system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why do muscles shake when tired? Doesn’t that take even more energy?
gpt
The basic mechanism that causes muscles to contract is very similar to a person reaching out their hand, grabbing on to a rope, then hauling on that rope.

If you want to picture how a muscle contracts, imagine a bunch of boats lined up front-to-end in a lake, with a group of people in the front and back of each boat, and each boat connected to the one before (and after it) with a rope. The ropes aren't tied to anything, the people in the boats are holding on to them like they were going to play tug-of-war. (the people in the front of one boat are paired up with the people in the back of the one ahead, and so on)

Suddenly someone gives an order, everyone pulls smoothly (nobody screws around and plays actual tug-of-war, they're cooperating) and the whole line of boats gets shorter - no one boat has enough rope to make a big difference, but if you have 10 boats with 20 feet of rope between each, and they all pull, the first and last boat will end up 180 feet closer together. If nobody screws up.

Now, imagine that everyone is tired after doing this for a very long time, and someone asks them to do it again. Some people are distracted and miss the orders, others are so tired their grip slips and they barely manage to grab the rope again, in one of the boats someone starts throwing up, etc., and they just barely manage to pull the boats together, but it's really slow and sloppy and uneven. That's the "shaking."