Turning 40, I had an experience that made me painfully aware of the importance of physical stamina.
Two years ago, when I caught COVID-19, I lost 5kg in a week and it took about a month for my strength to return.
I had assumed I was fine, but losing my stamina was far more serious than I expected, leaving me anxious about whether I could ever get back to normal.
That's when I realized what people in their 40s need isn't intense muscle training, but building an immune foundation that makes you less prone to colds and illness.
By "stamina monster," I don't mean someone who is just muscular.
Someone who rarely catches colds. Someone who doesn't carry fatigue into the next day. Someone who can recover quickly after feeling off. Someone who maintains good sleep, diet, exercise, and rest.
In other words, people who maintain their immune baseline through daily habits.
Of course, boosting immunity doesn't mean building a body that never gets sick.
What I want to convey is not pushing your body to the limit, but building a lifestyle foundation that prevents health issues from arising every day.
As a physical therapist who has observed many bodies, I had somewhat overestimated my own stamina.
So this time, based on my experience of losing stamina and feeling anxious, I'll introduce five habits of "stamina monsters" that I've learned and continue to improve upon.
1. More Important Than Muscles: Building 'Protective Stamina' That Prevents Colds
Once you hit your 40s, having muscles alone isn't enough.
Muscles are important, of course.
But what you really want isn't just looks—it's a body that resists fatigue and stays healthy.
Physical stamina includes things like strength and endurance—"stamina for movement."
On the other hand, there's also protective power, such as sleep quality, immunity, and stress resilience.
I call this "protective stamina."
In the past, when I worked hard, I went all out but postponed rest.
I pushed through even when tired. I kept moving even with insufficient sleep. I tried to power through illness with sheer willpower.
But that approach doesn't last long.
Instead, starting in your 40s, it's more important to build a body that doesn't easily fall ill rather than pushing yourself to the limit.
The relationship between exercise and immunity isn't simply "the harder you train, the better." Reviews suggest regular physical activity may benefit immune health, while temporary changes in immune function have been reported after prolonged, high-intensity exercise.
Furthermore, a study tracking 1,002 people over 12 weeks found that those who did aerobic exercise at least five days per week experienced 43% fewer days of upper respiratory infection symptoms compared to those who barely exercised. This isn't about "pushing to your limits," but rather
the importance of moderate movement habits
.
Stamina monsters aren't people who force themselves too hard.
They're people who build bodies that can move without forcing it.
2. Just Standing Up Is Enough: Protecting Your Immune Foundation Against Sedentary Time
To build a body that stays healthy, you don't need to rush to the gym right away.
The first thing to reconsider is how much time you spend sitting.
When I started using a smartwatch, I was shocked.
On days with lots of desk work, I wasn't even reaching 5,000 steps. Some weekends, I didn't even hit 2,000.
I knew I wasn't exercising.
But seeing the numbers was worse than I imagined.
That's when I realized that in your 40s, daily activity levels quietly drop before you even notice being sedentary.
In this state, trying to suddenly do intense cardio or weight training is hard to sustain.
Without basic stamina, time, and mental bandwidth, exercise won't fit into your life.
Even if you manage it, you might sacrifice sleep, accumulate fatigue, or disrupt your appetite, leading to eventual breakdown.
So start by reducing sedentary time.
Stand up once an hour. Go get water. Take the stairs. Use a farther bathroom. Stand while doing tasks that allow it.
Even these small actions help your body return to "movement mode."
Before making exercise an event, activate your body throughout daily life.
This is the first step toward becoming a stamina monster.
WHO guidelines on physical activity and sedentary behavior recommend reducing sitting time and replacing it with physical activity of any intensity.
You don't need to aim for perfection from the start.
First, reduce sitting time.
Even that alone is a big step for bodies in their 40s.
3. Squats: Switching On Your Body Temperature and Blood Flow
I believe squats are the most cost-effective exercise for people in their 40s.
No equipment needed. No space required. They engage thighs, glutes, and core simultaneously.
Plus, even if each rep is low impact, spreading them throughout your day increases overall activity.
I do 10 squats every time I use the restroom.
If no one's around, I sometimes do jump squats.
