ROOF BOLTERS AND SUPPLYMAN WERE LOADING SUPPLIES ON ROOF BOLTER. THE OFF SIDE OPERATOR WAS PUTTING BOLTS FROM SUPPLY CART ONTO HIS SIDE OF THE MACHINE WHEN HE FELT A TWINGE IN HIS BACK. HE STATED THE AREA WAS LOW AND HE WAS LIFTING NORMAL WEIGHT / LOAD. THE EMPLOYEE FINISHED THE SHIFT AS USUAL.