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