The Vatican states that proper respect is shown by bending down with our right knee until it touches the ground with a slight pause to show our humility and reverence before God. This is still the rule even during exposition, although the common practice is to bow on two knees when before the monstrance. The attitude with what you do it with is more important than the form, and it should never be done for show.
Additionally, if your knees are bad, you should not worry about this at all.