Physical ageing may begin earlier than many people expect. A major Swedish study has found that muscle endurance and aerobic fitness generally start declining around the age of 35, even among people who exercise regularly.

Researchers at Karolinska Institutet followed participants for nearly five decades, examining how their physical abilities changed throughout adulthood.

The findings, published in the Journal of Cachexia, Sarcopenia and Muscle, suggest that although ageing gradually affects physical performance, regular exercise can help maintain strength and fitness.

What The Long-Term Study Revealed

The Swedish Physical Activity and Fitness study followed 427 men and women born in 1958. Researchers assessed participants repeatedly over 47 years, beginning when they were 16 and continuing until they reached 63. Unlike studies comparing different age groups, this approach allowed scientists to observe changes within the same individuals. The assessments measured aerobic fitness, muscular endurance and muscle power.

Results showed that aerobic capacity and muscular endurance generally peaked between ages 26 and 36. Some abilities, particularly explosive muscle power, reached their highest levels even earlier.

How Quickly Does Fitness Decline?

Physical performance did not suddenly deteriorate at 35. Instead, researchers observed a gradual decline that accelerated with age.

By 63, average performance across the measurements had fallen by approximately 30–48% from peak levels. The pattern was similar among men and women, despite differences in individual fitness. Importantly, even participants who remained physically active experienced some decline. This suggests that reduced exercise is not the only explanation for age-related changes in physical performance.

Can Exercise Slow The Process?

One of the study's most encouraging findings concerned people who became active later in adulthood. Participants who increased their physical activity experienced improvements of approximately 5–10% in physical capacity.

Those who had been active since adolescence also tended to maintain higher fitness levels throughout adulthood. These results indicate that exercise remains beneficial even after the age when physical performance typically peaks. However, the findings do not establish that exercise can completely prevent biological ageing.

Why Muscles Change With Age

Scientists are still investigating the mechanisms responsible for declining physical performance. Possible explanations include changes in muscle cells, nervous system function and energy production.

Mitochondria, which help cells generate usable energy, may become less efficient with age. Muscles can also gradually lose mass and respond less effectively to growth and repair signals. Over time, these changes may make activities such as climbing stairs or carrying shopping more demanding.

Fitness May Also Protect The Heart

A related 2026 study involving the same Swedish participants found that better aerobic fitness at ages 34 and 52 was associated with less arterial stiffness at 63.

Flexible arteries help maintain healthy blood circulation, while increased stiffness is associated with cardiovascular risk. However, the findings demonstrate an association rather than proving that fitness directly prevents arterial ageing.

What This Means For Everyday Life

The researchers plan further assessments when participants reach 68. Meanwhile, the evidence supports maintaining an active lifestyle throughout adulthood. Walking, cycling, swimming and resistance training can all contribute to physical fitness when adapted to individual abilities.

For people starting exercise later in life, gradual increases in activity can make routines more manageable.

The central message is that reaching middle age does not eliminate opportunities for improvement. Even when physical performance naturally declines, regular movement can help preserve mobility, strength and independence.