EMPLOYEE SAID HE WAS RIDING IN THE MANTRIP TO #3 UNIT WITH HIS CREW AND THE MANTRIP HIT A BUMP IN THE ROAD. EE STATED THAT HE BOUNCED HARD IN THE SEAT CAUSING A STRAIN IN HIS MIDDLE BACK. EE CONTINUED TO WORK UNTIL 5/11/04 WHEN HE MISSED WORK BECAUSE HE WENT INTO THE HOSPITAL FOR BACK PAIN.