EMPLOYEE WAS PUSHING SURFACE WITH 9229 CAT. MATERIAL WAS HEAVY & WET. THE CAT HAD NO GROUSERS, WHICH MADE IT DIFFICULT TO PUSH. MATERIAL IN FIRST GEAR, STARTED USING 2ND GEAR WITH RUNNING START TO PUSH MATERIAL. EE FELT LOWER BACK POP. EE INDICATED THIS WAS AN OLD INJURY FROM 7-8 YEARS AGO.