This is the age when strength and fitness start to fade, long-term data shows

Physical deterioration does not wait for old age.

A new study followed the same people for nearly half a century and found that measurable declines in fitness and strength begin around age 35 and continue gradually into middle age.

The study is based on the Swedish “Physical Activity and Fitness” cohort, which began in 1971 with participants born in 1958. Researchers enrolled 427 people, a mix of men and women, and tested the same individuals repeatedly over a 47-year span, according to a news release.

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Models were used to estimate age- and sex-specific changes in the original cohort over the study period.

Follow-up examinations were carried out at various points throughout adulthood, allowing Karolinska Institutet researchers to observe changes within individuals rather than relying on comparisons between different age groups, according to the release.

At each exam, participants underwent standardized physical tests performed by trained professionals. Aerobic fitness was measured by tests that assessed maximal oxygen consumption, a common indicator of cardiovascular fitness.

Muscle strength and endurance were assessed using controlled performance tests, including grip strength and repetitive motion tasks. Height, weight, and other basic physical measurements were also recorded, and participants provided information about their physical activity habits.

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By using the same tests over time, the researchers were able to track long-term trends in physical performance.

The results showed that physical performance peaks in early adulthood and begins to decline around age 30.

This decline was seen across multiple measures, including aerobic fitness and muscle strength, and continued steadily into adulthood, the researchers shared.

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Although individuals who were physically active tended to maintain higher levels of overall fitness, activity did not completely prevent age-related decline.

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Still, the data showed that participants who increased their activity levels later in life improved their physical capacity by about 5% to 10%, showing that the body continues to respond to exercise beyond the peak years.

Limitations of the study

The study, which was published in the Journal of Cachexia, Sarcopenia and Muscle, consisted of people who were born the same year and lived in Sweden.

The researchers acknowledged that this may limit how the findings apply to other populations with different social, environmental or genetic backgrounds.

As with any long-term study, some participants dropped out over time, which could affect how well the sample reflects the original population.

Additionally, physical performance was measured at specific intervals, so short-term fluctuations between testing periods were not captured.

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Additionally, although physical activity was associated with better outcomes, the observational nature of the study means that other factors such as diet, occupation or health conditions may also have influenced the results, the researchers said.

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The team plans to continue following the cohort as participants get older, with the goal of linking changes in physical ability to health outcomes in the future.