What matters isn't powering through 30 minutes of exercise with gritted teeth.
It's placing small switches in your daily routine that activate your body.
For maintaining an immune baseline, I think it's crucial to avoid staying "cold," "stiff," or "inactive."
Press your feet into the floor, slowly squat down, then stand up.
Just this activates your body.
Japan's Ministry of Health, Labour and Welfare's "Guidelines for Physical Activity and Exercise for Health Promotion 2023" recommends adults engage in walking or equivalent physical activity for at least 60 minutes daily, exercise for at least 60 minutes weekly, and strength training 2–3 times per week.
Of course, squats alone won't instantly transform your immunity.
But moving large muscle groups is highly efficient for restoring activity levels.
On heavy days, move a little before overthinking.
Sometimes that alone changes your entire day.
4. Continue Without Overdoing It: Exercise Load That Doesn't Lower Immunity
When starting exercise in your 40s, the scary part is the gap between mind and body.
Your brain thinks you can still move quickly.
But your body lags behind.
Like dads sprinting at sports festivals and falling flat.
Your mind feels capable, but your body reacts slower. You fall. Next day, you strain something or suffer severe soreness.
Think of it like Amuro Ray's Newtype abilities exceeding Gundam's reaction speed—the pilot's mind outpaces the machine.
So instead of floor push-ups or full-speed running, start with wall push-ups.
If floor push-ups are tough, place hands on a wall and press.
That's enough.
What matters isn't applying strong load from the start.
Apply slightly more load than daily activities.
As you adapt, increase reps slightly, adjust angles, deepen movements.
Gradually acclimate your body this way.
This is how you create sustainable exercise.
Exercise for immune maintenance isn't about collapsing from overtraining.
It's about continuing loads that let you move again tomorrow.
Japan's Ministry of Health, Labour and Welfare's strength training information emphasizes the "principle of progressive overload"—applying loads beyond daily activity levels and gradually increasing them as adaptation occurs.
People who quit don't lack willpower.
Their initial hurdles are just too high.
Building fitness in your 40s requires design, not grit.
Start with sustainable loads.
That's safest and strongest.
5. Don't Sacrifice Sleep: Stamina Monsters Rest Well
If you're building an immune baseline, sleep is the final non-negotiable element.
Previously, when thinking about building stamina, I focused only on exercise.
But after COVID drained my strength and recovery took so long, I realized:
Bodies don't restore through movement alone.
They need rest to recover.
Work, smartphones, exercise—all may be important.
But if you keep sacrificing sleep for them, your body won't balance.
Stamina monsters aren't people who never rest.
They're people who rest well.
Ensure adequate sleep time. Avoid chronic late nights. Sleep early when tired. Lighten exercise when unwell. Schedule do-nothing days.
These are legitimate stamina-building practices.
Research on sleep and immunity shows people averaging under 7 hours of sleep were approximately 2.94 times more likely to catch colds than those sleeping 8+ hours.
So for immune maintenance, not sacrificing sleep is as important as exercise.
Building stamina in your 40s isn't just about moving.
It's about creating a lifestyle that restores condition.
Summary
What "stamina monsters" in their 40s do isn't extraordinary.
They simply maintain their immune baseline through daily habits.
Build "protective stamina" that prevents colds.
Reduce sitting time to restore activity levels.
Use squats to switch on your body.
Avoid overtraining; choose loads you can sustain tomorrow.
Don't sacrifice sleep.
Stamina monsters aren't just muscular people.
They're people who stay healthy and recover quickly when they falter.
It's never too late to start in your 40s.
Your immune baseline can be maintained through daily habits.
Today, just stand up once an hour.
That alone is a step toward becoming a stamina monster.
Reference Books + Recommended Resources
・『体力おばけへの道 頭も体も疲れにくくなるスゴイ運動』
Kazuki Sawaki / Mitsuhiro Kunimoto / KADOKAWA
・WHO Guidelines on Physical Activity and Sedentary Behavior
・Japan's Ministry of Health, Labour and Welfare "Guidelines for Physical Activity and Exercise for Health Promotion 2023"
https://x.com/hiroo_seitai/status/2074964001969312220
