system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why do muscles 'shudder' when lifting weights?
gpt
Each muscle cell is one really long cell that runs the entire length of the muscle. And anyone muscle cell can be either relaxed or flexing, but not any combination of the two.

So in order to modulate how much Force a muscle is generating, we have to change the number of muscle cells that are firing.  

But while we are lifting heavy things, inevitably some of our muscle fibers get tired and wear out, so they stopped firing and then we have to have another one jump in and replace it. This means that our brain has to do a lot of work to constantly be compensating for changes in the muscle and which fibers are firing, and that work takes a little bit of time, which makes our muscles wobbly when we are using them near their limits.