eli5 why do muscles get tired after holding the same position for an extended period of time?
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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.
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