What happens technically when we get exhausted ? is it the oxygen depletion ? or the build up of lactic acid ? why can’t we run/swim/climb forever ?


I was watching a documentary about high level athletes and it got me thinking about how extraordinary their feats are, can’t we tackle each symptom of exhaustion to make it so that the general public is more capable ? like an oxygen tank for the oxygen need, a cold water container for the thirst and heat and maybe an exoskeleton for an easier time, I’m asking why these notions are not feasible.

In: 55

There are a lot of components at play when your body gets tired.

You’re correct that oxygen (and blood flow, which is closely related in this context) is an important piece. This is why improving your cardiovascular fitness makes your endurance better — your heart and lungs can work more efficiently. You can strap an oxygen tank on someone, but there aren’t really many non-invasive ways to make their heart work more efficiently. I guess certain stimulants might have that type of effect.

Muscles are also important. And there isn’t all that much you can do, externally, to prevent muscle fatigue. Muscles have to be worked regularly to be strengthened. I suppose the external option here would be tools like crutches, which prevent your muscles from having as much work.

Energy is also important. You certainly can use energy to your advantage, by eating and drinking before and sometimes during movement. A long distance hiker will surely make it further with lots of water and electrolytes and salty snacks than they would without 🙂

And of course, humans need sleep. That’s a requirement, not an option. So you can’t keep moving forever.

So in short — **you could keep moving until your body absolutely requires sleep.** And you could use tools like hydration, additional oxygen, energy-filled foods, and physical equipment like crutches or hiking poles to keep you going for as long as possible.

But you will, eventually, be too fatigued to continue without sleep. Even if you’re a pro athlete!


The oxygen depleting isn’t necessarily an issue, at sea level the hemoglobin in your blood is fully saturated with oxygen. The probably is mostly the efficiency of your cardiovascular system.

As you seem to already know, your muscles require more oxygen when you exercise, so the heart now has to work harder to deliver blood to those muscles, but your heart itself also requires more oxygen to maintain the elevated rate of contraction.

If an untrained person tried to run a marathon, their heart wouldn’t be capable of delivering that much blood for that long to the person’s legs and eventually their muscles would be unable to continue running. As the person trains more, the heart grows stronger and is more efficient at delivering oxygen to the body, allowing muscles to work harder for longer.

Oxygen tanks wouldn’t really solve the issue because your blood is essentially always fully saturated with oxygen once it passes through your lungs. And even if you iced the muscles, that wouldn’t solve the loss of strength from the lack of oxygen

The limits to animal performance are:

Oxygen. This is determined by respiration efficacy, the size of airways and lungs compared to the body and other factors. Humans are pretty good here and can sustain quite high performance while replacing CO2 with fresh O2 via respiration.

Chemical energy. Animals move via chemical reactions, turning energy from food into motion. They don’t have an unlimited supply, and if they keep working at a rate where they can breath enough and keep up oxygen demands they will eventually run out of chemical fuel, forcing them to stop and recover, husbanding enough energy to keep producing more energy from food (digestion) and to keep oxygen coming (respiration).

Heat. Animals rely on chemical reactions that only work in a narrow temperature band, and the chemical reactions that let them move produce waste heat. If they have enough chemical energy and enough air, they can still be forced to stop if their body becomes too hot, restricting activity to what is needed for survival while they cool down.

Rest. Animals require sleep for reasons we don’t perfectly understand. Even if every other requirement is still met an animal will eventually have to sleep. The chemical reactions that allow animals to move also produce waste products. Scrubbing these waste products from the blood and tissues is best done while resting, and eliminating the resulting waste products from the body requires at least a short stop for most animals.

Your muscles store oxygen and energy when at rest, these reserves are depleted during exercise and the rate at which they can be replenished is lower than the rate at which they are consumed in intense exercise.

These rates are better in people with better cardiopulmonary health, but even the fittest people run into depleting oxygen and energy levels if their level of exercise exceeds this rate.

Lite exercise, you can do almost indefinitely- possibly for days on end, until you need to sleep, provided you are staying hydrated and consuming sufficient calories.