HE WAS TRYING TO LOOSEN TOP BOLT ON LOADER DRIVE SHAFT. THE SHAFT KEPT TURNING & HE TRIED TO HOLD IT STILL WITH ONE HAND WHILE LOOSENING BOLT WITH THE OTHER HAND. AS HE WAS DOING THIS HE FELT HIS BACK SLIP OUT OF PLACE. HE SAID HE HAS A PREEXISTING HISTORY OF A L-4 DISC. RUPTURE AND HERNIATED.