EE WAS DRIVING A HAUL TRUCK AND HE BACKED UP TO BE LOADED. THE LOADER OPERATOR'S HAND SLIPPED AND HE HIT THE BOOM LEVER INSTEAD OF THE BUCKET LEVER IN THE LOADER. THE FULL WEIGHT OF THE BUCKET HIT THE TOPSIDE OF THE UNIT CAUSING THE EE TO JARR HIS BACK. JARRING EFFECT SPRAINED THE EE'S LOWER BACK.