You get out of breath halfway up the stairs.
In the evening, it’s harder to shake off fatigue than it used to be.

While thinking, “It’s just because I’m getting older,” don’t you ever wonder?
At what age does physical fitness actually start to decline?

There is a study that brings us closer to the answer.
It is a study that followed the same people for 47 years, from age 16 to 63.
Today, I will introduce its contents as simply as possible.

https://youtu.be/yIAxLdwZVUM

When you think, “I never used to be like this”

You find yourself reaching for the handrail on the station stairs.
Your arms feel heavy for a while after holding your grandchild.
The day after returning from a trip, you are exhausted all day.

You think, “Maybe I’m not exercising enough.”
But where should you start?
And in the first place, is there any point in starting now?

I imagine this is a feeling many people can relate to.

The decline in physical fitness started earlier than you might think

Many people might think that physical fitness only starts to decline once you reach your 50s or 60s.

But what the study showed was a much younger age. And something even more important became clear.
It is that the individual differences appear not in when the decline begins, but in
how it declines after that.

Following the same 427 people for 47 years

The research was conducted by a team at the Karolinska Institute in Sweden.
It is a prestigious medical institution, also known for selecting the Nobel Prize in Physiology or Medicine.

In 2025, it was published in the medical journal ‘Journal of Cachexia, Sarcopenia and Muscle,’ which specializes in muscle atrophy.

The subjects were 427 people born in 1958.
They were not athletes, but people living completely ordinary lives.

The physical fitness of these people was actually measured five times: at ages 16, 27, 34, 52, and 63.
Studies that follow the same people for nearly half a century are quite rare.

They measured three types of physical fitness

1. Endurance (stamina)
   Measured by VO2 max, an indicator of how much oxygen the body can take in

2. Muscular endurance (muscle stamina)
   Measured by how many bench press repetitions could be performed

3. Explosive power (ability to move quickly)
   Measured by the height of a vertical jump

Result: The peak of physical strength was around 35 years old

Age at which the peak was reached

• Peak of endurance and muscular endurance … between 26 and 36 years old

• Women’s endurance … around 35 years old

• Men’s endurance … around 36 years old

The research team states that physical strength begins to decline from around age 35.

The rate of decline changes along the way

It is gradual at first.
However, once you pass 45, the rate of decline more than doubles.

At age 63

• Endurance and muscular endurance … about 65% of the peak

• Women’s explosive power … 48% lower than the peak

• Athletes who continue to exercise … maintained over 80% in endurance events

Even at the same age of 63, there is this much of a gap.

This is the biggest surprise. The age of the peak was the same whether you exercised or not

For both those who exercised and those who did not,
the age at which they reached their physical peak was almost the same.

In other words, the time when the downhill slope begins comes the same way for everyone.

However, things change at age 63.
The difference in physical strength between individuals had widened to 3 to 25 times compared to when they were 16.

Even if the start is the same, the way you go downhill after that creates a huge difference.

Moreover, physical fitness improved no matter what age you started at.

People who switched from a sedentary lifestyle to an active one saw
improvements in physical fitness at any age.

• Endurance … 6-7% increase

• Muscular endurance … 11% increase

• Explosive power … 4% increase

Also, those who exercised at age 16 maintained higher physical fitness until age 63.

The research team believes that
“even if you cannot change the timing of the peak, you may be able to change the rate of decline afterward through exercise.”

I will also note the limitations of the study.

• The subjects were Swedish people born in 1958. It is unclear if the same applies to Japanese people or the current generation.

• Whether they exercised or not was asked only as a “yes/no” question.

• The type and intensity of exercise were not distinguished. This study does not reveal what kind of exercise is effective.

• Measurements were taken only up to age 63. The situation after age 70 has not been verified in this study.

Therefore, please read this separately from claims like “doing this exercise will restore your physical fitness.”

Things you can do starting today

What can be said from this study is simple.

1. The decline in physical fitness comes to everyone around age 35.

2. The rate of decline varies greatly from person to person.

3. People who started exercising saw their physical fitness improve at any age.

What is important is not something special, but continuing to incorporate time for physical activity into your daily life.

Although not verified by the study, for example,
take the stairs instead of the elevator once.
Try walking for one train station distance.
I think starting with such small things is fine.

(If you have underlying health conditions or concerns about your knees or lower back, it is best to consult your doctor first for peace of mind.)

Summary: The decline is the same. How you descend is up to you.

I will summarize today’s talk in three lines.

As you age, your physical strength will decline.
However, you might still be able to change how fast it declines starting now.

So that your future self, 10 or 20 years from now, can climb stairs comfortably.
I believe today’s small step will lead to that.

Detailed information is also explained in this video.

Reference
Report by a research team at the Karolinska Institute in Sweden (published in the medical journal ‘Journal of Cachexia, Sarcopenia and Muscle’ in 2025). This article is based on the content of the explanatory video above.