leg extensions= like pushdowns for triceps, leg curl= like biceps curls its like saying if close grip bench press (Squats) is better than pushdowns (extensions) for developping triceps (quads).. same shit excepting that you can focus on one muscle
What about the hack squat machine? Should have good support for your back?trapz101, leg extentions get a bad wrap because of the way the weight sits on the ankle and how that puts "sheer" pressure on the knee, personally I do them and heavy and I have yet to have any problems from them.
Hacks are a good alternative to squats but with a bad back I don't know how they work for the op. I am up to five plates a side for 10. I've done leg extensions for yeeeeeears and have never had any knee issues.