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If the heart is a muscle, why doesn't it ever get tired? (reddit.com)
431 points by zem 9 days ago | hide | past | web | favorite | 155 comments

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.)

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:


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.


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.


Personal attacks aren't ok. Please review https://news.ycombinator.com/newsguidelines.html.

> 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.

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.

> he was always willing to bet his kids entire college tuition funds if you could show him a pill that would produce more mitochondria.




Good stuff!

> Activation of Rev-ErbA-α by SR9009 in mice increases exercise capacity by increasing mitochondria counts in skeletal muscle.

> Some companies are selling SR9009 online for human use as a 'research chemical'.

> "The drug [alters] the circadian rhythm (in mice) and we would need to assume in humans – and we don't know if it is beneficial or detrimental at this point."


It has "several issues that make it unsuitable for human use".

> Firstly, it has no oral bioavailability. I know the company selling this is indicating taking it orally is ok – but it doesn't even get into the blood.

> SR9009 has some functional groups that are known to have potential toxicology liabilities and it would never be developed as a drug.

> So bottom line – I would never recommend using this compound at this point. That being said – we are still working on improved compounds with one of the potential uses being sarcopenia – loss of muscle and strength due to aging.

Mitochondria are the powerhouse of the cell.

Mitochondria are the powerhouse of the cell x2 in the heart

I love this meme.

It's not a meme, it's an analogy.

Well, it definitely is a meme as defined by Dawkins[0]. In general, I would say that analogies and metaphors like this one have a huge impact on normal science, and are one of the main things that change during a paradigm shift. I recall a great example in how 150 years ago people thought of the body's physiology as analogous to a steam engine with downstream effects due to pressure building up, contrasted to how in today's computer age, we tend to think of physiology in terms of signal processing.

[0] https://en.wikipedia.org/wiki/Memetics

I agree, the imagery that we use in metaphor (in speech, visually, or in conception) influences how we think about things - and a popular analogy like that about the mitochondria is certainly a meme, a "cultural information transfer".

As you pointed out, a fairly recent meme is "the brain is the computer of the body", and maybe "the Internet is the brain of humanity".

I wonder what the trade-offs would be if you could amplify mitochondria in skeletal muscle?

The cells would consume more energy, so you'd need more food, so in conditions of food scarcity (i.e. 99% of our [pre]history) you would be at a severe disadvantage as your tribe would starve to death in conditions when more efficient homo sapiens would still survive.

Also require more oxygen and produce more heat.

As a guess: You would be unable to rest without utter exhaustion, and possibly even not then. You might need to keep flexing your muscles constantly.

There are other cell types that have a strongly increased amount of mitochondria like brown adipose tissue. They also don't have a "need" to perform some work constantly.

As with any cell function how much work mitochondria perform is highly regulated. Just because you have a lot of them doesn't mean you can't turn their function down.

Nadia Commenci was recruited as a gymnast at age five because she was too active and breaking the furniture at home. She made the first ever perfect 10 score in the Olympics. They didn't have a way to display it. It got displayed as 1.00 because they only had three spaces on the sign.

Her best friend became a professional ballerina.

I have heard that Chuck Norris does two workouts a day and if he skips one, he makes everyone around him crazy. If he does two per day, he's calm, cool and collected and fine to be around. If he doesn't, welp, he can't sit still and he can't control his mouth and he bounces off the walls and everyone can't stand him.

I also talked about the addiction process and mitochondrial function in a different comment.

> I have heard that Chuck Norris does two workouts a day and if he skips one, he makes everyone around him crazy. If he does two per day, he's calm, cool and collected and fine to be around. If he doesn't, welp, he can't sit still and he can't control his mouth and he bounces off the walls and everyone can't stand him.

this is a common theme among people who do lot's of sports. I did plenty of marathons, trail competitions in my time and a day without exercise felt like absolute agony mentally. There is probably also a psychological effect from missing the routine. Most sports addicts I know (including and especially body builders) have the problem of not giving their body enough recovery time between workouts causing them to actually perform less well, etc

I have a son that I put in gymnastics when he was a child. One of his teachers basically told us "Put him on Ritalin" because he never sat still in class. ADHD had already been ruled out, so this "Let's skip diagnostics and go straight to just drugging your kid for my convenience" attitude really pissed off my husband.

