A Swedish study that followed hundreds of people for 47 years has found that aerobic capacity, muscular endurance, and several other measures of physical performance generally peak during early adulthood and begin to decline around the mid-30s.
However, this does not mean the body suddenly loses strength when someone turns 35. These changes occur gradually and are influenced by various factors, including biology, physical activity, lifestyle, and overall health.
The encouraging news is that exercise continues to provide benefits even after this decline begins. Even people who have never maintained a regular workout routine can improve their strength, endurance, and ability to perform everyday activities by becoming more physically active.
The findings come from long-term research conducted by scientists at Sweden’s Karolinska Institutet, with results published in 2025 in the scientific journal Journal of Cachexia, Sarcopenia and Muscle. The study provides a rare look at how physical performance changes over several decades of life.
The research was part of the Swedish Physical Activity and Fitness study, known as SPAF. Researchers followed 427 men and women born in 1958, beginning when they were 16 years old and continuing their assessments until they reached the age of 63.
The duration of this project makes it particularly valuable. Unlike studies that compare people of different ages at a single point in time, the researchers observed the same individuals throughout a substantial portion of their lives.
This approach helped them better identify changes associated with aging while taking into account that people have different levels of physical ability from an early age.
Which Physical Abilities Were Measured?
The assessments covered several important aspects of physical performance.
Aerobic capacity indicates how effectively the body can use oxygen during physical activity. It is associated with the ability to walk briskly, run, cycle, and perform activities that require endurance.
Muscular endurance describes how long muscles can continue working without becoming excessively fatigued. This ability is important both during exercise and when carrying out ordinary daily tasks.
Muscle power refers to the ability to produce force quickly. It helps with movements such as rising from a chair, climbing stairs, and maintaining balance while moving.
The results showed that aerobic capacity and muscular endurance generally reached their highest levels between the ages of 26 and 36. Some abilities, particularly those requiring explosive power, may peak even earlier.
After this period, performance began to decline gradually. Initially, the changes were relatively small, but the decline became more noticeable over the years.
By the age of 63, average results across the measured physical indicators were approximately 30 to 48 percent lower than their peak levels, according to the study’s reported findings.
The general pattern was observed in both men and women, although baseline fitness levels and individual abilities varied from person to person.
Not necessarily in the way many people might imagine.
The age of 35 is not a strict biological cutoff or a moment when everyone begins losing strength in the same way. Instead, it represents an approximate period when certain physical abilities, on average, begin to decline after reaching their peak.
Someone who exercises regularly may have greater strength and endurance at 45 than another person at 25 who engages in little physical activity.
Similarly, people can improve their performance even after passing the age when physical abilities typically reach their highest levels.
For this reason, researchers emphasize the importance of distinguishing between biological changes associated with aging and physical inactivity. Both can affect performance, but they are not the same thing.
Exercise Can Improve Physical Fitness Even in Adulthood
One of the most important conclusions from the Swedish research is that physical activity continues to matter even after age-related decline has begun.
The study found that people who became more physically active during adulthood experienced improvements in physical capacity of approximately 5 to 10 percent.
Meanwhile, individuals who had been physically active since adolescence tended to maintain better levels of fitness, endurance, and muscle power throughout adulthood.
However, even active participants experienced a decline in performance over time. This suggests that exercise can help preserve and improve physical abilities, but it cannot eliminate all the biological changes associated with aging.
Maria Westerståhl, a lecturer in the Department of Laboratory Medicine and the study’s lead author, emphasized that it is never too late to start moving. According to her, physical activity can slow the decline in performance, even though it cannot stop it completely.
The researchers aim to better understand the mechanisms that cause physical abilities to peak during a particular period of life and why exercise cannot entirely prevent the subsequent decline.
What Types of Physical Activity Can Help?
For most adults, combining aerobic exercise with strength training can be a practical way to support physical health.
Brisk walking, cycling, and swimming are examples of activities that can help improve cardiovascular endurance.
Meanwhile, weightlifting, resistance-band exercises, and bodyweight workouts can help maintain and develop muscle strength.
Balance and mobility exercises may also be beneficial, particularly for older adults who want to preserve their independence in everyday activities.
Not everyone needs to begin with intense workouts. For people who have been inactive for a long time, starting gradually and increasing activity according to their individual abilities may be a more appropriate approach. People with health conditions or physical limitations should seek professional advice before starting a new exercise program.
Why Does the Body Begin to Lose Strength With Age?
Scientists are still investigating the precise reasons why some physical abilities begin to decline during early adulthood. However, the process likely involves several biological changes occurring simultaneously.
