Locale: ja
"I started doing strength training and I don't get tired at all in daily life. But I can't run a marathon." This seemingly strange phenomenon becomes easy to understand if we consider that the human body doesn't have a single "physical fitness" status. Muscle strength, muscle endurance, cardiovascular endurance, movement efficiency, and the durability of tendons and bones develop quite separately. The human body doesn't think about making "this person generally energetic." It only reacts to the stimuli it receives at the moment. It's a primitive biological system.
1. The Japanese word "体力" (physical fitness) is too broad, grouping everything together
When people say "you have good physical fitness," they are putting quite different abilities into the same box.
Lifting heavy furniture. Carrying a 10kg load. Walking all day on a trip. Climbing a mountain for hours. Running 5km. Completing a full marathon.
All of these seem to indicate "good physical fitness," but the underlying abilities are different.
Generating a large force in a single burst is maximum strength. Repeatedly exerting a lighter force is muscle endurance. Delivering oxygen throughout the body and producing energy over a long time is cardiovascular endurance. Reducing the energy needed to move at the same speed is movement efficiency, and enduring repeated movements requires resilience in tendons, bones, and joints.
In other words, the human body doesn't have a single "HP" but instead has a strange skill tree.
2. Even just strength training can make daily life suddenly easier
So why does strength training make you less tired during trips or shopping, even if you don't train for running?
One reason is that daily activities become relatively lighter compared to your maximum capacity.
For example, suppose climbing one step requires "10" units of force. If your leg's maximum capacity is 20, then each step is about 50% of your capacity. If your maximum capacity increases to 50, the same step becomes about 20%.
Although reality isn't this simple, the idea is similar. As muscle strength increases, "spare capacity" is created for daily activities such as standing, walking, climbing stairs, and carrying loads.
Therefore, even if marathon ability hasn't improved much, it's quite normal for "being able to play all day and not be as tired as before" to happen. Strength training doesn't just improve the ability to carry heavy objects, but also changes ordinary activities into lighter tasks for you.
3. Still, running makes you suddenly feel uncomfortable. Because they are on different servers
Here comes the inconvenient design of the human body.
It's fine to walk all day, but when you start running a bit faster, your heart rate suddenly increases and you feel uncomfortable. This is not a contradiction.
In running, the heart must pump blood, the lungs must deliver oxygen, the muscles' capillaries must receive it, and the mitochondria must continuously produce ATP. Furthermore, the running economy, which determines how low-cost it is to maintain the same speed, and the resilience of the feet to repeated landing shocks also play a role.
Even if your squats get stronger through strength training, this entire system does not automatically get stronger in the same proportion.
"Since my legs are stronger, you should also raise my cardiovascular system."
Human body: "Different departments."
It's very inconvenient.
4. Why "daily fitness" improves easily during hiking or circuit training — because many muscle groups are used
On SNS, there was a discussion about experiences such as, "Compared to when I used to spend long hours at the gym three times a week, I now feel less tired in daily life after doing hiking every other week and short-duration bodyweight circuit training frequently."
This is quite possible.
Hiking involves sustained low to moderate intensity movement. It uses leg muscle endurance, cardiovascular system, balance, and tolerance for long periods of activity. Moreover, when climbing uphill, strength for each step is also required.
On the other hand, circuit training with short rest periods uses the muscular system while keeping the heart rate stable. A meta-analysis of research on resistance training in circuit format has reported improvements in both muscle strength and cardiovascular fitness such as VO2max.[3]
In other words, it's not just "light strength training," but actually activating multiple muscle groups at the same time.
5. "Endurance" is not a single category
Here is another trap.
Although it is called "endurance," cardiovascular endurance and muscular endurance are not the same.
Cardiovascular endurance is the ability to continuously deliver oxygen throughout the body and supply energy. Muscular endurance is the ability of a particular muscle to exert force repeatedly.
Therefore, it is possible to have enough breath but heavy legs, or conversely, to have enough legs but cardiovascular system gets tired first.
"Ability to walk" is a combination of minimum muscle strength, leg muscular endurance, cardiovascular system, movement efficiency, and resilience of feet and tendons.
Humans have no status screen, yet too many internal parameters.