If he was exhausted enough, he occasionally sat still. So he was in gymnastics for a time and, when he was younger, I just made sure to take him to the park and stuff a lot so he could run around some every single day. Otherwise, there was no hope he would actually sleep.

He's also a kinesthetic learner (and thinker).

> this is a common theme among people who do lot's of sports. I did plenty of marathons, trail competitions in my time and a day without exercise felt like absolute agony mentally.

And then along comes coronavirus, and the privation for exercise addicts while under quarantine is so much less than what many are experiencing, but it's real—and no-one takes "it's hard not to exercise" seriously.

For a moment there I thought you were making a Chuck Norris _joke_ in a middle of a non-joke comment... ;)

Top endurance athletes do specifically targeted workouts which significantly increase their skeletal muscle mitochondria density. They don't appear to have trouble resting; rather the opposite.

Top endurance athletes.

You mean they rest just fine after routinely exhausting themselves with unusual feats of extreme endurance?

And you think this is somehow a rebuttal of my suggestion?

Endurance athletes do not routinely exhaust themselves during workouts. Instead they work out to build mitochondria (among other goals). Hence the popularity of polarized training programs. Exhaustion impairs proper rest and recovery. Your suggestion is not consistent with what we see happening in humans.

They don't remain endurance athletes by skipping workouts for months on end. I've seen articles where some body builder got the flu, couldn't workout for a week or so and lost significant amounts of muscle mass, like 25 pounds.

You develop capacity like that, you use it or lose it.

And maybe I'm not making my point very well, but everything I've seen in life suggests such people do have a real need to workout regularly. It's not a "nice to have" for them. It's a "must have."

There was even a woman who was eventually diagnosed with a genetic disorder whose disorder had long been de facto managed by her very active lifestyle.

But I'm going to step away from this discussion. It's starting to look like a pointless pissing contest and that's not really my cup of tea.

Losing “water weight” is a thing. Boxers do it as part of their weight management before their weigh-ins. When I am well-hydrated I lose ~1.5 lbs of water weight just by sleeping (by weighing myself before bed and right when I wake up). If I take a piss it’s an even 2. And we lose a lot of body fluids when we have the flu.

That wouldn’t account for all 25lbs (which is a significant sum to be losing in a week — is that even factual,? Did this guy weigh 350 lbs lean?), but it could account for a good amount of it over the span of two weeks if he was really sick and also not eating well due to lack of appetite, diarrhea, sore throat, etc. How quickly did he get his weight back? Did he lose any significant amount of strength?

The flu is not the same as being out of the gym for a week. Although we do know that you will begin to lose incrementally more and more muscle mass the longer you’ve been out of the gym, being out of the gym for one week shouldn’t lead a bodybuilder to lose 25 lbs, and sometimes it’s even part of good training strategy to take a one week break from the gym to let the nervous system recover, which gets taxed just like the muscles do.

Src: I studied Biological Science at university

Also, during my histology course when we covered different muscle types, we discussed the topic of fatigue in cardiac tissue, but I remember being left with a feeling that we didn’t really know the underlying physiology yet (this would have been nearly a decade ago) but the main arguments were that cardiac muscle is not the same as skeletal muscle, whose differences can be seen under a microscope, and that the heart has enough time to recover when it’s not contracting.

Here’s a similar question — does the esophagus ever get tired? How about the diaphragm? Do you ever get tired of breathing?

I really don't remember the details and the original article may not have listed weight loss. It may have listed inches around his biceps lost.

I just remember it being hyped a bit concerning "yeah, he looks amazing -- but you can't take a break at all because when he had the flu, he saw crazy declines in a short period of time." I want to say a week, but that could be wrong.

I'm seriously bad with remembering names, titles, etc.

I also remember a friend who took up weight training after giving birth. She said she lost four inches around her waistline and only lost four pounds because muscle mass is generally heavier than other tissues.

I've had similar conversations with other women who were trying to lose weight or otherwise work on their physique. Some are downright disappointed by such results because they are looking for a specific number on the scale.

Sort of tangential: I knew a woman who quit smoking. Smoking suppresses the appetite and meets oral needs. People who quit often take up snacking as a substitute and their weight shoots up.

She said she would weigh herself and cry and her husband would basically talk her out of taking up smoking again because she was so upset at the weight gain.

Anyway, my mind organizes info its own way and I'm not always happy about it and it sometimes gets me into all kinds of social hot water because it sounds like crazy talk to other people. So this is probably not going to get better from here.


