Most of what we know about how physical fitness changes with age comes from cross-sectional studies that measure people of different ages at a single point in time and compare them. It’s a useful approach, but it has a fundamental flaw: it compares different people rather than the same person over time. Individual differences in genetics, lifestyle, and health make these comparisons noisy.
True longitudinal research following the same individuals across decades is rare, expensive, and extraordinarily difficult to sustain. This is why a study recently published in the Journal of Cachexia, Sarcopenia and Muscle (2025) is so scientifically valuable.
The Swedish Physical Activity and Fitness (SPAF) cohort began in 1974, when 427 16-year-old teenagers were randomly selected to represent Swedish adolescents. Researchers from Karolinska Institutet followed these same individuals through five data collections at ages 16, 27, 34, 52, and 63, objectively measuring aerobic capacity, muscular endurance, and muscle power at each visit. The result is one of the longest and most detailed human fitness trajectories ever recorded: 47 years of data on the same people, from adolescence to the edge of old age.
What it found is both sobering and, if you understand it correctly, genuinely motivating.
When Does Physical Fitness Actually Peak?
The first major finding concerns timing. When does your body reach its physical best?
The answer, it turns out, is earlier than most people assume, and it varies by the type of fitness being measured.
Aerobic capacity (the ability of your heart and lungs to deliver oxygen to working muscles, which scientists call VO₂) peaked in the mid-30s. For men, peak relative aerobic capacity arrived at age 26; for women, at 31. Absolute aerobic capacity peaked slightly later, at 35–36 years of age for both sexes.
Muscular endurance, measured by the number of bench press repetitions participants could complete, peaked at 34 in women and 36 in men.
Muscular power measured by a vertical jump test peaked earliest of all. Men reached their peak in jumping at just 27 years old. Women peaked even earlier, at 19.
This is a striking finding. For many people in their 20s, the idea that their muscular power has already crested feels counterintuitive. But the data are clear: biological peak performance at least in terms of explosive muscle output arrives in early adulthood, often before most people give fitness a second thought.
The Fall: Gradual at First, Then Relentless
After the peak, the decline is not sudden but inexorable, and it accelerates with age in a pattern the study describes as non-linear.
In the early years after peak, the drop is modest: roughly 0.3% to 0.6% per year across all fitness measures. This is the kind of change that goes unnoticed in daily life. You feel, broadly, the same. Then, sometime after 40, the pace picks up. By age 63, the annual rate of decline in aerobic capacity and muscular endurance had reached 2.0% to 2.5%. For muscular power (jump height), the yearly loss was 2.2% by the early 60s.
Across the full arc from each individual’s peak to age 63, the numbers are substantial:
- Aerobic capacity fell by 30–40% from peak to age 63
- Muscular endurance dropped by 32–35%
- Muscular power declined by 41–48%
To put that last figure in perspective: women at age 63 retained only about half of their peak jumping capacity. Men retained about 60%. And the study notes that among master athletes who exercise throughout their lives, this level of power loss doesn’t occur until age 75 or beyond, suggesting that much of what the average person loses is driven by inactivity, not biology alone.
The Gap Between People Widens Dramatically With Age
One of the most illuminating findings concerns not just how much fitness declines on average, but how differently it declines across individuals.
At age 16, the participants in the SPAF cohort were broadly similar in fitness. By age 63, the spread had exploded. The variation in relative aerobic capacity was 25 times greater at age 63 than at age 16. Jump height variation increased nearly fivefold. Muscular endurance variance tripled.
This tells us that aging is not a uniform process. Some people at 63 are remarkably fit. Others have lost the majority of their physical function. The trajectory a person follows depends heavily on the choices they make and, crucially, on the habits they form across the decades in between.
This is where the study’s findings about physical activity become especially important.
Being Active in Adolescence Pays Dividends for Decades
The researchers examined whether leisure-time physical activity at age 16 was associated with better fitness outcomes later in life. It was consistently and significantly across every fitness measure tested.
