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Another part of the answer is that fatigue is largely in our heads. Your body wants you to minimize energy spending, probably because in cavemen days food was so hard to obtain. You will feel tired long before you hit real physical limits. If you've ever taken amphetamines, it becomes obvious. Suddenly, you can spend all night dancing more energetically than you ever did without feeling tired, work longer than ever, etc. Not suggesting that this is in any way sustainable long-term, it isn't, but for me, it really highlighted how my body is physically able to go several times beyond the point where I feel tired and uncomfortable. It's just that some core instincts really don't want me to spend energy liberally.

> Another part of the answer is that fatigue is largely in our heads.

Not in our heads, but in our nervous system. Some types of fatigue are due to feedback into our central nervous system that reduces muscle activation signals from your brain in order to avoid damaging the muscle. But if you're untrained, your body doesn't actually know where this point really is.

Most strength and endurance gains when you first start training are neurological, where the effect of this feedback loop is pushed back as your body learns the true threshold for muscular damage.

The heart does get fatigued though. You can run a horse into dying from a heart attack, for instance. Most people simply aren't fit enough to push it that far, as their other muscles would give out first.

My guess is that the heart rests between each beat.

Like a jazz musician.

like a serverless function

The concept that helped me most understand my own body was realizing the signal and the system are separate. In your car, the oil light is almost entirely separate from the actual oil system.

It's the same with our body. Feeling tired is separate from being tired. Feeling hungry is separate from being hungry.

Drugs work like putting electrical tape over a low oil light. Amphetamines and even caffeine are good at blocking out the signal. The underlying issue is still there.

But, sort of like you said, the signal comes really early. It's like a low fuel light that comes on when you get to a half tank.

>Drugs work like putting electrical tape over a low oil light. Amphetamines and even caffeine are good at blocking out the signal. The underlying issue is still there

That's only true for some drugs, those that mask pain. Those that stimulate the mind are like feeding an engine nitrous...and sometimes similarly dangerous!

Completely incorrect. Amphetamine releases massive amounts of norepinephrine, which is the direct precursor to epinephrine (adrenaline). This causes a cascade of physical effects, including suppression of insulin, increased blood glucose and a massive boost in cellular energy production.

That’s a far cry from “all in your head”. It’s a systemic response that affects every part of the body.

This looks like the so called Central Governor model. https://en.wikipedia.org/wiki/Central_governor

This makes sense "evolutionary", because it means that even if you're tired after a long day, you actually still have reserves for sudden fight/flight situations.

I'm pretty sure that you'd get rhabdomyolysis if you continued jogging because amphetamines prevent you from feeling tired.

> Another part of the answer is that fatigue is largely in our heads.

Military schools like the Ranger Indoctrination Program really emphasize this point. The body will go on far past the point you think it will stop.

I don't know if it was a top answer when you looked 8 hours ago, but a cardiovascular physiologist answered (14 hours ago) a very thorough, detailed answer with second-order explanations. It's now _the_ top comment, here is a direct link with tracking removed:

https://www.reddit.com/r/askscience/comments/fm2z8x/if_a_hea...

I was thinking of that answer too. It talks about the ways heart muscle is different from skeletal muscle, which is cool, but I don't see it carry those differences through into explaining about fatigue...

It mostly just mentions mitochondria and lactate though. I think it’s still subject to the same criticism.

The comments bridge these gaps. Production of lactic acid allows for the production of ATP in the absence of oxygen. It’s like a buffer for peak loads. The heart’s load is heavy, but relatively steady. It doesn’t have the ability to have peaks in output. It also can’t afford to not be supplied with a constant supply of lot of oxygen to feed the mitochondria.

As a bit of a side rant: biology is complicated, that doesn’t mean good work hasn’t been done. I notice many people discredit the field of biology, but eventually make fools of themselves when talking about it.

https://www.reddit.com/r/askscience/comments/fm2z8x/if_a_hea...

That depends on what you consider steady. My heart's load can vary by a factor of 4. But the load variance on skeletal muscles is even higher.

Would this mean that you could “boost” your workout by starting with some anaerobic exercises or reps?

The top answer is missing one part:

The heart is actually "resting" 60-70% of the time, the time between contractions, and it fires in a staggered fashion.

The high level of mitochondria means it can produce the ATP it needs during these short rests and is ready to go the next time the SA node zaps it.

i'll take the word of a Cardiovascular Physiologist in that reddit thread over some dude on hacker news who really doesn't know what he's talking about

I'm not saying anything there is wrong. I'm saying they don't answer the question - they're incomplete. They focus on listing differences between cardiac muscle and skeletal muscle, but those differences don't obviously explain why one gets tired and another doesn't.

> You can walk or jog at an easy effort for a long time and eventually your muscles will get tired and sore,

Perhaps this is because your lungs are getting tired--I wonder if there's lactate buildup in the lungs. I'm no biologist, but my understanding is our muscles need oxygen, to get this oxygen we need to do many things--breathe more air (v02 max), more efficiently distribute the oxygen through the body (heart rate and stroke volume), and finally the oxygen is delivered to the mitochondria in our muscles, so increased vascularity (vein size), would help.

Our muscles also need other supplies of energy--glycogen to produce atp or ketosis. I suspect if we're glycogen dependent we get tired a lot faster. There are many hyper marathon runners that are in ketosis, like Goggins:

> He was able to run 101 miles in 19 hours and 6 minutes—despite never having attempted to run a marathon previously

I think if the heart gets tired, you die, so the brain probably stops that from happening. You can die of exhaustion, many atheletes just die early from heart attacks.

Anecdotally, if I don't get lactate buildup in my muscles, since being in ketosis I don't really get tired. I went from a 5 minute bike being difficult, to a 1 hour bike and not getting tired near the end.

So it's been a while since I've studied this subject, but it works as most of our needs: physical activity activates some areas of our brain (in this case mostly the parietal lobe and the cerebellum) whereas other areas have their activity reduced. What follows in our senses is a response to the increased physical demand, through tiredness and soreness. Cardiac muscles and skeletal muscles communicate with different areas of the brain (consider how fear causes your heart rate to increase, for example), which are related to different sensations.