## 3 Rehabilitation Following NC-ACLs
### Mid-Stage  
The mid-stage of rehabilitation can begin only when all the objectives of the early stage have been achieved, including restoration of the joint ROM, swelling control, pain management, joint stability, and the athlete's psychological state [62]. The primary goal of mid-stage rehabilitation is to restore knee extensor strength, targeting strength within 20% of the contralateral limb [62]. Surgery reduces the neural responsiveness, strength, and volume of the quadriceps, making it crucial to restore its function during rehabilitation [132]. However, most patients fail to achieve this target within 6 months [132], and over 50% of athletes do not reach strength parity with the contralateral limb even after returning to sports [85; 132; 133]. Inadequate achievement of this goal compromises knee biomechanics, reduces functionality, and increases the risk of re-injury and progression to osteoarthritis [83; 86; 134; 135]. The predominant cause of this failure is AMI, which is triggered by injury and/or surgery at the neuromuscular level [86; 132; 136]. Various strategies to address AMI include pain and swelling management [137], BFR training [62; 138; 139], cryotherapy [89], NMES [95; 140; 141; 142], and active exercises [79; 136]. In the mid-stage, it is recommended to incorporate both open and closed kinetic chain exercises in addition to functional tasks [62]. Functional exercises improve coordination among muscle groups by simulating real neuromuscular tasks, reflecting how muscles function during athletic movements [143; 144; 145]. However, they do not prevent compensatory mechanisms wherein one muscle compensates for the weakness of another [62; 144; 145]. Conversely, open or closed kinetic chain exercises isolate individual muscles, enhancing their ability to generate force while minimizing these compensatory behaviors [146]. Given the controversy surrounding open kinetic chain exercises after ACL reconstruction [147], it is advisable to introduce them gradually with low loads and at safe flexion angles between 45\({}^{\circ}\) and 90\({}^{\circ}\)[148; 142]. Generally, these exercises should also be applied to the uninjured limb to preserve its strength and also to improve the injured limb itself, thereby ensuring an accurate limb symmetry index (LSI) [149; 150; 151]. This can be accomplished by performing the same exercises for the injured limb but at reduced volumes for the healthy limb [62]. The second goal is to restore the strength of the knee flexors, particularly in patients where the hamstrings have been used as a graft for ACL reconstruction [117; 118; 119; 120]. Strength deficits in the hamstrings are common and can persist for years post-surgery [112; 113; 114; 115; 116],and residual hamstring weakness is associated with an increased risk of re-injury [87]. This is because the hamstrings counteract anterior tibial translation and external rotation of the knee. The rehabilitation protocol for the hamstrings is similar to that for the knee extensor apparatus. However, if the hamstrings have been used as a graft, it is recommended to delay strengthening exercises for those muscles beyond 6 weeks to allow for tendon healing [102; 122; 123]. Following this period, isometric exercises should be introduced first, followed by a progression to open and closed kinetic chain exercises, as well as functional tasks [62]. By the end of the mid-stage, at least an 80% LSI should be achieved [62]. Simultaneously, when strengthening the knee flexor and extensor muscles, it is also important to reinforce the muscles that stabilize the joints proximal and distal joints of the knee [62]. These include ankle periarticular muscles, which generate a significant amount of force during running and jumping [152], and the triceps surae, which is a crucial stabilizer of anterior tibial translation, acting as an agonist to the ACL [153; 154; 155]. Similarly, the stabilizing muscles of the hip and pelvis, particularly the gluteus medius, can be weakened after surgery [126; 127].