Not sure if anyone here is familiar with the Strider plate loaded machines. They normally have them for biceps, and triceps, however they also have them for Leg Extensions as well. They are plate loaded in 3 different areas and allow you to hit the specific bodypart in 3-4 different areas. Needless to say after the reccomended 5 sets of Leg Extensions my quads are FRIED. If any of your gyms has one these machines for bis/tris, or Quads, I highly reccomend giving this a shot.
EDIT: Thinking this should have been posted in the Training section of the forums, sorry bout that!