No problem not here to attack. Hopefully something I shared helped!

It isn't physically possible to lose 25 pounds of lean muscle mass in a few weeks. That's off by an order of magnitude, check the energy and mass balance calculation.

But frequent exercise is certainly a "miracle drug" in that it effectively prevents or treats a host of different medical and psychological conditions.

It's possible I'm misremembering something. I can't find the story I remember seeing. I did trip across this:


But I really don't intend to post further. This is a completely pointless argument in my book and the world really doesn't need such at the moment.

Why would that be?

I'm basing it off of this comment:


And this follow-up comment sort of agrees with my inference -- at least the first part of my inference:


Edit: I will add that I am also, no doubt, basing it off of background knowledge about cell function. For example, mitochondria change and develop greater capacity to process X if you consume a lot of it, which is part of the addiction process and part of why withdrawal is a thing. If you consume a lot of, say, alcohol, you need to ramp down gradually so that the body can make changes at the cellular level to adjust to the lesser amount of alcohol and this is part of why cold turkey is so very hard and it is generally recommended that you taper off.

I used to ask smart people with PhDs and the like a lot of questions about cell function and read what I could get my hands on that could be followed by a lay person because I have a genetic disorder that impacts cell function.

I'm pretty sure this is what happens if you're a weight lifter?

Weight lifting isn't an endurance sport. If you want mitochondria obsession, look for anything long distance.

Non-repetitive weight lifting wouldn't be a good example, but neither is anything in 10s of km.

Mitochondria produce ATP as energy. You can't store much¹ ATP, therefore mitochondria must be capable of producing ATP at your near peak output (say 10s average).

I'd look at anything with high >10s peak power (probably a bonus if it's aerobic). Many bodybuilding exercises would qualify.

¹ Lactic acid is a byproduct of ATP production, therefore ATP supplies surely last less, than what it takes to start producing lots of lactic acid.

You become more chimp like.

you would need more red blood cells to feed them, and at sone point that puts you at risk for a heart attack because your blood is too thick

That's not a realistic risk unless you have some rare medical condition or are doping with a PED like EPO.

Aren't mitochondria captive cells from early evolutionary times? I googled and from nature.com "Mitochondria and chloroplasts likely evolved from engulfed prokaryotes that once lived as independent organisms. At some point, a eukaryotic cell engulfed an aerobic prokaryote, which then formed an endosymbiotic relationship with the host eukaryote, gradually developing into a mitochondrion."

I am going to hijack this top comment.

> If the heart is a muscle, why doesn't it get tired?

Because it's illogical to assume muscles fatigue. They physically don't get to the point where they can no longer operate -- under normal everyday use and exercise.

What does happen, that may seem like fatigue, is a gradual shifting of fuel sources by muscle cells depending on exertion levels.

Suffice to say, it goes like this:

- First: Creatine Phosphate is the first energy source in your muscles, the most powerful one, and the least plentiful. It's the first to go during exercise and is partly why you focus on 1-5 reps for strength-focused lifting (that's roughly how long it takes to get used up)

- Second: Glycogen in your muscles and liver. This is fairly plentiful, fairly powerful, and rapidly mobilizeable. After your creatine phosphate stores are emptied, glycogen takes over. This is still a very powerful fuel source, but its metabolism creates a negative feedback loop on itself. You can only sustain moderate exertion (see: sprinting or 8-15 reps moderate weight) for around 60-90 seconds, before glycogen is no longer easily accessible (note: accessible, not depleted. It's almost impossible to deplete in a single workout)

- Finally: Fat. Once you've exhausted glycogen, your body turns to fatty acid oxidation. This is the least powerful but most plentiful. In normal exercise you use a mix of fat and glycogen depending on how hard you exert yourself. Harder: more glycogen. Easier: more fat. If you want to know how it feels like to run on only fat: do a marathon -- then hit the wall. What you experience is literally the complete depletion of glycogen, and a transition into "low-power" mode as your body starts running on the only fuel source it has left: fat. You have months of this fuel source on your body, you won't run out. You can keep walking for days without having eaten anything or slept, but you won't be able to run at any pace that resembles a jog. I know, because I have.

What's the point of all this? It's to illustrate that muscle "exhaustion" is a misnomer. Muscle exhaustion is, in reality, a depletion of power-generating fuels leading to a state of minimal exertion.

Coincidentally, that's the mode the heart operates in 24/7.

