EE WAS OPERATING A SKID LOADER TO PICK UP BALLS TO CHARGE A MILL. EE HAD TO OPERATE THE MACHINE AGGRESSIVELY TO PICK UP THE BALLS. EE COMPLAINED OF A SORE KNEE AND IT WAS REPORTED TO HIS SUPER VISOR ON MONDAY. HE THOUGHT HE DIDN'T NEED MEDICAL ATTENTION UNTIL IT STARTED TO BOTHER HIM INCREASINGLY ON 5/3/00.