Worker's comp? I had to take one years ago after a more minor surgery on my right knee. My TKR is the left one (I switched bad knees over the years). It actually wasn't too bad. They'll put you through the paces, just give it your best try. Let them know if something is painful. They'll rate how long you can be on your feet, various work tasks you can or can't do, etc. Then they'll make the decision about the percentage of permanent disability you might have. My right work-related knee was rated at 40% partial permanent disability of the entire leg. I received a settlement, still work at the same company.
That 40% disabled knee got me through my TKR on the other knee. But now it's fussing more. Don't worry so much about the FCE -- just make your best honest effort and make sure if you might need work restrictions, you get them.
--Jackie