EMPLOYEE WAS EXITING THE REAR OF THE FORD F450 SERVICE TRUCK. THE TRUCK WAS EQUIPPED WITH A CABLE STEP MOUNTED TO THE REAR BUMPER. AS HE STEPPED ON THE CABLE STEP, THE WELD CONNECTING THE CABL E AND RETAINING NUT BROKE. AS THE STEP GAVE WAY THIS CAUSED A TWISTING MOTION TO HIS BACK RESULTING IN A BACK MUSCLE STRAIN. HE WAS DIAGNOSED WITH MUSCLE SPASMS AND RESTRICTED FROM WORK.