Coincidentally, the heart has an asinine amount of mitochondria to fuel non-stop fatty acid oxidation.

The heart doesn't fatigue because it's not physically possible. It doesn't need creatine phosphate or glycogen (very very little) to pump blood. It's not a strenuous task.

An aside: if you had as much mitochondria in all your cells, as you do in your heart's, you would waste away.

Some anecdata but, I suffered from very bad muscle injuries from one summer in 2014. I skateboarded hard for two days in a row. Ran 3-ish miles on both days. Two days later I was exhausted but took two ibuprofen and I went to a beach with friends anyway. Where I skated hard at a park for a few hours, then swam, then ran. A day or so later, after not getting the best sleep, on an hour drive home, I pull both quads. The day after I had to call out for a week. I was exhausted and my obliques, core, quads, and arms were beyond sore. I laid around for most of the week.

Your muscles may not get "exhausted" but it seems like there must be some breakdown in the cells that outpace recovery at some point.

When they work, muscles break fibers. The more intensely you work out, the more they break. So you have to rest for them to be rebuilt (and more fit to the exercise), otherwise at some point they give in. The more frequently you train, the more they can withstand.

Did your pee turn brown?

That seems unlikely given that if he had rhabdo he’d have felt bad enough to need to go to hospital. 20% death rate due to kidney failure is not to be sneezed at.


No, nothing like that fortunately.

Not true. It is possible to run mainly on fat.

Also, what else would you call heavy legs after a long run other than fatigued?

> The heart doesn't fatigue because it's not physically possible.

You can exert yourself to a heart attack, much like a horse can be run to death. If it's not the heart that's fatigued, what's the cause of death?

You can't really exert yourself to the point of a heart attack unless there is some other pre-existing condition like a congenital heart defect, arterial blockage, history of cocaine use, etc.

Thanks! That's more useful than many other answers.

> you would waste away

- what does that mean? why?

> he was always willing to bet his kids entire college tuition funds if you could show him a pill that would produce more mitochondria.

Did he mention whether faculty get free family tuition or not?

His children will be made to forego college in order to trailblaze the up and coming trade of mitochondria hunting.

I read it as him claiming that investing in it would be a sure bet.

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?

Because the trade-offs are not optimal for most people. Might require huge caloric budgets not available (until now). At present, energy efficiency is actually detrimental to reproduction for most people in first world countries, so it's conceivable if we wait another million years or something, our muscles might have a lot. Assuming not getting tired leads to reproductive success, of course.

For a vast amount of human history food has been much harder to obtain and less calorie-dense than it is today, so evolution has optimized for energy efficiency rather than raw power.

Do chimps/apes have a greater muscular power-density than humans?

Significantly, and a large part of the reason why is energy efficiency. Humans have large brains that take a lot of calories to sustain, and because of their large brains didn't have nearly as strong evolutionary pressure to maintain musculature.

Probably the wrong way to look at it. They can exert greater force for shorter periods but the great apes have far worse endurance than humans. A chimp is about a third stronger than a human of equivalent weight but we can run marathons and they can’t. We have a higher proportion of slow twitch muscle fibres compared to fast twitch than the great apes.

I think the major downside is energy costs. You can only support so much high-energy tissue with a constant caloric budget.

Think of evolution as Uncle Fester from the Addams Family.

You never know quite what you'll find him working on, but every now and again, he'll have just the thing for the job.

But by and large, it's most likely something completely insane. Evolution has no intent. No direction. It's more a descriptive statement of an ongoing process than a end to which some force is constantly seeking.

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

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

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

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.

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"

Neat, but why don't all the cells in our body do that? What's the trade-off that goes the other way for cells most likely to get cancer (prostate, breast, lungs, lymph nodes)?

yes ! this is the nature of superoxide dismutase. basically free radicals are a price paid for generating ATP.



Is skeletal muscle cancer common? I haven't heard of it though perhaps it has another name.


Free as in radicals, not free as in beer.

As others pointed out, an increased output would not be sustainable long term within the normal body. The physiology behind action potential[1] and refractory period[2] explains how they compliment each other to make sure our system function properly and effective.



Not unless having vast amounts of mitochondria is a competitive survival/reproductive advantage favoring natural selection of individuals in their environments

This resonates with my understanding: evolution is lazy, not optimal.

Or: evolution is lazily optimising. Survival doesn’t imply perfection, as it is my definition a process.

