MECHANIC WAS CHANGING TRANSMISSION SCREEN LOCATED UNDER PUMP IN D11 TRACTOR. HAD TO GO THRU FLOOR BOARD OPENING TO GET TO THE TRANSMISSION SCREEN, WHILE CLIMBING DOWN THRU THE OPENING, LOST FO OTING ON OILY HOSE, AND FELL, WHILE TRYING TO CATCH HIMSELF CAUSED HIM TO STRAIN HIS LOWER BACK MUSCLES.