eli5 why do muscles get tired after holding the same position for an extended period of time?
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Because muscles are fighting gravity. Gravity is constant, which means that if you want to hold your arm up in a way that contradicts the direction of gravity, you will have to activate your muscle. Activating your muscle requires energy.
Inside your muscle are proteins that attach and pull on the muscle fibers, generating force. However, they don’t stay attached: they cycle off (which uses energy) and then reattach. *Probably* this is so that your muscle can tense and release quickly (but as with many things in biology it is hard to know exactly why it evolved in the way it did). In principle this is not necessary – since only *releasing* the muscle requires energy, in principle you ought to be able to hold a position without using any energy and getting tired, which is what actually happens in rigor mortis – but a constantly-on muscle is not very useful.
Our muscles simply aren’t very efficient. Even though physics says there’s no energy needed to maintain a certain position our muscles don’t actually work like that. They require constant energy to maintain a certain force.
When you work out you burn glycogen (“sugar”) and produce lactic acid. Your body is good at restoring your muscles to resting state but can’t do it as fast as your muscles consume+produce those resources.
So after sometime, your muscles get tired and start burning
Probably this info needs corretion (I read it long ago), but in this case, first thing that gets tired is not your muscles, but neurons. For keeping muscles in unusual position neurons need constantly send signals. And been active for extended amount of time is really burdening for neurons
Some don’t! You have two types of muscles in your body:
– Smooth muscle
– Skeletal Muscle
Smooth muscles are all the muscles you don’t control and these ones don’t really get tired! These include the bowls, stomach, anus, heart and a few others. Think: your anus muscle stays clenched 24×7 and it never gets tired! (Thank god! Could you imagine how messy that would get). Likewise your heart very rarely gets tired unless you have an underlying condition, or you’re doing extreme endurance training.
Skeletal muscles are the ones you’re thinking of and the muscles you can consciously control. They do get tired and usually via two main mechanisms: neural fatigue and metabolic fatigue. The system that sends signals (neurons) get tired from signalling the muscles to contract and the muscles, consuming energy, get tired from contracting (metabolic fatigue).
Both forms of fatigue are underpined by the same basic problem: not enough energy to fuel the work OR too much waste which is interfering with using incoming energy (ATP).
It’s a lot like a car. A car uses fuel + air (aka ATP for muscles) and produces exhaust gas (lactic acid and other byproducts for muscles) to make power. If the exhaust gets blocked or restricted and the exhaust gas can’t clear the engine, power is lost because exhaust gas interferes with the fuel + air mixture. You do this long enough the car will stop. Likewise if you don’t have enough fuel in your tank because you’ve been driving for a long time (aka using your muscles for a long time) your car will also stop.
Controlling the muscles takes energy. Picture the muscles as very weak-minded soldiers that can only stand still as long as you yell at them. They let go as soon as you get tired of yelling.
I can’t give you the exact science behind it, but muscles are controlled by nerves which use up ions (iirc mainly sodium) when controlling them. Once they run out of the ions, you feel it as losing strength and being tired. Part of the strength training is being efficient at controlling your muscles in addition to having more of them.
In electronics, you can picture it as normally open switch that’s closed and is opened only when voltage is applied.
What you’re asking is why, if we are not doing any “work” in the physics sense, do we need energy (and get tired) holding a static position. The answer is that even if, say, the arm is not moving noticeably, the actual muscle contractile proteins are constantly attaching, pulling, releasing, cocking, and attaching again. This is not a perfectly smooth operation, so you will notice shaking happen if this goes long enough. Practice can minimize this shaking by smoothing out the neural pathways and making all the muscles (fibers, really) fire at the same time.
They get tired from contracting without a break. This is partly because the waste products from contractions have trouble getting flushed out when they’d constantly contracting without a break. Those waste products are acidic and painful.
I think I see the issue.
So, your legs aren’t rigid like a table’s legs. If you’re standing still, you’re actually pretty precariously balanced, and you might just tip over. For example, people with inner ear infections struggle to stay standing sometimes.
Your bones aren’t perfectly stacked, you’re top heavy, etc.
So your body is *constantly* making adjustments to keep you from tipping over. Your toes need to flex, your legs shift a bit side to side, etc. All of this actually uses a lot of power over time.
A table on weird hinged legs would need to be precariously balanced, and then you’d still need someone standing there fixing it constantly.
For a different issue, if you’re holding your arms out to the side, again, there’s nothing rigid holding it up. It’s floppy bags of meat and bones connected with string. Unlike a rigid frame, if the muscles stop working, it’s just floppy.
This is actually good for us, because if we were rigid, we couldn’t move. And we spend a lot more time moving than holding one position for hours at a time, at least historically.
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