How exactly do muscles’ ability to apply force emerge from chemistry / physics. I “know” pretty vaguely that the elasticity of muscle tissue is fundamental to how it works, but how does the body actually chemically create that elasticity, and what phenomenon of chemistry is elasticity emergent from?
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The way it works is that each muscle cell physically contracts. It physically pulls itself together from the inside into a shorter shape, causing the overall muscle (made of many cells) to shorten and change its shape too, pulling any tendons and bones along with it.
(I’m honestly not entirely sure what you mean by elasticity. There’s multiple possibilities, I just don’t know which one you mean.)
So how does that contraction work?
How that works is, they’ve got a bunch of different filaments of protein inside of them, in two types, myosin and actin. These protein filaments ultimately [slide past one another](https://en.wikipedia.org/wiki/Sliding_filament_theory) in order to contract. What happens is that the mysoin head on the [myosin](https://en.wikipedia.org/wiki/Myosin) filament binds to the [actin](https://en.wikipedia.org/wiki/Actin) filament and sort of throws the actin filament past itself; since the filament is covered in these active myosin heads, this “concerted throwing” ultimately leads to the two filaments sliding past one another.
So how does that throwing past work?
How that works is, the [myosin head](https://en.wikipedia.org/wiki/Myosin_head) is activated and derives its energy from a compound called ATP that it [hydrolyzes](https://en.wikipedia.org/wiki/ATP_hydrolysis). ATP is an energy store in the body, and it has to physically bind to the myosin head. The binding changes the shape of the myosin head, but then the force of the unbinding changes the shape again, in an energetic way that can perform physical work.
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