People who were physically active as teenagers maintained higher aerobic capacity, better muscular endurance, and greater muscle power throughout the entire observation period, up to age 63. This wasn’t a marginal difference. It persisted across every follow-up visit, at every age, in both men and women.
This finding has important implications for how we think about youth sports and physical education. The habits formed in adolescence aren’t just good for teenagers; they appear to establish a biological and behavioral foundation that influences physical capacity for life.
It’s Never Too Late to Start
But here’s the critically important counterbalance to that finding: becoming active at any age also works.
The researchers tracked individuals who changed their physical activity status between visits, going from inactive to active, or vice versa. Those who transitioned from inactivity to regular physical activity showed measurable improvements across all fitness outcomes:
- Aerobic capacity increased by 6–7%
- Muscular endurance improved by 11%
- Muscle power increased by 4%
These are not trivial gains. They represent the kind of functional improvement that, at older ages, can make the difference between independent living and dependency. And critically, this effect held regardless of the age at which the transition occurred. The study directly contradicts the fatalistic assumption that early inactivity sets an irreversible course.
The rate of decline after peak performance, the slope of the fall, appears to be modifiable by physical activity. You may not be able to push your peak later (the evidence suggests that timing is largely biological), but you can slow how steeply you descend from it.
Education, Socioeconomic Status, and Fitness
The study also found that having a university degree was associated with higher absolute aerobic capacity and better muscular endurance throughout life, though not with muscular power. The reasons for this association are likely multifactorial: access to recreational facilities, time for exercise, health literacy, lower physical job demands, and greater awareness of preventive health behaviors may all contribute.
This connection between education level and physical capacity mirrors findings from other research linking lower socioeconomic status to higher rates of sarcopenia, the age-related loss of muscle mass and function that becomes clinically significant in the 60s and 70s. It suggests that physical fitness across the lifespan isn’t purely a matter of individual willpower; structural and social factors also shape trajectories.
The Sarcopenia Warning Built Into These Numbers
Sarcopenia, the condition in which muscle loss becomes severe enough to threaten mobility, balance, and independent living, typically becomes clinically diagnosable in the 60s and 70s. But this study argues that the process underlying it begins decades earlier, quietly and without obvious symptoms.
The researchers frame their findings explicitly in this context: the decline in physical capacity beginning around age 35–40 in the general population, if left unchecked, can progress to clinically significant dysfunction. The 47-year trajectory they’ve mapped is essentially the pre-clinical history of sarcopenia in real time.
Currently, physical activity is the only evidence-based intervention that can meaningfully delay sarcopenia’s onset and progression. This study adds powerful longitudinal support for that conclusion and shifts the question from “how do we treat sarcopenia?” to “how early can we prevent it?”
The answer implicit in the data is that the time to start is before the decline accelerates, which means now, regardless of your current age.
What This Means for You
Whether you’re in your 20s, 40s, or 60s, this 47-year dataset offers clear guidance:
Your physical peak is real; it arrives earlier than you expect, and decline begins in your 30s, long before it becomes noticeable. That decline, while inevitable in direction, is not fixed in rate. Physical activity consistently and significantly moderates the rate of fitness decline.
Being active as a teenager provides long-term advantages that persist into your 60s. If you weren’t, becoming active now still produces real, measurable gains in aerobic fitness, muscle endurance, and power. The slope of the fall is something you can influence.
The widening gap between people at age 63 compared to age 16 is not random. It reflects decades of choices. The most powerful of those choices is whether to remain or become physically active.
Reference
Westerståhl, M., Jörnåker, G., Jansson, E., Aasa, U., Ingre, M., Pourhamidi, K., Ulfhake, B., & Gustafsson, T. (2025). Rise and fall of physical capacity in a general population: A 47-year longitudinal study. Journal of Cachexia, Sarcopenia and Muscle, 16, e70134. https://doi.org/10.1002/jcsm.70134