I'm trying to understand how muscles contraction/tension works but getting loss in on the cellular level. From my understanding, when muscle tissue need to contract, the cells are flooded with calcium? Does that mean they get filled up with calcium? If so, are the cells hollow or does the calcium replace the fluid that was originally there? Or does the calcium cause the reaction in the cell for it to firm up?
I'm asking because there's an alien character who's quite thin but is also very strong and I was wondering about the science of that. I know them being alien means that their biology wouldn't necessarily follow earth rules but they could also share similar traits.