system
Given the following post, respond as if you're commenting on Reddit:
human
[Marvel/DC] When people have superhuman strength, where exactly does the strength come from? A lot of them don't have muscles much larger than humans who work out a lot.

Spider-Man is usually portrayed as pretty muscular, but his muscles aren't big enough for me to understand how they could do some of the things he does. 

With the hulk I can understand it because his limbs are the size of tree trunks and I can therefore see where the power comes from. But other characters with super strength, that still more or less look like normal people, where does the strength come from? Are their muscles really dense or something?
gpt
Muscle density is the big one. But it's not just that. Super dense muscles taking on a heavy load might very well break the bones of the lifter. 

When super strength is a thing you'll also be wanting a complete overhaul of he entire skeletal system.

Mystic sources also exist and those don't necessarily need to follow the laws of physics.

For the highest tiered super strengths the mechanism may be a sort of tactile telekinesis.