When Does Fitness Decline Start? Key Lessons from a 47-Year Study
Key insights: 60-second read
- Peak physical capacity comes relatively early: Aerobic capacity and muscular endurance peaked between about 26 and 36, while muscle power peaked earlier.
- Decline begins before 40: The study found a gradual decline after peak capacity that accelerated with age.
- The decline is substantial: From peak performance to age 63, the estimated decline ranged from 30% to 48%, depending on the measure.
- Physical activity matters: People who were active earlier in life had higher physical capacity, while becoming active later in life was also associated with measurable improvements.
What does this 47-year study tell us about ageing?
A 2025 Swedish study followed 427 people from age 16 to 63 and repeatedly measured aerobic capacity, muscular endurance and muscular power. It found that physical capacity peaks relatively early, declines gradually after that, and that the rate of decline becomes steeper with age.
- Peak capacity: Aerobic capacity and muscular endurance peaked between approximately 26 and 36 years.
- Earlier peak for power: Jump performance peaked at 27 in men and 19 in women.
- Decline: The decline began before 40 and accelerated later in adulthood.
- Physical activity: Being active earlier in life was associated with better physical capacity, while becoming active later was also associated with improved performance.
As a physiotherapist, one of the most common concerns I hear is that losing strength, fitness and physical capacity is simply an unavoidable part of getting older. A fascinating new study provides a useful perspective - because while some decline appears to be part of normal ageing, the researchers also found substantial differences between individuals and meaningful associations with physical activity.
The SPAF study, published in the Journal of Cachexia, Sarcopenia and Muscle, followed the same Swedish participants from adolescence into early old age. This is particularly valuable because longitudinal research can show how physical capacity changes within the same people over decades, rather than simply comparing different age groups at one point in time.
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When Does Physical Fitness Peak?
The study found that aerobic capacity and muscular endurance generally peaked between the mid-20s and mid-30s, while muscular power peaked earlier. After peak capacity, performance gradually declined, with the rate of decline becoming progressively steeper with age.
- Aerobic capacity: Relative VO2 max peaked at approximately 26 years in men and 31 years in women.
- Muscular endurance: Bench press performance peaked at approximately 36 years in men and 34 years in women.
- Muscular power: Vertical jump performance peaked at approximately 27 years in men and 19 years in women.
The study suggests that physical capacity follows a gradual, non-linear trajectory, with measurable decline beginning before 40 and accelerating later in adulthood.
A physio's perspective
This is one reason I think it is useful to view fitness as something worth building and maintaining across the lifespan. Ageing is not something we can stop, but our starting point and the rate at which our physical capacity changes are not identical from one person to another.
How Much Does Fitness Decline With Age?
By age 63, the estimated decline from each measure's peak ranged from 30% to 48%. Muscular power showed the largest decline, while aerobic capacity and muscular endurance also declined substantially.
| Measure | Peak age | Decline to age 63 |
|---|---|---|
| Relative aerobic capacity | 26-31 years | 37-40% |
| Muscular endurance | 34-36 years | 32-35% |
| Muscular power | 19-27 years | 41-48% |
The annual rate of decline also changed with age. For aerobic capacity and muscular endurance, the researchers estimated relatively small declines initially, accelerating to around 2%-2.5% per year later in the observation period. Muscle power showed a similar acceleration, reaching approximately 2.2% per year.
Can Exercise Change How We Age?
Based on this study, yes, but with an important qualification: this study does not show that exercise stops age-related decline. Instead, it found that physical activity was associated with higher physical capacity and that people who became physically active later in adulthood showed measurable performance improvements.
Participants who reported being physically active during leisure time at age 16 had higher aerobic capacity, muscular endurance and muscular power throughout the observation period. The researchers also found that participants who switched from an inactive to an active lifestyle improved their performance across all three fitness measures.
The estimated improvements associated with switching from inactive to active were approximately:
- 6%-7% for aerobic capacity
- 11% for bench press performance
- 4% for vertical jump performance
That is an important message. You do not need to have been highly active throughout your entire life to benefit from becoming more active. The study suggests that physical capacity remains adaptable even later in adulthood.
What Are the Limitations of This Study?
This is a particularly interesting study, but it is important to have perspective.
