Leg extensions are hugely damaging to the knees.
Especially when the resistance is anything appreciable.
Squats, done properly, are much safer and more effective. A deep knee bend is a more natural movement for the human species.
Think about it. Where in everyday life do you have knee flexion without ANY hip involvement? The torquing forces on the knees when doing leg extensions is huge.
There might be a minimal amount of muscle-building effect performing leg extensions (especially if you're heavily gassed), but the damage-to-benefit ratio makes the movement not worth it.
