The ACL is located in the knee joint and connects the femur to the tibia. Its main function is to limit the excessive forward displacement of the tibia, and it works together with other structures in the knee joint to maintain the stability of the knee joint and enable the body to perform various complex and difficult lower limb movements. ACL injuries are common in sports trauma and traffic accidents, and rupture can produce significant knee instability and seriously affect knee function, which requires timely surgical treatment and appropriate post-operative rehabilitation to restore joint stability, enhance joint proprioceptive function, and ultimately restore normal motion. Repeated sprains of the joint can easily cause damage to the articular cartilage, meniscus and other important structures, leading to premature aging of the joint and the development of osteoarthrosis. After ACL surgery, traditional home rehabilitation training is still flawed. Patients do not recover well on their own without real-time supervision, guidance or timely correction of incorrect posture by a therapist or physician during training. Knee extension is a contraction of the quadriceps muscle through the tendon of the quadriceps, patella, patellar ligament and tibial tuberosity. Decreased knee extension can lead to joint instability, repeated joint injuries and increased exudation, and cause pain, resulting in: pain – restricted quadriceps movement – quadriceps atrophy – knee instability – ligamentous laxity This results in a vicious cycle of: pain – restricted quadriceps movement – quadriceps atrophy – knee instability – ligament laxity – further injury – increased pain. Therefore, after ACL reconstruction, swelling and pain are common problems after ACL surgery, and it is essential to interrupt the vicious cycle by reducing the swelling and pain as soon as possible and restoring and maintaining the quadriceps in good condition during rehabilitation. Because of the swelling and pain most patients will be in keeping the knee joint in flexion position. Long-term knee flexion and restricted movement not only leads to contracture of the joint, but also causes atrophy of the quadriceps muscle and enters the vicious cycle mentioned above. Therefore, it is extremely important to reduce swelling and pain at an early stage. After discharge from the hospital, patients who undergo rehabilitation at home are usually relieved of pain by applying ice. However, if the patient is trained in the rehabilitation department, physical therapy is given to reduce swelling and pain at the same time, which will make the rehabilitation training twice as effective. From this point of view, in-hospital rehabilitation has its absolute advantage of being irreplaceable. Long-term non-weight bearing can lead to quadriceps atrophy, and insufficient quadriceps strength is the cause of the last 10 – 20 degrees of extension limitation. The quadriceps muscle is located on the anterolateral aspect of the thigh and it consists of four heads, namely the rectus femoris, the medial femoral muscle, the external femoral muscle and the internal femoral muscle. In particular, atrophy or decreased muscle strength of the internal femoral muscle can severely affect terminal knee extension. Therefore, after routine ACL reconstruction, training of quadriceps muscle strength plays a very important role in the whole rehabilitation program and is carried out throughout the whole rehabilitation training. However, during the training process or follow-up, we found that, due to the early postoperative period, restriction of weight-bearing on the affected limb as well as the opening movements, most patients do not try to continue the training of knee extension to about 10 to 20 degrees, thinking that they have reached the straight position, so that the quadriceps (mainly the internal femoral muscle) muscle fibers within 0-10 or even 0-20 degrees are difficult to be adequately trained in time. In addition, most of the patients only paid attention to the knee flexion exercises and not to the maintenance of the joint extension angle. In contrast, in patients trained in the rehabilitation unit, these errors were detected and corrected by the therapist in a timely manner, thus fully exercising the quadriceps muscle strength and preventing the phenomenon of muscle atrophy. The stretching of the N cord muscle and triceps calf muscle as well as the muscle strength exercises can prevent the shortening of the N cord muscle as well as the postoperative pain. In most patients, it is difficult to stretch the knee sufficiently to limit knee extension when practicing at home. In short, without the guidance of a professional therapist, the same rehabilitation procedures are difficult for patients to complete well and adequately, which greatly reduces the effectiveness of rehabilitation and subsequently prolongs the rehabilitation time. This prevents patients from returning to work early. Even, the long-term limitation of knee extension leads to damage to the habitual joints, recurrent pain, etc., which seriously affects the patient’s daily life. The burden on the family and society is also not negligible.