
If the heart is a muscle, why doesn't it ever get tired? - zem
https://www.reddit.com/r/askscience/comments/fm2z8x/if_a_heart_is_a_muscle_why_doesnt_it_ever_get/
======
bo1024
I don't think any of the top answers are good at all. I suspect part of the
reason is how poorly we understand muscle fatigue.

Many answers talk about lactate and how it builds up in muscles but not the
heart. However they don't say anything about how lactate is related to
fatigue.

A lot of the answers talk about the high mitochondria concentration in the
heart. This allows it to use more oxygen and produce more ATP. That's fine,
but again, what is the connection between mitochondria and fatigue?

One example to show these answers are inadequate: You can walk or jog at an
easy effort for a long time and eventually your muscles will get tired and
sore, without ever going anaerobic or lactic -- i.e. without your leg muscles
ever needing more energy than your mitochondria can provide or ever producing
more lactic acid than your bloodstream can clear away immediately.

So mitochondria and lactic acid can't be the full story.

I don't know the true answer and I'm not a biologist, but I suspect the heart
has evolved to never be the weakest link. Maybe exercise always strengthens
the heart at a higher rate than the other muscles. Or maybe part of the answer
is that we don't have nerves to feel soreness/tiredness in the heart? (I have
seen research that the heart does experience temporary damage from e.g.
marathons, so it does get tired in some sense. Of course then it gets
stronger.)

~~~
tachyonbeam
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.

~~~
naasking
> 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.

~~~
swixmix
My guess is that the heart rests between each beat.

~~~
zarmin
Like a jazz musician.

~~~
mygo
like a serverless function

------
docbrown
As we were told during high school biology: cardiovascular muscle contains
vast amounts of mitochondria as compared to other muscle types. Mitochondria
are good. Therefore, more and stronger mitochondria = more efficient working
muscle tissue.

One of my favorite professors holds a PhD in kinesiology (exercise physiology
concentration) and wrote his dissertation on the mitochondria. As he stated
many times in class: if you want to have a superhuman composition, acquire
more mitochondria. he was always willing to bet his kids entire college
tuition funds if you could show him a pill that would produce more
mitochondria.

~~~
hyperdunc
Then I guess the obvious question a non-physiologist would ask is, why don't
all our muscles already have vast amounts of mitochondria? If it's that
beneficial with no major downsides then surely evolution would have stumbled
on that configuration already?

~~~
ivalm
Mitochondria do produce a lot of free radicals. Probably energy expensive to
maintain cells like that.

~~~
rolph
not only energy expensive but free radicals do genetic damage, and a this
requires the second budget for a set of enzymes to counteract the damages and
scrub out FR's

~~~
lumost
Why isn't Heart Cancer more common if it's producing so many free radicals?
Are there other mechanisms blocking genetic damage?

~~~
dodobirdlord
Muscle cells don't replicate past early childhood development, they just get
bigger and add more nuclei. So muscle cancers are extremely rare compared to
cancers of other tissues.

~~~
rolph
each nucleus contains a full genomic content. muscles are more of a
"myosynsynctium" than a group of cells. there are cohorts of cells that are
more prone to becoming cancer due to thier developmental origins and the
complement of developmental mechanisms that can be reactivated.

things such as cell adhesion, and cell migration through tissue dermal cells
are good at this as they do these things over the course of development.

muscles are built where they will live so these features are inhibited
somewhat more completely and harder to "switch on or off"

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jxramos
There's three types of muscles: smooth, skeletal, and cardiac. They share a
lot in common but are very distinct to each other. Might not be an apples to
apples comparison because fatigue is a notion taken from our experience of
skeletal muscles which is not cardiac muscle. It would be interesting to
compare the energy spend of each, or rather the power each is capable of. As
enduring as the heart is its likely limited in its power output because its
just circulating fluid against resistance and gravity. I don't know how much
energy is required for that. Bench pressing a hundred pounds, what's the power
in that and how many heart beats would equal that work?

~~~
TheSpiceIsLife
Someone with more maths chops than me should be able to tell you how much
energy it takes to lift 50kg 50cm over 5 seconds at Earth’s surface.

