Getting that knee nice and straight again will take time. It’s a matter of stretching the big, thick tendons at the back of the knee, commonly called the hamstrings. I remember doing a dissection of them in anatomy class and they are thick and tough.
Does the history of the knee impact progress in extension? You bet. I can use my own two knees as an example. They were operated on the same day. Before surgery I did those exercises where you press the back of the knee down on the floor (table, bed). My right knee could do it; my left knee, however, which was my worst knee and could not bend enough to go up a stair, could not do that exercise. It hurt terribly to even try. The knee would not go flat, and because it could not go flat, my left hamstrings had shortened. After surgery, my right knee very quickly, within a week or two, was at 0 extension. Good knee! But my left... it didn’t like going flat anymore than it had before surgery... because my hamstrings needed to stretch back to a length that would let my knee do that.
Anyway, long story short, I didn’t worry about it and set about just doing some knee presses on my ottoman when I thought of it. And I walked a lot. Lots of walking. My OS said walking would get my extension back, and so did all the folks here at Bonesmart, so I decided to trust nature. And sure enough, at around five months my left knee’s extension was back to normal, at least 0. It came back on its own just like everyone said it would.
But it came back five months later than my right leg, which never got shortened hammies due to its different history.
It’s best not to put weight on the knee while doing hamstring stretches. Just not necessary, and you risk straining the hamstring (muscle or tendon), which would be unfortunate. TKR is painful enough without having to deal with something like that at the same time.
