Aging may begin to affect physical abilities earlier than you expect, remarkable 47-year Swedish study found fitness, muscle endurance, performance declining around 35.
But findings also offer encouraging message, although exercise cannot completely prevent physical changes associated with aging, becoming more active can produce meaningful improvements.
Researchers at Karolinska Institutet in Sweden spent nearly five decades tracking how people’s physical abilities changed as they grew older significantly and systematically. The Findings published in 2025 in Journal of Cachexia, Sarcopenia and Muscle, help explain when physical decline begins, how it progresses, why staying active remains important.
Research was part of Swedish Physical Activity and Fitness study SPAF, followed 427 randomly selected men and women born in 1958 over 47 years. Participants underwent physical assessments over 47 years, beginning at age 16 and continuing through age 63, repeatedly examining same individuals as they aged.
Why Following Same Individuals Matters
Unlike most earlier research on physical aging comparing different ages at single point, this project repeatedly examined same individuals as they grew older.
Distinction matters because physical abilities vary considerably from person to person, following individuals over decades allowed observation within same population rather than generations.
Assessments included aerobic fitness, muscular endurance, muscle power, describing oxygen use, how long muscles continue working, ability to produce force quickly.
47-Year Study Reveals When Fitness Peaks
Results revealed consistent pattern, aerobic fitness and muscular endurance generally reached highest levels between ages 26 and 36, performance beginning to decline mid-30s. Some abilities, particularly explosive muscle power, peaked even earlier, decline initially modest but became progressively faster with age, offering important insights for longevity.
By age 63, average performance across measurements had fallen by roughly 30 to 48 percent from peak levels, yet exercise still helped maintain abilities. Importantly, pattern appeared in both men and women, although individual fitness levels differed, overall rates of age-related decline similar between sexes notably.
Exercise Can Slow Physical Decline Even After 35
One of most striking findings was regular physical activity did not entirely prevent decline associated with aging, even active people showed diminishing performance.
However, exercise made important difference in how well people maintained their abilities over decades, providing encouraging good news for all ages.
Participants who became physically active during adulthood experienced improvements in physical capacity of approximately 5 to 10 percent, demonstrating meaningful benefits from starting late. Those already active during adolescence also tended to maintain better fitness, endurance, muscle power throughout adulthood, showing early activity provides long-term protective advantages.
Biological Changes Contribute Beyond Inactivity
Findings suggest physical decline not simply result of becoming less active with age, biological changes also appear to contribute, although exercise can reduce impact.
It is never too late to start moving, study shows physical activity can slow decline in performance, even if it cannot completely stop it.
Lead author Maria Westerståhl, lecturer at Department of Laboratory Medicine, said researchers will look for mechanisms behind peak at 35 and why activity slows loss.
Peak Does Not Mean Sudden Weakness
Distinction important, reaching average physical peak mid-30s does not mean everyone suddenly becomes weaker at that age, individual improvements still possible through training. People can continue improving individual fitness and strength through training, even after age when population averages begin to decline, offering hopeful message for everyone.
Why Body Starts Losing Strength
Scientists still investigating exactly why physical performance begins deteriorating relatively early in adulthood, several biological processes thought to contribute to gradual decline. As people age, changes in muscle cells, nervous system, body’s ability to produce energy can gradually reduce physical performance and functional capacity. One possible contributor involves mitochondria, tiny structures inside cells converting nutrients into usable energy, changes in mitochondrial function may affect muscle energy production efficiency.
Over time, muscles can also lose mass and become less responsive to signals triggering growth and repair, more advanced deterioration contributing to sarcopenia condition.
Although processes often become more noticeable later in life, Swedish findings suggest some underlying changes in physical function begin decades earlier than expected.
Researchers hope to determine which biological mechanisms drive decline and why physical activity can delay, but not completely prevent, loss of performance.