One factor involves how muscles function and how the body produces the energy needed for movement.
Changes in Muscles and Energy Production
Inside cells are structures called mitochondria, which help produce usable energy for the body. Changes in how these structures function can affect how efficiently muscles perform during physical activity.
The nervous system also plays an important role because it coordinates movement and sends signals that help muscles generate force.
As people age, changes in these processes can affect physical speed, power, and endurance. However, the importance of each mechanism and the way it interacts with physical activity remain subjects of ongoing research.
Another concern is the gradual loss of muscle mass and strength. At older ages, this can contribute to sarcopenia, an age-related condition characterized by declining muscle mass and function.
Sarcopenia can make ordinary activities such as getting up from a chair, carrying objects, or climbing stairs more difficult. However, it does not affect everyone in the same way and should not be confused with every minor decline in performance that may occur during adulthood.
Maintaining physical activity, eating an appropriate diet, and taking care of overall health are important for supporting muscle function as the body ages.
Physical Fitness in Early Adulthood May Be Linked to Heart Health
The benefits of good physical fitness extend beyond the muscles. They may also be associated with cardiovascular health later in life.
A follow-up study published in 2026 in the journal Scientific Reports analyzed data from the same group of Swedish participants.
Researchers found that people with better aerobic capacity at ages 34 and 52 tended to have less stiff arteries at age 63.
Arteries are blood vessels that carry blood from the heart to different parts of the body. Their elasticity helps them adapt to the pressure created as blood circulates.
When arteries become stiffer, they lose some of their ability to expand and contract. Greater arterial stiffness is associated with a higher risk of cardiovascular problems.
According to the study’s reported findings, the association between earlier aerobic capacity and arterial health at age 63 remained statistically significant even after researchers accounted for several risk factors, including obesity, smoking, and blood pressure.
However, this finding demonstrates a statistical association and does not, by itself, prove that good physical fitness directly causes better arterial elasticity. Nevertheless, it reinforces scientific interest in the role of physical activity in long-term health.
Other Studies Show That Improvement Is Possible Even After 60
Evidence that physical activity remains beneficial during aging also comes from other research.
A study published in 2026 in the journal GeroScience followed 82 older adults who continued cycling over a nine-year period.
Despite maintaining their activity, participants experienced an average decline of 19.7 percent in maximal aerobic capacity. This shows that even an active lifestyle does not completely prevent age-related decline.
Nevertheless, their fitness levels remained very high compared with those of other people in the same age group. Almost everyone who completed the exercise tests achieved results at or above the 90th percentile compared with reference populations.
Another study, published in 2026 in The American Journal of Medicine, examined 49 men aged 60 to 82 who had not previously participated in structured exercise programs.
Over four years, participants who performed resistance exercises every week improved their upper- and lower-body strength. Those who combined resistance training with aerobic activity also improved their aerobic capacity.
The study was relatively small, and participants selected their own exercise programs, so the results should be interpreted cautiously. Nevertheless, the findings support the idea that measurable improvements in physical abilities can be achieved even at older ages.
What Can We Do to Maintain Strength as We Age?
The findings from these studies suggest that there is no single age after which exercise loses its value. On the contrary, regular physical activity can help preserve the abilities needed for an active and independent life.
To support strength and endurance, adults can focus on several sustainable habits:
- Daily movement: Walking, taking the stairs, and breaking up long periods of sitting can help reduce inactivity.
- Strength training: Weightlifting or bodyweight exercises can help maintain muscle mass.
- Aerobic activity: Brisk walking, cycling, and swimming can help improve endurance.
- Balanced nutrition: Getting enough protein and essential nutrients supports muscle function and recovery.
- Recovery and sleep: Adequate rest helps the body handle physical demands and recover after exercise.
These changes do not all need to happen at once. A routine tailored to a person’s age, health, and current activity level may be easier to maintain over the long term.
The 47-year Swedish study shows that certain physical abilities peak during early adulthood and begin to decline around the mid-30s. However, this process should not be viewed as an immediate and inevitable loss of strength.
People differ from one another, and physical activity levels can significantly influence how well performance is maintained over the years. Although exercise cannot completely stop biological aging, it can help improve strength, endurance, and aerobic capacity.
The researchers plan to continue following the Swedish group, with further assessments scheduled when participants reach the age of 68. These findings may help scientists better understand the relationship between lifestyle, aging, and long-term changes in physical performance.
The main message remains clear: turning 35 is not a deadline for strength or physical fitness. Whether someone starts exercising in their 30s, 50s, or later in life, appropriate physical activity can provide important benefits and help preserve independence for longer.
