Arthroscopic minimally invasive surgery to reconstruct the ACL has become the standard of care for improving knee instability after ACL rupture, but clinical outcomes do not only depend on the precise surgical approach, but also on proper postoperative rehabilitation, which can facilitate the recovery of joint motion. In recent years, postoperative rehabilitation is becoming more and more radical, but there is a great disagreement among different scholars, and there is no gold standard to guide postoperative rehabilitation. This article discusses some of the controversial issues in the selection of rehabilitation training programs, and provides further insight into how to develop an optimal postoperative rehabilitation program. 1. Issues of postoperative braking and early activity Before the 1990s, a conservative rehabilitation program was favored after ACL reconstruction to prevent stress damage to the ligament stops after reconstruction. The conservative rehabilitation program emphasized prolonged postoperative braking and weight-bearing restriction of the knee joint, which generally took 12 months to achieve full extension and gradually return to normal activities. However, the high complication rate of loss of knee range of motion and the prolonged rehabilitation process associated with the conservative rehabilitation approach have attracted attention, and different opinions have been expressed by Roth et al. who produced an animal model of goats reconstructed after ACL resection and found that the animals with 6 weeks of knee braking after reconstruction had decreased muscle strength and activity and showed severe destruction of patellofemoral cartilage. The authors were earlier to suggest that long-term knee braking after ACL reconstruction is detrimental, and Muneta et al. found, through experimental studies in a rabbit ACL reconstruction model, that the early active group of grafts had faster ligament regeneration, and microangiography also revealed faster recanalization in this group. This study also supports the idea of protected subacromial joint activity immediately after surgery. In recent years, studies have been conducted to investigate the molecular biology level of the effect of stress on ligaments. majima et al. demonstrated through a study of rabbit medial collateral knee ligaments that MMP-1 gene expression was significantly lower in stressed tissues than in the unstressed group, and modern studies have demonstrated that MMP-1 is capable of degrading collagen types I-III and X Arnoczky et al. also showed that “stress deprivation” would cause an increase in MMP-1 expression in rat caudal tendon cells, and that stress application would inhibit MMP-1 expression in a significantly inverse relationship. These scholars have revealed the importance of maintaining appropriate stress after ligament reconstruction and the danger of “stress deprivation” from the basic research level, and opened a new way to understand and evaluate the effect of stress on the mechanical properties of ligaments, but deeper research is rarely conducted at home and abroad, especially the lack of relevant human study data. 2. Application of knee rehabilitation braces The use of rehabilitation braces in the short term after ACL reconstruction has been advocated by physicians who prefer more conservative postoperative rehabilitation, as it allows a gradual restoration of knee range of motion, but with the development of rehabilitation concepts, the need for postoperative knee rehabilitation braces has become an issue of interest to many scholars. Vadalà et al. concluded from a bone tunnel CT scan study that aggressive rehabilitation after ACL reconstruction without a brace exacerbates bone tunnel enlargement. They randomly divided 45 patients who underwent ACL reconstruction with autologous N cord tendon into 2 groups, with a conservative postoperative rehabilitation program in the control group and an aggressive rehabilitation program in the study group, during which joint motion was restricted without the use of a brace. The study index was the change in the diameter of the femoral and tibial bone tunnels, and two bone tunnel scans were performed on all patients at the time of surgery and at the mean postoperative date of 10 months. As a result, the extent of femoral and tibial tunnel enlargement was significantly higher in the study group than in the control group at month 10 postoperatively. However, several high confidence randomized controlled clinical studies have not concluded that there is a significant benefit of postoperative knee rehabilitation bracing, and that postoperative bracing is associated with reduced patient compliance and increased medical costs. Both groups were fully weight-bearing immediately after surgery. Both groups were fully weight-bearing immediately after surgery. The incidence of complications such as early postoperative knee swelling, intra-articular blood accumulation and wound effusion was lower in the former than in