EE REPORTED WHILE CHANGING A TIRE ON A CATERPILLAR 938 LOADER IN THE UNDERGROUND SHOP, HE WAS PULLING DOWN A BREAKER BAR TO LOOSEN THE WHEEL STUDS. A CO-WORKER ADDED WEIGHT TO THE BAR CAUSING THE EE TO LOSE BALANCE AND FALL ON HIS KNEES RESULTING IN SWELLING OF THE RIGHT KNEE. LOST WORKDAYS BEGAN ON 09-02-03.