@Enexin
I assume you mean the black markings whenever they get angry.
No, I don't believe it has been explained.
There's apparently a prequel to this series, that may have explained it? Not sure. It's never been TL'd and I don't know the name of it off the top of my head.
I assume it's related to the Alien Invasion talked about several chapters ago in Roy vs Jeanne's fight. How humans became physically stronger, and some changed so much they were called "abnormals".
It's likely that those black stress lines are a form of abnormalsy. Which also explains why they show up when they're angry since we know abnormals have anger problems in their transformations.
Edit: Or it could all just be an artistic choice, who knows really.