EE was roof bolting in 5 right on the active mmu 001-0, a piece of rock binder on top of the coal seam loosened up from rib and struck the EE on the back of both lower legs. EE suffered a compressed right calf muscle. Cause of this injury was awareness.Compliance with rules/regs not a factor, mining equip/systems not a factor, miner proficiency could be a factor.