EE WAS REPLACING THE LINER PLATE IN THE TRUCK BOX OF 1554. HE WAS IN THE PROCESS OF GRINDING OFF SOME WELD MARKS. HE WAS GRINDING FOR ABOUT 5 MINUTES USING A 1# GRINDER. AFTER GRINDING OFF THE THIRD AREA OF MARKS. EE WENT TO STAND UP AND FELT PAIN IN LOWER BACK. HE WAS EVALUATED BY A PHYSICIAN AND TAKEN OFF TO WORK.