But the findings also offer an encouraging message. Although exercise cannot completely prevent the physical changes associated with aging, becoming more active during adulthood can still produce meaningful improvements in fitness and strength.

Researchers at Karolinska Institutet in Sweden spent nearly five decades tracking how people's physical abilities changed as they grew older. Their findings, published in 2025 in the Journal of Cachexia, Sarcopenia and Muscle, help explain when physical decline begins, how it progresses, and why staying active remains important throughout life.

A 47-Year Study Reveals When Physical Fitness Peaks

The research was part of the Swedish Physical Activity and Fitness study (SPAF), which followed 427 randomly selected men and women born in 1958. Participants underwent physical assessments over 47 years, beginning at age 16 and continuing through age 63.

Unlike most earlier research on physical aging, which compared people of different ages at a single point in time, this project repeatedly examined the same individuals as they grew older.

That distinction matters because physical abilities vary considerably from person to person. Following individuals over decades allowed researchers to observe how fitness changed within the same population rather than relying on comparisons between different generations.

The assessments included aerobic fitness, muscular endurance, and muscle power. Aerobic fitness describes how effectively the body uses oxygen during exercise, while muscular endurance measures how long muscles can continue working. Muscle power reflects the ability to produce force quickly, which is important for activities such as jumping, climbing stairs, and maintaining mobility.

The results revealed a consistent pattern. Aerobic fitness and muscular endurance generally reached their highest levels between ages 26 and 36, with physical performance beginning to decline around the mid-30s. Some abilities, particularly explosive muscle power, peaked even earlier.

The decline was initially modest but became progressively faster with age. By age 63, average performance across the measurements had fallen by roughly 30 to 48 percent from peak levels.

Importantly, the pattern appeared in both men and women. Although individual fitness levels differed, the overall rates of age-related decline were similar between the sexes.

Exercise Can Slow Physical Decline, Even After 35

One of the most striking findings was that regular physical activity did not entirely prevent the decline associated with aging. Even people who remained physically active showed evidence of diminishing performance as they grew older.

However, exercise made an important difference in how well people maintained their abilities.

Participants who became physically active during adulthood experienced improvements in physical capacity of approximately 5 to 10 percent. Those who were already active during adolescence also tended to maintain better fitness, endurance, and muscle power throughout adulthood.

These findings suggest that physical decline is not simply the result of becoming less active with age. Biological changes also appear to contribute, although exercise can help reduce their impact.

"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. 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," says Maria Westerståhl, lecturer at the Department of Laboratory Medicine and lead author of the study.

The distinction is important. Reaching an average physical peak in the mid-30s does not mean everyone suddenly becomes weaker at that age. People can continue improving their individual fitness and strength through training, even after the age when population averages begin to decline.

Why Does the Body Start Losing Strength?

Scientists are still investigating exactly why physical performance begins deteriorating relatively early in adulthood.

Several biological processes are thought to contribute. As people age, changes in muscle cells, the nervous system, and the body's ability to produce energy can gradually reduce physical performance.

One possible contributor involves mitochondria, the tiny structures inside cells that convert nutrients into usable energy. Changes in mitochondrial function may affect how efficiently muscles produce the energy required for movement.

Over time, muscles can also lose mass and become less responsive to the signals that trigger growth and repair. More advanced muscle deterioration can contribute to sarcopenia, an age-related condition involving the loss of muscle strength and mass that can make everyday activities increasingly difficult.

Although these processes often become more noticeable later in life, the Swedish findings suggest that some underlying changes in physical function begin decades earlier.

The researchers hope to determine which biological mechanisms drive this decline and why physical activity can delay, but not completely prevent, the loss of performance.

Follow-Up Research Links Early Fitness to Heart Health

The importance of maintaining physical fitness throughout adulthood extends beyond muscle strength.

In a 2026 follow-up study published in Scientific Reports, researchers examined additional health outcomes using the same Swedish cohort. They found that people with better aerobic fitness at ages 34 and 52 tended to have less arterial stiffness at age 63.

Arterial stiffness occurs when blood vessels become less flexible, making it harder for them to expand and contract as blood moves through the body. Greater stiffness is associated with increased cardiovascular risk.

Notably, the relationship between earlier fitness and later artery health remained significant even after accounting for several established cardiovascular risk factors, including obesity, smoking, and blood pressure.

The results do not establish that better fitness directly prevented arterial stiffening. However, they suggest that maintaining aerobic capacity during early and middle adulthood may have benefits that extend well into later life.

More Evidence That Staying Active Pays Off

Other research has reinforced the idea that exercise remains valuable even when physical performance inevitably declines with age.

A 2026 study published in GeroScience followed 82 older adults who continued cycling over a nine-year period. Despite maintaining active lifestyles, participants experienced an average 19.7 percent decline in peak aerobic capacity.

Yet their fitness remained remarkably high. Nearly all participants who completed the exercise test performed at or above the 90th percentile for their age when compared with reference populations.

The findings suggest that although regular exercise cannot eliminate biological aging, it can help people maintain unusually high levels of physical function as they get older.

Another 2026 study in The American Journal of Medicine followed 49 men ages 60 to 82 who had not previously participated in structured exercise training.

Over four years, participants who performed weekly resistance exercises improved their upper and lower body strength. Those who combined resistance training with aerobic exercise also improved their aerobic fitness.

Although the study was relatively small and participants chose their training programs, its findings support the idea that meaningful physical improvements remain possible well into older adulthood.

It's Never Too Late to Improve Your Fitness

The original Swedish research is continuing, with investigators planning another round of assessments when participants reach age 68.

The next stage is intended to explore how long-term changes in physical performance relate to lifestyle, overall health, and the biological processes responsible for aging.

Together, the findings point to an important distinction between getting older and becoming physically inactive. Some loss of fitness and strength appears to be a natural part of aging, but the extent of that decline can vary substantially.

While the body may begin losing some of its physical capabilities around age 35, those changes do not eliminate the benefits of exercise. Whether someone begins exercising in their 30s, 50s, or later, becoming more active can still help improve physical performance and preserve the abilities needed for a healthier, more independent life.