EMPLOYEE AND WORKERS WERE IN THE PROCESS OF BUILDING A ROCK BOX AT NEW #6 HEAD. EMPLOYEE AND CO-WORKER LIFTED A 4'X 8' PIECE OF 1/4"" SHEET METALAND HE FELT A BURNING SENSATION IN HIS LOWER BACK. HE FINISHED HIS SHIFT AND FILLED OUT AN ACCIDENT REPORT AT END OF SHIFT. NEXT DAY HE WENT TO HOSPITAL AND WAS DIAGNOSED WITH A MUSCLE STRAIN TO HIS LOWER BACK.