HE WAS ROOF BOLTING AT TIME OF INCIDENT, IN THE PROCESS OF INSTALLING INSIDE BOLT ON THE RIGHT SIDE OF BOLTER. AT THIS TIME, A ROCK 8'X8'X8"" THICK FELL INBY THE ATRS AND CANTILEVERED 3' OUTBY THE ATRS. HE TOOK OFF RUNNING & THE ROCK STRUCK HIM ON THE BACK OF BOTH LEGS. IT STRUCK HIS RIGHT HEEL, THE BACK OF HIS LEFT LEG AT ABOUT THE TOP OF HIS BOOT, CAUSING HIS KNEE TO JERK FORWA