First, the participants came from a Swedish population cohort born in 1958, so the results may not apply identically to every population. Second, the study measured several important aspects of physical capacity, but these tests do not capture every component of health, physical function or quality of life.
Physical activity was also assessed relatively simply as whether participants reported being physically active during leisure time. That means the study cannot tell us exactly how much exercise, what type of exercise, or what training program is optimal for preserving physical capacity.
Most importantly, associations between physical activity and better physical capacity do not prove that activity alone caused every difference between individuals. Other lifestyle, environmental and socioeconomic factors may also contribute.
What Should You Do With This Information?
You cannot prevent ageing, but you can invest in your physical capacity. This study reinforces the value of staying physically active across the lifespan rather than waiting until a noticeable decline has already occurred.
- Start wherever you are: If you are currently inactive, becoming more active can still be worthwhile. The study found measurable improvements in people who became active later in adulthood.
- Build a broad base of fitness: Aerobic capacity, muscular endurance and muscular power are different qualities, so maintaining physical capacity is unlikely to be about one type of exercise alone.
- Don't ignore power: Muscle power declined substantially in this study. As appropriate for your age, experience and health, maintaining the ability to produce force quickly may be an important part of staying physically capable.
- Prioritise consistency: The most useful exercise program is one you can perform regularly and adapt as your circumstances change.
- Individualise your approach: Pain, previous injury, medical conditions, training history and current capacity all matter. Your exercise program should reflect the person doing it, not simply your age.
The bigger picture
For me, one of the most interesting findings is the increasing difference between individuals as they age. The researchers found that the variation between people increased substantially over the 47 years, with relative aerobic capacity showing a 25-fold increase in variance. Ageing affects everyone, but we clearly do not all follow the same physical trajectory.
The journey of ageing is universal, but our physical capacity is highly individual. We cannot stop the clock, but there is good reason to keep investing in the body you want to have as you get older.
- Grant
I genuinely hope this article offers a fresh perspective - or at least one useful takeaway. If you have a different issue, or simply want to learn more about how your body moves, head over to the Your Wellness Nerd YouTube channel. Subscribe if you feel inclined, and let me know in the comments what you'd like me to cover next.
- Grant
Frequently Asked Questions
At what age does physical fitness peak?
In this 47-year Swedish study, relative aerobic capacity peaked at about 26 years in men and 31 years in women. Muscular endurance peaked at about 36 years in men and 34 years in women, while jump-based muscular power peaked earlier, at 27 years in men and 19 years in women.
When does physical decline begin?
The study found that physical capacity began to decline after peak performance, with the decline becoming more apparent before age 40 and accelerating with increasing age. The exact trajectory differed depending on the physical capacity being measured.
How much does fitness decline by age 63?
From peak performance to age 63, the estimated decline ranged from 30% to 48%. Relative aerobic capacity declined by about 37% in women and 40% in men, muscular endurance by about 32%-35%, and vertical jump performance by about 41%-48%.
Can you improve your fitness if you start exercising later in life?
Yes. In this study, participants who transitioned from an inactive to an active lifestyle showed measurable improvements in aerobic capacity, muscular endurance and muscular power. The study therefore supports the idea that physical capacity remains adaptable in adulthood.
Does exercise stop age-related decline?
No. The study found that physical capacity declined with age even though physical activity was associated with better performance and a slower rate of decline. Exercise should therefore be viewed as a way to build and maintain physical capacity, not as a way to stop biological ageing.
Why is muscle power important as we age?
Muscular power declined substantially in this study, with vertical jump performance declining by approximately 41%-48% from peak to age 63. Power is the ability to produce force quickly and contributes to many everyday movements, so maintaining this physical quality may be relevant to preserving physical function as we age.
One key insight from this research
Physical capacity naturally declines with age, but the trajectory is not identical for everyone. This study found that staying physically active was associated with higher physical capacity, while becoming active later in adulthood was also associated with measurable improvements.
Reference:
Westerståhl M, Jörnåker G, Jansson E, et al. (2025). Rise and Fall of Physical Capacity in a General Population: A 47-Year Longitudinal Study. Journal of Cachexia, Sarcopenia and Muscle, 16(6), e70134. Read the full study on PubMed Central.
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