EE WAS USING A 1 1/2"" AIR IMPACT TOOL WHILE STANDING ON LADDER AND REACHING OVER HEAD TO TIGHTENCOLLET BOLTS OF 922-C CAT FRONT END LOADER BOOM PINS. EE FELT SMALL PAIN TO LOWER BACK, NOT SIGN IFICANT AT TIME. REPORTED 2 DAYS LATER AS STRAINED LOWER BACK.