Knee strengthening exercises?

I recently suffered a minor knee injury it has left my left knee feeling slightly weak. What are some good exercises to condition my knees so something like this will not happen again?
Answer:
I was told to strengthen the knee to use an eliptical trainer which I do now every other day and it really worked wonders. I had a full on kneecap fracture and due to this I am almost back in marathon shape

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very light leg extensions. but don't do them yet. wait until you feel no pain.

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squats

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The first thing you should do (if you haven't already) go see a doctor. A doctor will make sure it's OK to do the exercises and send you to a physical therapist who will show you which exercises, how many of each to do and how often you should do the exercises.

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