INJURED AND 3 OTHERS WERE LOADING BAGS OF ANFO EXPLOSIVE. EACH MAN CARRIED TWO 25LB. BAGS FROM INSIDE OF BOX TRAILER AND DROPPED INTO BED OF A P.U. TRUCK. INJURED THE STEPPED DOWN ONTO THE BUMPER OF THE TRUCK AND THEN DOWN TO THE GROUND.INJURED FELT PAIN TO LOWER BACK AND HAD NUMBNESSIN RT. LEG. X-RAYS AND MRI INDICATED A BULGING DISC.