The posterior thigh muscles are very important for ACL recovery. An ACL injury causes anterior tibial translation, or anterior instability. During the examination, the doctor will pull the calf forward to determine the degree of ACL injury by bending the knee 15 degrees and 90 degrees and then pulling the proximal calf forward, if the proximal calf moves more than normal, the ACL injury will be determined. The purpose of ACL reconstruction is to restore anterior stability to the knee, which means that the reconstructed ligament is used to control the forward movement of the tibia (i.e., the proximal calf). However, the ACL is not the only structure that maintains the anterior stability of the knee joint. The stability of the knee joint consists of both ligaments and muscles, with static stability determined primarily by the ligaments and dynamic stability determined primarily by the muscles. Clinically, some patients with ACL injuries can also engage in sports such as running and jumping, mainly because their muscles are strong and compensate for some of the static stability of the ligament with the dynamic stability of the muscle (this is not good, as prolonged compensation can aggravate articular cartilage and meniscal damage). The muscles at the back of the knee are the dynamic structures that maintain the anterior stability of the knee joint, and strong muscles can control the anterior tibial translation. After an ACL injury, most patients have decreased motion, and muscle atrophy is especially evident at one point of the injury. After surgery, there is a greater need to strengthen muscle training, especially posterior muscle training, to allow the atrophied muscles to recover and to make the muscles stronger for better dynamic stabilization and better ligament protection after weight-bearing activities. Currently, it is common internationally to use the autologous N cord tendon to reconstruct the ACL. The N cord tendon is the main structure of the posterior medial knee joint, and after taking the tendon, there is still a short-term effect on the strength of the posterior knee muscles. Therefore, it is more necessary to strengthen the muscle training of the posterior side of the knee. The specific exercise method is not complicated, early on it is mainly isometric contraction of the muscles on the posterior side of the knee. The back side of the thigh muscles are tense in the case of straight knees, in accordance with the 10-second principle, tense for 10 seconds, relax for 10 seconds. 10 for a group, 90 seconds between the group and the group – the so-called 90-second principle. It is important to note that the time and intensity of the training of the posterior muscles should be more than that of the anterior muscles. The main goal of the first phase of rehabilitation is still to allow the reconstructed tendons to grow together with the bones, which requires 6 weeks of controlled knee motion, with all knee flexion and extension contact required under brace control. The general principle is 4 weeks of practice to 90 degrees, during which time no strength training is required.