## 3 Rehabilitation Following NC-ACLs
### Early Stage
[PERSON]'s study [78] supports aquatic therapy and emphasizes the role of low-impact conditioning exercises in reducing the stress on the knee while enabling continued muscle strengthening. This approach is particularly beneficial for athletes as they progress into sport-specific training since aquatic exercises can help maintain conditioning without compromising joint integrity, making it an effective option for sustaining gains made during earlier rehabilitation stages. Once pain and swelling are well controlled, muscle strengthening should begin. Following ACL reconstruction, quadriceps muscle atrophy and anthropogenic muscle inhibition (AMI) are common and should be promptly addressed [79]. Weakness in the knee extensor muscles is associated with altered biomechanics, instability, persistent pain, and a risk of early-onset osteoarthritis [80; 81; 82; 83]. The study by [PERSON] et al. [80] points out that managing pain and swelling is crucial for early mobilization and knee extension recovery. It emphasizes that strategies such as cryotherapy, elevation, and compression should be prioritized immediately post-surgery to set a solid foundation for later rehabilitation stages. This is especially important as extension deficits can lead to long-term biomechanical issues and affect gait patterns, potentially complicating further rehabilitation. Additionally, quadriceps weakness can impact long-term outcomes and the return to sport, with greater initial weakness leading to more difficulty in regaining strength post-surgery [84; 85; 86; 87]. In particular, [PERSON]'s study [82] suggests that quadriceps strength is not only essential for knee stability but also crucial for slowing osteoarthritis progression. This supports the view that rehabilitation programs should include targeted quadriceps strengthening from the early stages to minimize long-term joint deterioration and improve functional outcomes, particularly for athletes wishing to resume prior sports levels. The degree of muscle weakness depends on various factors, such as the type of surgery, the graft used, the severity of AMI, swelling, pain, and inflammation [81; 87; 88; 89]. Initially, recruiting the quadriceps may be challenging. This can be facilitated by using cryotherapy before training [89; 90; 91; 92], transcutaneous electrical nerve stimulation (TENS) [93; 94], neuromuscular electrical stimulation (NMES) [95], blood flow restriction (BFR) training [96; 97], and fatigue exercises [98; 99]. These methods should be combined with active training, starting with isometric exercises [63]. Once sufficient quadriceps recruitment is achieved, progression to open and closed kinetic chain exercises is possible. There are some concerns regarding open kinetic chain exercises as they have been thought to overload the new ligament [100; 101]. However, recent evidence suggests that this risk is minimal [100]. Nonetheless, it is recommended to start these exercises with a reduced range of motion (45-90 deg) and low load [102; 103; 104; 105; 106]. In the early stage, strengthening exercises should always be performed with a low load to the point of fatigue [63]. This is crucial to respect the biological healing timeline of the new ligament, which can take several months, progressing up to two years post-surgery [107; 108; 109]. The new ligament is particularly vulnerable between 6 and 12 weeks post-operation due to its remodeling process and may not tolerate overloading during this phase, posing a risk of re-injury [102; 103; 110; 111]. As with the extensor apparatus, hamstring weakness following ACL reconstruction is very common when using autograft, as well as hamstring strains, and can persist for years [112; 113; 114; 115; 116], with the failure to recover increasing the risk of re-injury [87]. Therefore, the recovery of hamstring strength and function should be initiated promptly.