Fitness and strength start to decline around age 35, even in people who keep training. That’s the main finding of a Swedish study that followed the same group of adults for 47 years, from age 16 to 63. The decline is gradual at first and speeds up with age, but it isn’t fixed: people who became active as adults improved their physical capacity by 5 to 10 percent. Here’s what the study measured, what it means, and how to train to slow the drop.
What the 47-Year Swedish Study Found
The research comes from the Swedish Physical Activity and Fitness study (SPAF), run at Karolinska Institutet in Stockholm and published in the Journal of Cachexia, Sarcopenia and Muscle at the end of 2025. It followed 427 randomly selected men and women born in 1958, about half of them women, testing them repeatedly from adolescence to age 63. The researchers measured aerobic capacity, muscular endurance, muscle power and strength, using tests such as the bench press and vertical jump.
Three results stand out. Physical capacity peaked in the late twenties to mid-thirties and began to fall from around 35, regardless of how much people had trained earlier in life. The decline then continued gradually and accelerated with age. And men and women differed in their absolute levels but declined at similar rates. In other words, the general population follows the same curve already documented in elite athletes, just from a lower starting point.
Why This Study Matters More Than Most
Most of what we know about aging and fitness comes from cross-sectional studies, which compare different people at different ages at one point in time. That approach mixes up the effects of age with differences between generations. SPAF tested the same individuals for almost half a century, which shows how each person actually changed over time. Very few studies in the world have that kind of data.
It has limits worth stating. Several hundred participants is a modest sample, all born in the same year in one country, and activity habits were partly self-reported. The study also describes what happens, not exactly why. The research team has said it will now look for the biological mechanisms behind the peak at 35 and examine participants again at age 68.
Exercise Slows the Decline, It Doesn’t Stop It
The most useful finding is the 5 to 10 percent improvement in people who became physically active as adults. Lead author Maria Westerståhl summed it up plainly: activity can slow the loss of performance even if it can’t halt it completely. That matches other recent research. A 2026 study in GeroScience followed 82 older adults who kept cycling for nine years and still found an average drop of about 20 percent in peak aerobic capacity. Staying active doesn’t freeze the clock, but it means you decline from a much higher level.
That higher level is what matters for daily life. Aerobic capacity and strength set how easily you climb stairs, carry groceries, recover from illness and stay independent later on. A large study published in the British Journal of Sports Medicine in June 2026 linked 90 to 120 minutes of weekly strength training with the lowest mortality risk, especially when combined with regular aerobic exercise. Honestly, the takeaway from all of this is simple: 35 isn’t a reason to slow down, it’s the age when training starts paying the biggest dividends.
How to Train Each Quality That Declines
The study tracked several physical qualities, and each one responds to a different type of training. Here’s how to cover all of them in a normal week.
| Quality that declines | Why it matters | How to train it | Weekly dose |
|---|---|---|---|
| Aerobic capacity | Strongest fitness predictor of long-term health | Easy zone 2 cardio plus one interval session | 150 min moderate or 75 min vigorous |
| Maximal strength | Lifting, carrying, independence | Compound lifts with progressive overload | 2 to 3 sessions, 90 to 120 min total |
| Muscle power | Fast reactions, fall prevention | Jumps, throws, lifting with fast intent | 2 short blocks inside strength sessions |
| Muscular endurance | Repeated effort without fatigue | Higher-rep sets, circuits, carries | 1 to 2 blocks per week |
Power deserves special attention. It typically falls faster than strength with age, and it’s the quality most people stop training first. Adding a few sets of jumps, medicine ball throws or lifts performed with fast intent keeps it trained without extra sessions.
A Simple Weekly Plan for Adults Over 35
This structure covers every quality the study measured in about four hours a week:
- Two or three full-body strength sessions built around squats, hinges, presses, rows and carries, adding weight or reps over time.
- Two to four sets of jumps or throws at the start of each strength session, while you’re fresh.
- Two easy cardio sessions of 30 to 45 minutes at a pace where you can still talk.
- One interval session, such as 4 x 4 minutes hard with 3 minutes easy between efforts.
- Daily walking and 5 to 10 minutes of mobility or balance work.
If you’re starting from zero, begin with two strength sessions and two walks a week, then build up over two to three months. Anyone with heart disease, high blood pressure or joint problems should get medical advice before adding intense intervals. For more detail, see our guides to strength training over 50, VO2 max for beginners, how fast you lose muscle and balance training after 40.
FAQ
At what age does fitness start to decline?
A 47-year Swedish study from Karolinska Institutet found that fitness and strength begin to decline around age 35, then continue to fall gradually, with the decline accelerating at older ages.
Not completely, but it slows it substantially. In the same study, people who became physically active as adults improved their physical capacity by 5 to 10 percent.
Is it too late to start exercising after 40?
No. Adults who start training later in life still gain strength and aerobic fitness, and the long-term health benefits of strength and cardio training apply at every age.
Do men and women lose fitness at the same rate?
According to the Swedish study, men and women had different absolute fitness levels but declined at similar rates as they aged.