6. Why doesn't it become universally strong? Because "the system that is used gets developed" is the basic principle
There is a basic principle in exercise physiology called training specificity. In short, the body tends to adapt in the direction of the given stimulus.[4][5]
Repeating heavy loads leads to adaptations in the direction of force production, such as neural activation and muscle fiber size. The 2026 ACSM resistance training guidelines also organize how load settings vary depending on the purpose, such as muscle strength, muscle hypertrophy, and power.[1]
During long-duration endurance exercise, the body adapts towards oxygen delivery, utilization, and long-term ATP supply.
The body does not say in the central command,
"Let's make a person who is strong for traveling. Increase muscle strength, cardiovascular system, bones, and tendons by 12% each."
It responds to signals such as mechanical tension, metabolic stress, and energy demand that occur locally.
It is an intuitive organism.
7. So, if you just eat a meal, everything will get stronger. Unfortunately, the meal was just construction material
By this point, it's natural to think:
"Since it's too troublesome, just eat and sleep, and my strength, heart and lung capacity, and bones will all get stronger automatically"
I understand the sentiment.
However, food mainly provides energy and materials. Even though proteins, carbohydrates, fats, vitamins, and minerals are necessary, they cannot replace the exercise stimulus that determines "what to strengthen."
If you order a lot of construction materials on Amazon and pile them up in your yard, they won't automatically become a two-story house.
Muscles need stimulation for muscles, heart and lungs need sufficient load, and bones need appropriate weight.
Of course, if you are nutritionally deficient, adaptation will be difficult. Therefore, food is not unimportant, but rather "materials are necessary, but not design instructions."
8. Including "immunity" in physical fitness makes the discussion even more confusing
It's a bit risky to summarize "improved physical fitness = increased immunity" in one sentence.
The immune system is not a single gauge, and it's impossible to compress various aspects such as resistance to infectious diseases, inflammation control, and response to vaccines into a single number.
It is certain that regular physical activity is associated with many health benefits, and the WHO recommends both aerobic activity and muscle-strengthening activity for adults.[2]
However, when explaining the feeling of "not getting tired easily in daily life," it is easier to look at strength, muscle endurance, heart and lung capacity, movement efficiency, and recovery.
By throwing everything into the convenient term "immunity," you end up creating another huge physical fitness gauge.
9. The closest thing to universal physical fitness is ultimately "gaining a little of multiple skills"
If the human body were a skill tree, the way to become more versatile is simple.
Instead of making one ability 999, raise multiple abilities used in daily life to a sufficient level.
The WHO recommends adults to engage in 150-300 minutes of moderate-intensity aerobic activity, or 75-150 minutes of high-intensity activity, and to perform muscle-strengthening activities on two or more days per week.[2]
This is not about becoming a marathon runner. To cover a wide range of daily activities such as walking, climbing stairs, traveling, playing, carrying loads, and long-duration activities, it is more convenient to have both aerobic and strength components.
There are countless combinations, such as hiking, brisk walking, cycling, circuits, bodyweight training, and weight training.
If the goal is "not getting tired after a day of traveling," then a full marathon record is not necessary.
10. Conclusion: The human body is not a comprehensive HP system like an RPG, but a legacy system full of patches
Because humans use the term "vitality," it seems as if there is a single vitality gauge inside the body.
In reality, it's different.
If you build muscle strength, daily activities become relatively easier. If you train for endurance, your oxygen supply and ability to sustain long periods of activity improve. Some people feel that activities like circuit training or hiking improve overall physical fitness because they use multiple abilities at once. On the other hand, it's common for someone with strong muscles to have low marathon ability.
Also, not everything is automatically enhanced by diet alone.
The human body is not a product designed to make your life convenient. Evolution is not an engineer who looks to the future and designs for overall optimization, but rather the accumulation of changes over generations.
That's why the design is like this.
Lifted something heavy?
Muscles: "Will I get stronger?"
Ran for a long time?
Cardiovascular system: "Is this my turn?"
Ate a meal?
Body: "Thanks for the materials."
Will it all be made convenient together?
Human body: "That function is not implemented."
Rather than a defective organism, it's a legacy system that has been patched for hundreds of millions of years.
It works. But the design philosophy is quite rough.
