With Knott's support on this, I'd have to say that I'm really not a fan of knee extensions for a person with patellofemoral problems. I'm with Knott, you should be doing almost all of your exercises with your foot planted on something so that you have a better chance at acheiving good alignment of the Hip-knee-foot. The exception to the rule tis the straight leg raise with the knee turned out and that's only an exception because it heavily biases the VMO.