EE WAS ASSISTING IN CHANGING TOOTH TIPS ON DRAG-LINE BUCKET. EE WAS SWINGING A 12 LB. SLEDGE HAMMER DRIVING THE PINS OUT OF THE TIP AND STATED HE FELT SOMETHING PULL IN HIS BACK AND HIP AREA. GROUND WAS LEVEL AND LIGHT GOOD. A HEAVIERSLEDGE HAMMER IS NORMALLY USED TO DRIVE THE PINS(LESS BOUNCE AND LESS FORCE). EE STARTED TO MISSWORK ON 2-20-02, SOUGHT MEDICAL AID.