EE WAS OPERATING A 657 CATERPILLAR SCRAPER HAULING TOPSOIL TO A STORAGE PILE. WHILE TRAVELING W/A LOAD, HE HIT A SLICK SPOT IN THE HAUL ROAD. SCRAPER SLID SIDEWAYS TIL IT HIT DRY MATERIAL. WHE N IT STRUCK DRY MATERIAL, THE SUDDEN TRANSFER OFENERGY PRODUCED A JERKING MOTION TO UPPER BODY OF OPERATOR RESULTING IN BACK PAIN. PAIN GOT WORSE & HE WAS UNABLE TO WORK STARTING 2/18/02.