My 7th grade teacher taught me this so I could be wrong. Your muscles are made up of lots of fibers. If you used all your fibers at once, it would be way to much force and rip your muscle so your brain doesn’t let you (in extreme cases you can ex. Pinned under a car). When you’re muscles are doing work, only use some of them. If those fibers arnt strong, they “get tired quickly” and then the other fibers have to contract and they switch back and forth causing the shake
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