Need some help /fit/ >2 years ago >making great gains

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Need some help /fit/

>2 years ago
>making great gains
>squats new favorite exercise
>one day, last set, ass to ground
>feel sharp pain in my knee
>go to the doctor
>reoccurring injury from when I was a teenager
>hurts all the time
>morale destroyed
>slowly stop going to the gym
>getting fat now

I'd like to start lifting again but I can only do minor, low weight excersies. Running is still fine. What leg exercises can I do to get decent results? Running? Biking? Swimming?

I'd rather look like I do now instead of pic related.