Injured was hauling from shovel in pit #6. The material was gray mud. When dumping the material would come out all at once, and jar the truck and driver. Injured was bracing his leg against the floor when dumping and the jarring strained his knee. Was seen by a Dr. on 1/24/07, an MRI was done on 1/25/07 and he was released to return to work on 1/26/07.