And the more optimised an organism is the more fragile it’s continued existence.

This just reminded me of the game Parasite Eve.

He would have lost his sons college thition then. Mitochondrial _inhibition_ is actually more correlated with longevity than the other way around.

If more mitochondria meant more efficient working muscle tissue, then that's what we would have evolved, there's always a compromise. Are you sure you can keep the same power output for a given mass and volume of muscle if you keep increasing mitochondrial numbers? Even the reddit answer clearly says that the heart muscle is rate limited by oxygenation (they have more mitochondria but are also highly vascularized to power the mitochondria). More mitochondria===more power doesn't mean it's better for us, physiology is rarely that simple.

> if you want to have a superhuman composition

You responded to a bunch of claims the comment didn’t make.

Specifically it is due to reproductive defects in mitochondrial dna as one ages. Anti-aging researcher Aubrey de Gray has a talk on this specific issue:


> Mitochondrial _inhibition_ is actually more correlated with longevity than the other way around.

Longevity is overrated. The light that burns twice as bright lives half as long. I'd rather be a superhuman for 40 years than feeble for 120 years.

As I understand it, longevity in the life-extension sense is generally taken to mean years of good health.

Edit: fixed a word.

Maybe yes, maybe no.

But evolution didn't decide on the structure and behavior of our throw-away-cup bodies in order to make our puny minds sitting on top our puny brains happy.

For better or for worse, in humans (we are not turtles, for example, they have their own success strategy), AVERAGE 2nd generation reproductive success is the measure of our existence. The collection of strategies to maximize that are different for human men and human women. But aligned. On average. 2nd-generation reproductive success is our "purpose" in a universe made mostly out of hydrogen. Nothing more - nothing less.

I'd rather be a superhuman for 40 years than feeble for 120 years.

But that’s not a choice you can actually make, it’s out of your control. What if you could choose by taking a pill?

I'd rather be healthy for 120 years like I am now, yet could live with never being a superhuman.

Steroids let you control this trade-off

Human evolution happened almost exclusively during times where food was scarce.

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?

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.

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

PE = M g h

245 joules

245J/5s == 49W

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


OP hypothesis seems to be correct.

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.

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.

I'd actually be interested to know how many mitochondria are there in the heart of other species when compared to humans, because the mere assertion that the heart doesn't get tired because otherwise we would die does little to explain the mechanisms behind it.

This. Beating heart on an ultrasound is widely considered the first true sign of a healthy pregnancy. Almost before anything else, the heart develops and immediately begins beating, and does so until you die.

There's a reason why the cliche X is the beating heart of Y.

> 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.

True. My answer was more of a cri de coeur.

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.


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

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.

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.

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

Yes. Also this:


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

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.

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.

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

This is a pretty cool technology.


It squeezes the legs to help the heart pump blood.

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?

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.




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/

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."

> 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.

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.

I routinely (especially in the summer in the Southwest) experience near immediate hypotension after finishing a (running) race. The last time it happened, the medic explained that my legs add significant circulatory capacity— when they’re moving. I now walk around after races until vasoconstriction happens, and I fair much better.

I don’t know enough to corroborate the ram pump theory, but I don’t doubt that there are other mechanisms aiding circulation

Sorry, I was hoping my insertion of the word 'generally' implied that it wasn't the only driver of circulation in all circumstances. Yes, there are check valves in the circulatory system and peripheral muscle contractions contribute as well, but comatose people don't, as far as I know, die because their muscles aren't contracting. The body doesn't seem to require peripheral muscle contractions to circulate blood.

I always think of it as a peristaltic pump: big tube, squeeze one side, squeeze the other to circulate blood.

This paper might be of interest: "A review of selected pumping systems in nature and engineering--potential biomimetic concepts for improving displacement pumps and pulsation damping" https://www.ncbi.nlm.nih.gov/pubmed/26335744

From the paper I was thinking of(link added to parent comment):

"the velocity of the blood in the embryonic heart lumen exceeds the speed of the peristaltic wave moving through the lumen"

which is interesting if you think of the heart as a peristaltic pump.

Oh hey, this reminds me of one of my favourite songs https://www.youtube.com/watch?v=Y5OpvDdVtYU

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.

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.

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

The heart does experience hypertrophy from exercise.


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.

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).

Ah, TIL.

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.

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

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

Yogis would posit it's prana shakti.

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.

It does rest after every beat

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.

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.

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

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

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