black line edge 

Remember those individual muscle fibers that make up the big, whole muscle... the one with a name? As you work out, if you use enough weight, you will be black line edge  exercising those individual fibers, some of which are not very strong to start with. As you exercise rep after rep, you will be working, and tearing, those weaker fibers, and they will "fail".

https://fairsupplement.com/black-line-edge