I’d imagine it’s at least a magnitude of more than the heart would expend in 5
seconds.

~~~
drdeca
50 kg * 50 cm * 9.8 (m/s^2)

PE = M g h

~~~
anticensor
245 joules

~~~
ORioN63
245J/5s == 49W

This website [1] shows several sources. The range of estimated power outputs
is between 1W and 5W.

[https://hypertextbook.com/facts/2003/IradaMuslumova.shtml](https://hypertextbook.com/facts/2003/IradaMuslumova.shtml)

OP hypothesis seems to be correct.

~~~
bagacrap
yes, but the original question was "how many heartbeats". First source in the
link says 75bpm and 2w, 245J/2w = 122.5 seconds or ~2 minutes of 75bpm, 150
beats.

------
downerending
It doesn't because it can't. Any hearts that got tired died, and their owners
did not reproduce.

It's like being Atlas--you're holding up the world. It's not a virtue, a flaw,
a conceit, nor anything at all. It is because it must be.

~~~
naasking
> It doesn't because it can't. Any hearts that got tired died, and their
> owners did not reproduce.

They're asking why the heart can't get tired. You answered why we evolved such
that the heart can't get tired, which is not the same question.

~~~
downerending
True. My answer was more of a _cri de coeur_.

------
bad_user
Missed from that first comment is that... the heart not getting tired is a
problem.

You can overwork your heart which can lead to lesions. Endurance athletes who
exercise excessively can have heart problems.

[https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3538475/](https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3538475/)

That muscles get tired and sore is a feature because it prevents damage.

~~~
hinkley
Atrial fib is no joke. I don't know who needs to hear this, but if you are
considering becoming an endurance athlete, and especially if you are
considering transitioning to ultra-endurance, go see a fucking doctor before,
after and during.

Source: father had a stroke 20 years later.

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huffmsa
Everybody always wonders about the heart and never about the intestines.

Shit in shit out, day after day after day. If not for you guts never tiring,
your heart wouldn't get any fuel, you'd get a fistula and die of sepsis, you'd
back up like a lot of people's pipes this week because they're flushing
unflushable wipes.

So here's to the real unsung smooth muscles of the body. We salute you.

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luc_
Wow. Incredibly reasonable question that I've never given much thought to.
Thanks for sharing!

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jaked89
[https://pubmed.ncbi.nlm.nih.gov/29798816/](https://pubmed.ncbi.nlm.nih.gov/29798816/)

~~~
01100011
Yes. Also this:

[https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5215277/](https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5215277/)

It may be that the heart is multi-functional. It may serve as a regulator, a
pump, or some combination of the two.

------
perlgeek
Isn't it funny how many meanings the word "why" has?

My first explanation, on hearing the question, was: because, evolutionarily
speaking, no organism would have relied so heavily on the heart if it got
tired easily.

Or looking on it another way, if the heart was easily fatigued, we'd be all
dead, or be very different.

~~~
hinkley
There have been a few astrophysics shows where they talked about 'why are we
here?', with varying definitions of 'why' or special emphasis on _here_ (as
opposed to elsewhere).

In most of them, someone says words to the effect of "well if we weren't here,
we wouldn't be having this conversation". Survivor bias, in a very literal
sense.

~~~
perlgeek
... also know as
[https://en.wikipedia.org/wiki/Anthropic_principle](https://en.wikipedia.org/wiki/Anthropic_principle)

------
6510
The heart does get tired eventually. Poor blood circulation will do terrible
things.

This is a pretty cool technology.

[https://billionsinchange.in/en/solutions/renew/](https://billionsinchange.in/en/solutions/renew/)

It squeezes the legs to help the heart pump blood.

~~~
elric
Wouldn't that increase mean arterial pressure? Would the heart reduce its
output as a response to the external pressure? And what's with their claims of
it clearing "trash" from your brain?

~~~
6510
I honestly have no idea. The marketing material suggests old people show
health improvements. I'm no expert but so far it seems officially it doesn't
do anything but looks promising.

[https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3021100/](https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3021100/)

[https://www.ncbi.nlm.nih.gov/pmc/articles/PMC385349/](https://www.ncbi.nlm.nih.gov/pmc/articles/PMC385349/)

[https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3383356/](https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3383356/)

------
01100011
I hate to spread a wacky idea, but I ran across something on Facebook the
other day that I initially wanted to disprove but then started to seem
plausible. The general consensus is that the heart is a pump and generally the
sole driver of circulation in mammals. There is an alternative theory, which
seems easy to disprove, that the heart is more of a regulator and operates
more like a hydraulic ram. I can post more links later. Curious if anyone has
heard of that and if there is a rebuttal to the idea.

