EMPLOYEE AND 2 FOREMEN WERE TRYING TO UNLOAD PUMP FROM THE BACK OF A DODGE TRUCK WHERE IT WAS NEEDED TO BE CHANGED OUT. WHILE TRYING TO SLIDE THE PUMP TO THE BACK OF THE TAILGATE AND POSITION IT UPON A STACK OF 12"" PIPE THE EMPLOYEE TOLD THE OTHERS HE NEEDED TO GET A BETTER FOOT PLACEMENT. WHILE TRYING TO DO SO HE SLIPPED UPON THE PIPE RIGHT FOOT LODGED AND TWISTING RIGHT KNEE.