Follow-Up Research Links Early Fitness To Heart Health
Importance of maintaining physical fitness throughout adulthood extends beyond muscle strength, follow-up study in 2026 published in Scientific Reports examined same Swedish cohort. Researchers found people with better aerobic fitness at ages 34 and 52 tended to have less arterial stiffness at age 63, suggesting long-term cardiovascular benefits.
Arterial stiffness occurs when blood vessels become less flexible, making harder to expand and contract as blood moves through body, associated with increased cardiovascular risk.
Notably, relationship between earlier fitness and later artery health remained significant even after accounting for several established cardiovascular risk factors including obesity, smoking, blood pressure.
Cyclists Maintain High Fitness Despite Decline
Other research reinforced idea exercise remains valuable even when physical performance inevitably declines with age, study in GeroScience followed 82 older adults cycling over nine years. Despite maintaining active lifestyles, participants experienced average 19.7 percent decline in peak aerobic capacity, yet fitness remained remarkably high compared to peers.
Nearly all participants who completed exercise test performed at or above 90th percentile for their age when compared with reference populations, demonstrating exceptional maintenance.
Resistance Training Works Well Into 60s And 80s
Another 2026 study in 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 upper and lower body strength, those combining resistance with aerobic also improved aerobic fitness.
Although study relatively small and participants chose training programs, findings support idea meaningful physical improvements remain possible well into older adulthood and beyond.
It’s Never Too Late To Improve Fitness
Original Swedish research continuing, investigators planning another round of assessments when participants reach age 68, exploring long-term changes relating to lifestyle and biological aging. Together, findings point to important distinction between getting older and becoming physically inactive, some loss natural but extent varies substantially based on activity.
While body may begin losing some capabilities around age 35, changes do not eliminate benefits of exercise, whether beginning in 30s, 50s, or later. Becoming more active can still help improve physical performance and preserve abilities needed for healthier, more independent life, offering encouraging good news for everyone.
Q&A
Q: When does physical fitness generally begin declining according to 47-year Swedish SPAF study of 427 people?
A: Study found aerobic fitness and muscular endurance generally peak between 26 and 36, physical performance beginning to decline around mid-30s, initially modest then faster.
Q: Can becoming more active during adulthood still improve fitness even after age 35 decline begins?
A: Yes, participants who became physically active during adulthood experienced 5 to 10 percent improvements, showing meaningful benefits and it is never too late to start.
Q: Does maintaining aerobic fitness in early and middle adulthood benefit heart health later according to follow-up research?
A: Yes, people with better aerobic fitness at ages 34 and 52 tended to have less arterial stiffness at age 63 even after accounting for obesity, smoking, blood pressure.
FAQ
Q: How much does physical performance decline by age 63 compared to peak levels in Swedish study?
A: By age 63, average performance across measurements had fallen by roughly 30 to 48 percent from peak levels, though exercise helped maintain unusually high function.
Q: Is decline around age 35 similar for men and women according to Swedish Physical Activity and Fitness study?
A: Yes, pattern appeared in both men and women, although individual fitness levels differed, overall rates of age-related decline were similar between sexes.
Q: Can people in their 60s, 70s, and 80s still improve strength and aerobic fitness through exercise?
A: Yes, study of men 60-82 showed weekly resistance exercises improved strength, combined resistance plus aerobic also improved aerobic fitness over four years.
Q: Why does body start losing strength relatively early, what biological mechanisms are involved?
A: Changes in muscle cells, nervous system, energy production, mitochondria function, muscle mass loss, reduced responsiveness to growth signals may contribute, leading to sarcopenia eventually.
Q: What will next stage of Swedish research investigate when participants reach age 68?
A: Next stage intended to explore how long-term changes in physical performance relate to lifestyle, overall health, and biological processes responsible for aging and decline.
Disclaimer: This article provides educational information based on 47-year Swedish SPAF study and follow-up research and does not constitute professional medical advice, diagnosis, or treatment recommendation.