Edit: The paper I was thinking of, if anyone is interested, is here
[https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5215277/](https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5215277/)

Edit #2, from the paper: "Gladwin et al. estimated that 25% to 30% of basal
human blood flow can be attributed to red blood cell-induced production of
nitric oxide by vascular endothelium."

~~~
thaumasiotes
> The general consensus is that the heart is a pump and generally the sole
> driver of circulation in mammals.

It's certainly not the general consensus that the heart is the sole driver of
circulation in mammals. Your heart drives the flow of blood out through your
arteries; it's much less effective at driving blood back through your veins.

~~~
TheSpiceIsLife
Veins contain one-way valves, so veinous return is aided by movement and
muscle contraction.

The lymphatic system doesn’t have a pump at all and is entirely driven by
movement + muscle contraction + fluid pressure as a result of tissue
integrity.

------
yawaramin
Oh hey, this reminds me of one of my favourite songs
[https://www.youtube.com/watch?v=Y5OpvDdVtYU](https://www.youtube.com/watch?v=Y5OpvDdVtYU)

------
havella
It does. Try riding a bike above FTP at altitude for as long as you can. Then
describe how your heart feels for the reminder of that day.

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ck2
Any runner or cyclist knows your heart gets tired.

A beat is not guaranteed volume, a rested heart can move more blood at the
same heartrate than a tired one.

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econcon
Also, why heart doesn't get bigger/thicker walls through cardio? Like we can
do enough reps and sets to achieve hypertrophy?

~~~
science4sail
The heart _does_ experience hypertrophy from exercise.

[https://en.wikipedia.org/wiki/Athletic_heart_syndrome](https://en.wikipedia.org/wiki/Athletic_heart_syndrome)

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ummonk
Isn't a heart attack basically the heart muscle getting too fatigued to keep
going?

It doesn't seem to ben the only muscle that doesn't get tired - e.g. if it
weren't for sleeping I suspect the neck muscles could hold up the head
indefinitely. The diaphragm obviously never gets tired either, unless you have
asthma.

I can also type pretty much indefinitely without feeling muscle fatigue in my
fingers. climbing on the other hand will exert them enough to quickly fatigue
them.

~~~
01100011
A heart attack is caused by an interruption in the supply of blood to the
heart muscle(or by an electrical problem, i.e. arrhythmia).

~~~
ummonk
Ah, TIL.

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aaaaarghZombies
The heart does get tired, it can operate at very high levels but only for a
short period of time.

It also has a pretty good blood supply.

------
macawfish
It does can get tired. This tiredness is just experienced differently from
tiredness of other muscles.

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macawfish
Hasn't anyone here ever heard of a broken heart? A broken heart is a tired
heart.

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tummler
Yogis would posit it's prana shakti.

~~~
carapace
Aye. The heart is not really a pump for blood. There's a kind of signal that
comes in from above through the Sun to the Earth and then up into each beings
heart. The organ is more of a resonator or tuned waveform guide for this
pulse, which then flows through the body moving the blood.

There's a kind of meditation you can do that involves becoming aware of the
pulse throughout your body. The subjective experience is very powerful, so
much so that I've personally have not gone "all the way" as I fear it would
dissolve me. Metaphorically, direct awareness of the blood pulse is like
looking into the sun, it's too potent.

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polytronic
It does rest after every beat

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sebow
Doesn't it though?As we grow older, the heart rate usually slows down.And
since it is a muscle, it shows that working out can improve heart rate and
other vascular conditions.

"Tired" not as in the other muscles, but you can't function at 100% for long
period of times.

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akiselev
Because all known multicellular organisms, with the exception of a few (<10
out of millions) microscopic species, use the electron transport chain to
produce ATP and the animals whose primary organ responsible for supplying the
entire body's metabolism suddenly stopped all died on the spot.

Now, _how_ those animals survived is a much more interesting question.

------
herbstein
This isn't suuuuper relevant, but the title instantly made me think of one of
my favorite folk-punk songs by Ramshackle Glory called "Your heart is a muscle
the size of your fist". The album version is really good[0], but I do vastly
prefer the energy of this phone-recorded live version[1].

    
    
      [0] https://youtu.be/pC3IrqUpm9U
      [1] https://youtu.be/ubX9JaaqBXo

~~~
milofeynman
Not: Your Heart Is a Muscle by Carly Rae Jepsen?

