I'm assuming you weren't actually pushing on your knee with your hands, but were straightening it or whatever it's called when you stretch it a little for extension. Anyway, I haven't done any exercises at all, but decided to try one time what I think she had you do; I could do it, but it wasn't easy. I also tried it with my good knee and it was a breeze, so it's something my right knee isn't ready to handle. I think it's something that your knee isn't ready for quite yet either since it was such a difficult thing to manage, and I'll not be doing it again. I'm not agreeing that your leg is weak due to a lack of that exercise, IMHO it's more likely that it's weak because it's still recovering from all the abuse it had to go through during surgery. You may be one who takes longer to recover, or maybe you're right on "schedule" for an average recovery; whichever it is, you're still in the early days, and your knee is letting you know that. I'm still limping quite a bit as my extension is not all the way to straight yet. My knee bends better than it straightens, but my understanding is that extension generally comes later than flex. Hopefully the next four weeks are good for your knee, but I also hope you don't get impatient, or nervous, about the time frame, and overdo things.