Sharla, I take it you mean lying on your back lifting your legs straight. Not a good exercise if you have a bad back. Hanging leg raises are better. Although they work your quads most of the contractions around the knee are isometric (non-moving) so are ok,in general, for injured knees.

God you ask a lot of questions lol. You need to get an anatomy book or find a site and understand the difference between agonist, antagonist and synergist. Also, concentric, eccentric and isometric muscle contractions. It would take too long to explain fully. I'm sure in the first instance google could help.

Has your physio given you knee extensions two legs up, one leg down?