As most of you know, I try to refrain from giving advice on bad knees. I'm not a doctor, nor am I a physical therapist. Then there's the fact that even if I were, I haven't examined or even laid eyes on your particular knees.
So, generally, I'm not going to second-guess a treatment regimen prescribed by someone who has actually inspected your knees and discussed them at length with you.
In creeps a little exception.
I have become aware of at least two cases where patients with painful knees have been told to do one-legged dips as part of their exercise routines. (If you've never done one: While balancing on one foot, you bend your leg, dipping down, while trying to maintain a straight line.)
Let's imagine that your buff physical therapist advises you to do one-legged dips to help fix your ailing knees.
Here's what you should do after he says this:
Look into his face. Is it flushed? Are his eyes bloodshot? His pupils dilated? His breathing stertorous? These all can be signs of recreational, judgment-impairing drug use.
Surreptitiously gauge the distance to the door. Could you beat him to the door, even with your bad knee(s), if he made a move to try to restrain you and force you to do a couple of one-legged dips "just for practice"?
If he does suggest you try doing one under his careful supervision, demur gently, perhaps with a line such as, "Oh, maybe not right now, I just ate -- why don't you show me and I promise I’ll do a few as soon as I get home?" (Make sure your fingers are crossed behind your back.)
After your visit ends, when the receptionist says, "And when would you like to schedule your next appointment for?", snap your fingers as if you just thought of something. Then say, "Ah, you know, I just remembered my work unit is being transferred to Tokyo. Oh well! Sayonara!"
Okay, seriously now: I can't think of a good reason why someone with chronic knee pain -- someone who probably has some difficulty climbing a set of stairs -- should be encouraged to do a tricky exercise that involves balancing while putting his or her entire body weight on a weak and possibly unstable knee joint.
Before anyone jumps up and says, "Oh, I know why physical therapists prescribe one-legged dips!" let me be clear: I'm pretty sure I know why they do. In brief, it's basically structuralist thinking run amok. So that's not the issue.
If you really care about fixing your bad knees, the issue is that you have to slowly ease them back to greater and greater strength. The key to getting better isn't doing some demanding exercise that many people would struggle to do with fully healthy knee joints. That just strikes me as absurd.