I had my Acl reconstruction surgery 1.5 years back, now my knee joint is back to normal functioning, I can cycle, I can jog but after doing any physical activity the muscles behind my knee make cracking sound when I stretch my knee to full extension and there is pain. In the morning the full extension pains allot but after a few moves it relaxes a little bit. It also pains when I do calves stretches. My muscle around the knee Hamstring muscles are not fully developed as compared to my fully functioning knee, but I can do most of the basic leg exercises with my injured knee.