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#1-do you have full knee extension? I’m assuming that’s a no based on what i read.
#2-I’m also assuming by “PT” you mean physical therapist, and if so, did he/she do a full eval to check ROM of the hip/thigh/lower leg/ankle/foot? That would help key in with where to start.
#3-Does said “pop” hurt?
Pain being the symptom, unless you abruptly did something traumatic like a knee sprain during running or jumping (at any point), I would say its probably assocaited with some sort of dysfunction up or down the chain. Address ROM of the muscles surrounding hip/knee/ankle/foot. Don’t torque on the knee joint, like i said, chances are its due to lack of ROM of the surrounding musculature.
The two big ones i would see about:
Your hamstring flexibility on each leg separately (where you’re unable to keep a flat back).
You hip flexibility, predominantly the rectus femoris.
I used to have a “click” near full knee extension (not painful), which could also be called a “pop”, following my most recent ACL surgery. I found addressing hip weakness and mobility in the musculature surrounding the knee and hip to be most effective in eliminating this and restoring proper ROM to my knee joint.