Knee probs

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Can anyone give me some modifications on exercises for knee problems?? Due to deteriorating cartilage(sp) in my knees, I cannot do lunges or squats without an enormous amount of pain, and I'm at a stand-still when it comes to a lower body workout. It's depressing having all of these workout dvd's that concentrate on doing lunges to work the thighs and butt knowing if I attempt the full lunge I will be stuck and in agony trying to stand back up!