Well this is a new one, I just noticed it over the last week while walking around the airport.
If I walk for more than ~300yds at a go (or ~3min on an elliptical), my left knee will refuse to lock straight, forcing me to use my leg muscles to support my bodyweight. Every time I try to get it to lock, the joint will hurt, and tremor front and back (the joint wants to back away from the pain, but I'm trying to get through it, so it bounces back and forth). A few minutes rest and I'll be back to normal until I go for another long walk.
Before this, I've had stiffness in my knee and lack of mobility (couldnt move the joint more than 90deg under its own power, a strap would bring it to about 120deg), but I never had this tremor.
I would see a doc.