 63 |
|---|---|---|---|---|---|
| Aerobic capacity | Objective aerobic-fitness testing | Between ages 26 and 36, in both sexes | About 0.3%–0.6% per year | About 2.0%–2.5% per year | Within the study’s overall 30%–48% range |
| Muscular endurance | Bench-press repetitions | Between ages 26 and 36, in both sexes | About 0.3%–0.6% per year | About 2.0%–2.5% per year | Within the study’s overall 30%–48% range |
| Muscle power | Sargent jump test | Age 27 in men; age 19 in women | About 0.2%–0.5% per year | About 2.2% per year | Within the study’s overall 30%–48% range |
The percentages are estimated average trajectories, not a timetable that applies identically to every person. Individual differences became substantially wider with age. The study reports that total average decline from each measure’s peak to age 63 ranged from 30% to 48%, depending on the outcome.
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Why “around age 35” is only a shorthand
Headlines often cite age 35 because aerobic capacity and muscular endurance reached their highest levels in the late 20s to mid-30s. That does not mean a switch flips on a person’s 35th birthday. Power—the ability to produce force quickly—peaked earlier, especially in women in this cohort. Different abilities follow different curves, and the transition from peak performance to decline is gradual.
The pattern was initially modest loss followed by a steeper drop at older ages. A person can therefore remain near their peak for years, then experience faster change later. The cohort averages cannot predict the exact year when an individual will notice a difference in lifting, climbing stairs, running or recovering from exertion.
How physical activity changed the outlook
Activity in adolescence
People reporting higher leisure-time physical activity at age 16 had better measured aerobic capacity, muscular endurance and power later in the study. This is an association, not proof that one specific teenage sport caused a lifelong advantage.
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Becoming active during adulthood
Participants who became more active as adults also tended to perform better on the objective tests. The result supports starting—or restarting—regular activity after adolescence. It does not show that exercise removes the effects of aging; active participants still followed age-related decline.
What the study did not test
The investigators did not compare a particular workout plan, resistance-band routine, supplement, wearable or piece of gym equipment. The results therefore cannot establish that one training method is superior. They show a relationship between activity and better capacity, not a guaranteed percentage improvement from a named intervention.
What to do with the findings
The practical message is to treat fitness as something worth maintaining before a decline becomes obvious, while recognizing that improvement remains possible later. These are sensible applications of the activity associations in the study, not protocols tested by SPAF.
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Build resistance progressively
Use exercises that challenge major muscle groups and can become harder over time. Body-weight movements, resistance bands, free weights and machines can all provide progression when the load, repetitions or range of motion are adjusted. Beginners should start with a manageable level, learn safe technique and increase difficulty gradually.
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Keep an aerobic base
Walking briskly, cycling, swimming or another sustained activity can help maintain aerobic work capacity. Choose an option that is accessible enough to repeat consistently; the study linked higher activity with better performance but did not identify a required weekly dose.
Do not neglect power
Because power peaked earlier and declined faster at older ages in the study, include appropriately scaled movements that require controlled speed—such as rising from a chair briskly or performing a light medicine-ball throw—only when balance, joints and medical status make them suitable. Older adults or anyone with pain, dizziness or a chronic condition should ask a clinician or qualified professional how to train safely.
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Track your own trend
Repeated, comparable measures are more informative than a single test. You might record walking time, sit-to-stand repetitions or a consistent strength exercise, keeping the equipment, technique and rest periods similar. A personal baseline can reveal change even though it cannot reproduce the study’s laboratory-style longitudinal design.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Important limits on the age estimates
- The sample consisted of Swedish adults born in 1958, so the exact peak ages and rates may not transfer to every country, generation or socioeconomic group.
- The study was observational. It estimates trajectories from repeated measurements but cannot prove that aging alone caused a fixed percentage loss in each individual.
- Differences in birth cohorts, physical-activity habits, education and health-care settings may affect comparisons with people born decades later or living elsewhere.
- “Strength” is broader than any one test. The study’s bench-press repetitions and Sargent jump results do not represent maximal lifting strength, balance, flexibility or every daily-life task.
The clearest answer to “When does strength decline?”
For several common measures of physical capacity, peak performance arrived between the late 20s and mid-30s, with explosive power peaking earlier. Decline started gently and became faster with advancing age; by 63, average losses from peak ranged from 30% to 48% across the tested outcomes. Staying active—or becoming active in adulthood—was associated with better performance, but it did not stop aging-related decline.
“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,” Maria Westerståhl, lecturer at Karolinska Institutet, said in a summary of the study.
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