EMPLOYEE WAS OPERATING A BROOKVILLE 9 PERSON MANTRIP WITH HIS RIGHT KNEE PROTRUDING FROM THE OPERATOR'S COMPARTMENT. HE STRUCK A TIMBER THAT WAS INSTALLED BETWEEN BELT AND TRACK FOR ROOF CONTR OL PURPOSES AT BREAK #48 ON MAINLINE. HE WAS TAKEN TO LOGAN GENERAL HOSPITAL WHERE HE WAS DIAGNOSED WITH LIGAMENT DAMAGE.