First of all, knee extension is more important than knee flexion in terms of knee function, because if a person has normal knee extension and only 90 degrees or so of knee flexion, he or she can walk without any problem. But even if the knee can be flexed to the full angle, but the knee extension is 10 degrees worse, then the person must be walking with a limp. So the focus of our post-operative exercises should be: knee extension! For patients with ACL reconstruction, knee extension exercises are even more important, because first of all, many of us take the tendon of the N cord muscle, which is an injury in itself, so after this injury, we must strengthen the knee extension. Many people will complain of soreness and pulling pain on the back side of the knee, which is normal for tendon removal. Other patients are unable to straighten their knee before surgery because of a meniscus card or stricture, or swelling, so it is even more important to strike while the iron is hot in the early stages after surgery!