This has been plaguing me forever, even when I was at peak physical condition when I was an athlete (martial artist).
Basically, any exercise or movement that requires legs to stay straight, I feel some strain in the back of my knee, and I can't figure how to fix this, specially now that it is severely getting in the way of exercises I am trying to learn (example: it seemly makes straight leg L-Sit impossible)
Because of... mistranslation, I always believed THOSE were the hamstrings, and kept searching how to improve hamstrings, only some time ago I found out hamstrings are the muscles in the back of the leg.
I also found out then how to test hamstrings flexibility and strength, and both are actually fine, only the tendons themselves behind the knee that are feel short and stiff.
So I don't even know the name of whatever I want to fix, so where do I get started?