EMPLOYEE WAS GRADING THE DRAGLINE BENCH. WHILE DOING THIS, THE BLADE HIT THE TOP OF A LARGE ROCK THAT WAS PARTIALLY BURIED CAUSING THE EMPLOYEE TO BE JARRED. HE STATED HE WAS WEARING A SEAT BE LT. OVER THE NEXT FEW DAYS, HIS BACK BECAME STIFF AND SORE. HE WORKED 7/26 AND 7/28 - WENT TO DR ON 7/28 AND THEN WAS OFF.