Patients after ACL reconstruction are asked to practice passive knee extension the day after surgery, why is this? First of all, we all know that knee extension is more important than knee flexion in terms of knee function, because if the knee is not fully extended, walking is bound to be lame. Therefore, after surgery, we all put extra emphasis on the function of knee extension. Then for ACL reconstruction, the significance of practicing knee extension in the early postoperative period is mainly the following three points: 1. Restriction of knee extension caused by joint injury: In the early stage of ACL injury, almost all patients will have severe swelling of the knee joint. The joint capsule of our knee joint is more relaxed at an angle of 30°, so many patients prefer to rest at this angle, then over time, the knee joint cannot be straightened. In other patients, the knee injury will be accompanied by damage to the meniscus or cartilage, causing intra-articular entrapment (such as a barrel stem-like tear of the meniscus causing joint locking), when the joint will also be significantly limited in motion. Therefore, in the early postoperative period, we should actively practice straightening. 2. Prevent scar occupation: scar proliferation is a common phenomenon after arthroscopy. After ACL reconstruction, if knee extension exercises are not performed at an early stage, some scar will grow between the anterior to intercondylar fossa of the reconstructed ACL, which will form an occupation over time and cause dysfunction of knee extension. 3, take the tendon: at this stage, most of our ACL reconstruction using the graft, are autologous N cord tendon, take the tendon is also equivalent to an injury, for this injury, similar to muscle strains, is to be fixed in the early extension position, to prevent scar contracture healing. Many patients prefer to rest with the knee slightly flexed after surgery, which over time can result in limited knee extension. I have heard before that a number of people, even medical professionals, are concerned that practicing extension early after ACL reconstruction will have the effect of causing impingement or laxity of the graft. This is not really a concern. Nowadays, surgeons are more and more precise about the bone tracts and points of the ACL, and are getting closer to anatomical reconstruction. As long as the patient does not have severe joint laxity, there is no need to worry too much.