EE HAD JUST SET ATRS ON ROOF BOLTER AND SAW IMMEDIATE ROOF CRACK. EE STARTED RAISING AND LOWERING ATRS IN ATTEMPT TO BREAK THE ROCK. AFTER SEVERAL ATTEMPTS THE ROCK BROKE FREE AND RODE BACK TO THE OPERATOR CONTROLS, STRIKING EE IN HIS LEFT SIDE LEG AND HIP AREA. ROCK SIZE WAS 30"" LONG X 20"" WIDE X 4"" THICK.