TL;DR
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A Swedish study tracked 427 people for 47 years and found that aerobic fitness and muscular endurance generally peaked between ages 26 and 36, with average physical performance declining from around the mid-30s. The study also found that people who became active in adulthood improved their physical capacity, although activity did not fully prevent age-related decline.
A 47-year Swedish study that repeatedly assessed 427 people from age 16 to 63 found that aerobic fitness and muscular endurance generally peaked between ages 26 and 36, followed by a gradual decline that became faster with age. Researchers also found that becoming more physically active in adulthood was linked to improvements in physical capacity, even though activity did not entirely prevent the decline associated with aging.
The research was part of the Swedish Physical Activity and Fitness study, or SPAF. Researchers at Karolinska Institutet followed 427 randomly selected men and women born in 1958, assessing them repeatedly over 47 years. Participants were first examined at age 16 and followed through age 63. The study’s long timeframe let researchers track changes within the same people, rather than infer age-related change by comparing different age groups at one point in time.
Assessments covered aerobic fitness, muscular endurance and muscle power. Aerobic fitness describes how effectively the body uses oxygen during exercise; endurance concerns how long muscles can keep working, while power measures the ability to produce force quickly. The researchers reported that some abilities, particularly explosive muscle power, reached their highest levels earlier than the broader fitness measures. Across the measures, average performance at age 63 was about 30% to 48% below peak levels.
The report said the broad pattern appeared in both men and women, with similar rates of decline despite differences in individual fitness. Participants who became physically active during adulthood improved their physical capacity by approximately 5% to 10%. Those active as adolescents also tended to maintain better fitness, endurance and power through adulthood. The findings were published in 2025 in the Journal of Cachexia, Sarcopenia and Muscle.
What the Findings Mean for Lifelong Fitness
The results distinguish an average population trend from an individual limit. A peak around the mid-30s does not mean everyone begins to lose strength suddenly at 35, or that people cannot improve after that age. The study’s results on adult activity indicate that fitness can still improve later in life, even as average performance changes with age.
The findings also suggest that declining activity alone may not explain the pattern: the report says physically active participants still showed age-related changes. Researchers believe biological processes contribute too, but the study does not establish exactly which processes drive the decline. For readers, the practical relevance is measured: activity may help preserve or improve physical capacity, but it should not be presented as a way to stop aging or guarantee a specific outcome.
Physical capacity matters beyond exercise performance. The source material notes a 2026 follow-up study using the same cohort that examined other health outcomes and reported links between aerobic fitness at ages 34 and 52 and heart health. The provided report does not give the follow-up study’s detailed results, so it cannot support more specific claims about the size or nature of those links.
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How SPAF Tracked Participants Over Decades
Many studies of physical aging compare people of different ages at a single time. SPAF instead made repeated assessments of the same cohort, beginning in adolescence and continuing into later adulthood. That design gives researchers a record of how performance changed within this group over time, although it does not mean every person followed the same trajectory.
The report describes a pattern in which aerobic fitness and muscular endurance peaked across a range of ages, rather than at one exact birthday. The peak range of 26 to 36 and the approximate start of decline around the mid-30s are averages from the group. Individual fitness levels varied, and the results do not establish a universal age when a person’s strength must begin to fall.
The researchers have suggested that changes involving muscle cells, the nervous system and energy production could help explain age-related changes in performance. Mitochondria, which help cells convert nutrients into usable energy, are one possible area of study. These mechanisms were presented as potential contributors, not as conclusions proven by the cohort findings. The report also refers to sarcopenia, an age-related loss of muscle mass and strength, but does not say the study diagnosed that condition in participants.
“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.”
— Maria Westerståhl, lead author and lecturer at Karolinska Institutet’s Department of Laboratory Medicine
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What the Study Cannot Establish
The study identifies patterns in one Swedish cohort, not a precise age at which every person loses strength. Its reported peak range and later declines are group averages; the source says individual fitness levels differed. The findings also do not show that activity will produce the same improvement for every person, or specify how much activity participants did and what kinds of exercise led to the reported gains.
The biological causes remain unresolved. The report lists changes in muscle cells, the nervous system and energy production as possible contributors, but says researchers are still investigating the mechanisms. The figures for the decline by age 63 are reported across the study’s measurements; the source material does not provide separate figures for each measure or additional statistical details.
The 2026 follow-up is described only in brief in the material provided. It says the researchers examined additional health outcomes and found that people with better aerobic fitness at ages 34 and 52 had links to heart health, but the available account ends before giving the full finding. Its precise results and limits are therefore unclear here.
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Researchers Seek Biological Explanations
The SPAF researchers say they plan to investigate the mechanisms that may explain why physical performance peaks in early adulthood and why activity can slow, but not fully halt, decline. That work could help clarify how much of the observed pattern reflects biological aging and how much is shaped by other factors.
The 2026 follow-up study, which used the same cohort to examine health outcomes, adds another line of research, but the source material does not provide enough detail to assess its specific results. Readers should treat the age range and percentage changes as findings from this cohort, rather than as a timetable that predicts an individual’s future. The confirmed takeaway remains that physical capacity changed across adulthood in the study, while participants who became more active could still make gains.
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Key Questions
At what age did physical fitness peak in the study?
Aerobic fitness and muscular endurance generally peaked between ages 26 and 36. Some measures, including explosive muscle power, peaked earlier. These are group patterns, not a fixed peak age for every person.
Does the study show that everyone starts losing strength at 35?
No. The researchers reported that average physical performance began declining around the mid-30s. Individual fitness levels and trajectories differed, so the study does not establish an exact age when every person starts losing strength.
Can exercise prevent age-related decline?
The study found that physical activity did not completely prevent decline, but adults who became active improved their physical capacity by approximately 5% to 10%. The report does not establish that every person will see the same result.
How many people took part, and how long were they followed?
Researchers followed 427 men and women born in 1958, with assessments beginning at age 16 and continuing through age 63—47 years in total.
Do researchers know why performance declines?
Not yet. The report identifies changes in muscle cells, the nervous system and energy production as possible contributors, but says researchers are still investigating the biological mechanisms.
Source: rss
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