INJURED AND CO-WORKER HAD REPLACED SKIRT RUBBER ON RBC-1A CONVEYOR BELRT AND WERE IN THE PROCESS OF LOCKING THE HOLD DOWN BARS. INJURED CLIMBED UP ON BELT STRUCTURE TO GAIN BETTER VANTAGE POIN T. AS HE LEANED OVER TO CONTINUE INSTALLING THE HOLD DOWN BAR, HE FELT A PAIN IN HIS LOWER BACK.