I had quite the "snoozy quad" (Bonesmart terminology) after LTKR last year. In my case it was atrophy combined with an old VMO tear plus muscle inhibition. I had pain and difficulty when attempting the SLR or simply tightening the muscle. I couldn't manage a SLR at all for three weeks, and then I'd shake all over from the effort. My quad finally woke up, but it took a long time. Also, flexion returned slowly with that knee, unlike the relatively pleasant recovery I am experiencing with my recent RTKR. Hang in there- you're doing great for just a couple weeks out!