If the heart is a muscle, why doesn't it ever get tired?
reddit.com
reddit.com
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.)
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.
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!
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.
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.
That’s a far cry from “all in your head”. It’s a systemic response that affects every part of the body.
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...
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.
https://www.reddit.com/r/askscience/comments/fm2z8x/if_a_hea...
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.
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.
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.
https://news.ycombinator.com/item?id=22643333
And this follow-up comment sort of agrees with my inference -- at least the first part of my inference:
https://news.ycombinator.com/item?id=22644577
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.
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.
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.
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
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).
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.
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?
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.
But frequent exercise is certainly a "miracle drug" in that it effectively prevents or treats a host of different medical and psychological conditions.
https://www.webmd.com/diet/news/20140529/fast-weight-loss-ma...
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.
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 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.
Cheers.
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".
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"
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.
1:https://en.wikipedia.org/wiki/Action_potential
2:https://en.wikipedia.org/wiki/Refractory_period_(physiology)
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.
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.
You responded to a bunch of claims the comment didn’t make.
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.
Edit: fixed a word.
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?
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.
Did he mention whether faculty get free family tuition or not?
> 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.
Your muscles may not get "exhausted" but it seems like there must be some breakdown in the cells that outpace recovery at some point.
Also, what else would you call heavy legs after a long run other than fatigued?
> you would waste away
- what does that mean? why?
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?
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3737409/
aka
> 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.
I’d imagine it’s at least a magnitude of more than the heart would expend in 5 seconds.
PE = M g h
This website [1] shows several sources. The range of estimated power outputs is between 1W and 5W.
https://hypertextbook.com/facts/2003/IradaMuslumova.shtml
OP hypothesis seems to be correct.
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.
There's a reason why the cliche X is the beating heart of Y.
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.
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/
That muscles get tired and sore is a feature because it prevents damage.
Source: father had a stroke 20 years later.
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.
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.
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.
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.
This is a pretty cool technology.
https://billionsinchange.in/en/solutions/renew/
It squeezes the legs to help the heart pump blood.
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3021100/
"Tired" not as in the other muscles, but you can't function at 100% for long period of times.
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."
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.
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.
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
"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.
I don’t know enough to corroborate the ram pump theory, but I don’t doubt that there are other mechanisms aiding circulation
A beat is not guaranteed volume, a rested heart can move more blood at the same heartrate than a tired one.
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.
It also has a pretty good blood supply.
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.
Now, how those animals survived is a much more interesting question.
[0] https://youtu.be/pC3IrqUpm9U
[1] https://youtu.be/ubX9JaaqBXo