the latter, but there was no significant difference in the long-term clinical outcome. Harilainen et al. reported the results of a 5-year follow-up randomized controlled study of postoperative brace application with or without brace, in which one group wore partial weight bearing immediately after surgery, full weight bearing after 3 weeks, 0-90 degrees of mobility for the first 3 weeks, 0-120 degrees of mobility for the next 3 weeks, full range of motion at 6 weeks, and removal of the brace at 12 weeks. In the other group, no brace was applied throughout the rehabilitation process, with partial weight-bearing for the first 2 weeks and a range of motion of 0-90 degrees, followed by full weight-bearing and full range of motion training of the knee after 2 weeks. The results showed no significant differences in knee stability, Lysholm score, and Tegner score between the two groups after 1, 2, and 5 years. A study by Möller et al [20] also confirmed that no significant advantage was observed in the postoperative brace application group and that the unrestricted activity group instead obtained a better Tegner score at 6 months postoperatively. Other studies by other authors also did not find differences in clinical outcomes with or without brace application, and the unapplied brace group tended to achieve earlier full joint range of motion and functional recovery without significant complications such as knee instability [21,22]. 3. closed-chain training and open-chain training Both are rehabilitation training modalities to promote the recovery of joint motor function, but they are different in nature and application. Closed-chain training is a joint movement of the hip, knee, and ankle, in which the foot is fixed to the ground, platform, or pedal. Typical training methods for closed chain training include shallow squat exercises, stationary bike exercises and leg press exercises. Open chain training refers to the movement of the knee joint that does not depend on the hip and ankle joints, when the foot is unrestricted and can move freely. Quadriceps isometric contraction, straight leg raise, and isometric training are typical training methods for open chain training. Previous studies have shown that full range of OKC of the quadriceps in flexion from 30° to full extension causes forward deviation of the tibia and excessive tension on the graft resulting in graft injury. CKC can promote the co-contraction of quadriceps and N cord muscles to maintain the stability of the knee joint and reduce the forward displacement of the tibia caused by increased pressure loading, thus reducing the shear force on the knee and the tension on the ACL, and CKC can significantly reduce the force generated on the patellofemoral joint compared with OKC at 60-90° of knee flexion, reducing postoperative anterior knee pain. CKC training is safe, and this view is shared by many scholars [23,24]. Therefore, in the past, clinicians mostly avoided OKC training in the early postoperative period after ACL reconstruction. However, Perry et al [25] compared the effects of closed-chain and open-chain training on knee stability and limb function after ACL reconstruction, measuring joint laxity and evaluating limb function at 8 and 14 weeks postoperatively, respectively, and found that there was no significant difference in knee stability and limb function between the two groups of patients with closed and open-chain training, respectively, after surgery.Morrissey et al [ 26] supported the same idea in their study. Mikkelsen et al [27] made a randomized controlled study of postoperative CKC training alone and combined CKC and OKC training, and the results showed no significant difference in joint stability between the two groups at 6 months postoperatively, and they found that the peak quadriceps moment was significantly better in the combined OKC training group than in the CKC training group alone at 6 months, and most patients in this group regained their pre-injury level of motion in The time to return to pre-injury levels of motion was 2 months earlier in this group than in the CKC-only group. Although these studies have demonstrated the safety and effectiveness of early OKC training, further large sample and long-term clinical studies are needed before it can be recommended as a standard for clinical application. Even though early OKC training is not yet widely accepted, early training of the N cord muscle group is necessary and safe to promote functional recovery, because the stress on the ACL caused by the contraction of the flexor muscle group is very weak. The rehabilitation program after ACL reconstruction is still very controversial among surgeons. A scientific rehabilitation program should promote the recovery of limb function as soon as possible without exceeding the safety range of the graft. The mechanism of stress on the mechanical properties of the graft needs to be further explored, so there is still room for extensive clinical and basic research in this area.