At what age does your body start losing strength? What a 47-year study found
A Swedish study published in late 2025, back in the news this week, followed the same people for 47 years. Its conclusion fits in one sentence: fitness and strength peak around 35, then decline, faster and faster. Its good news too: it is never too late to start.
What the study measured
Since the 1970s, researchers at Karolinska Institutet in Stockholm have followed several hundred randomly selected Swedish men and women. They measured them several times between the ages of 16 and 63: cardiorespiratory fitness, muscle strength and muscular endurance.
That is what makes the study rare. Most studies compare young and older people at the same moment. This one followed the same people for 47 years, so it sees how each person actually changes.
What it found
Fitness and strength peak around age 35. After that, they start to decline, slowly at first, then faster and faster with age. The decline shows up in everyone, whatever sport they did when they were younger.
In other words, the body does not start losing its abilities at 60 or 70: the process starts much earlier, without you noticing it day to day.
The good news: it is never too late
Participants who became active as adults improved their physical capacity by 5 to 10%. "It is never too late to start moving," says lead author Maria Westerståhl. She adds that physical activity can slow the decline in performance, even if it cannot stop it completely.
The participants will be tested again at 68, to understand how lifestyle and health shape this decline.
How to know where you stand
The decline is invisible from one day to the next. The only way to track it is to measure yourself, then measure again a few weeks later. Three simple tests are enough, at home and with no equipment:
- the 30-second chair stand, for leg strength, the kind the study talks about;
- getting up off the floor without hands, for mobility;
- standing on one leg, for balance.
Each guide gives you the usual values for your age and a free calculator to see where you stand.
What you can do
The World Health Organization recommends muscle-strengthening exercise for adults at least two days a week, on top of 150 to 300 minutes of moderate activity such as brisk walking. After 65, it adds activities that work balance and strength at least three days a week.
For the legs, the simplest move is the test itself: stand up from a chair without your hands, slowly, then sit back down with control. A few sets, two or three times a week, already work the right muscles. If you have pain, an injury or any doubt, ask a health professional first.
What the study does not say
It covers a few hundred Swedes, and 35 is an average age: everyone has their own curve. It shows a trend, not what will happen to any one person. And the 5 to 10% gains apply to people who became active as adults; they do not say how much each person can gain.
In BodyYears
BodyYears measures your strength, balance and mobility with your iPhone camera, combines them into a body age, then suggests ten-minute sessions that target the ability that needs it most. The monthly test then shows whether the curve is moving. The method explains the whole calculation.
Sources
- Westerståhl M, Jörnåker G, Jansson E, Aasa U, Ingre M, Pourhamidi K, Ulfhake B, Gustafsson T. Rise and Fall of Physical Capacity in a General Population: A 47-Year Longitudinal Study. Journal of Cachexia, Sarcopenia and Muscle, 2025 (doi 10.1002/jcsm.70134).
- Karolinska Institutet. Long-term study reveals physical ability peaks at age 35, December 16, 2025.
- Bull FC et al. World Health Organization 2020 guidelines on physical activity and sedentary behaviour. British Journal of Sports Medicine, 2020.