## 4 Discussion
Consistent with the results of this study, applying systematic rehabilitation exercises to patients with an ACL injury may reduce knee joint build and muscle atrophy, improve muscle function and reduce muscle defect rates. Additionally, an increase in joint ROM appears to contribute to knee joint stabilization. In this way, early rehabilitation exercises can improve the knee joint function score, and it is thought that a greater effect will appear, especially when BFR is added. Improving muscle function and muscle activity after ACL surgery is important, and [PERSON] et al. [28] reported that BFR exercise for 16 weeks significantly improved knee extensor and flexor muscle power in ACL patients, and [PERSON] et al. [29] reported that BFR exercise increased knee extensor strength and endurance. In addition, [PERSON] et al. [27] reported that hamstring muscle strength was significantly increased through BFR rehabilitation exercise after ACL surgery for 12 weeks, showing results consistent with this study. Several studies have been reported on the improvement of muscle function in relation to BFR, but there is a lack of research related to muscle activity in ACL patients. [PERSON] et al. [30] found that a low-intensity electrical stimulation program resulted in significant improvements in VMO, VLO and RF, and [PERSON] et al. [31] compared muscle activation through an 8-week biofeedback exercise program, showing relative reports that there was a significant increase in active in VMO and VLO. VMO and RF were significantly improved in this study as well, and it is judged that the BFR exercise program is effective for muscle activity in relation to the improvement of isokinetic muscle function, as in previous studies. [PERSON] et al. [26] showed significant improvement in Y-balance performance as a result of combined treatment with BFR and resistance exercise for 8 weeks after ACL surgery, and [PERSON] et al. [32] reported that BFR and low-intensity resistance exercise for 3 weeks. As a result of the combined treatment, the Y-balance performance was significantly improved. In addition, [PERSON] and [PERSON] [33] reported that the Y-balance test significantly improved as a result of the combined treatment of physical therapy and BFR exercise for 4 weeks. In particular, more effective results were found in the group treated with BFR, and the results of this study also showed that the Y-balancing performance was better in the group treated with BFR than in the general rehabilitation group, showing consistent results. In order to bring about improvements in muscle function, metabolic stress induced through proper muscle mechanical tension and exercise, damage to the integrity of individual muscle fibers by destroying the skeletal muscle structure and induction of cellular responses through stimulation of the mTOR pathway are proposed [34]. In this way, given that the improvement of muscle function occurs with appropriate stimulation, we carefully predict that the stimulation of BFR exercise implemented in this study was an appropriate program that could show positive changes in muscle function and muscle activity improvement. A key challenge in rehabilitative exercise programs is to design optimal training programs that promote neural and muscle adaptations while keeping biological healing constraints and safety in mind. In BFR, muscle function improves, muscle hypertrophy occurs, the intramuscular environment changes to hypoxia and acidity, metabolites accumulate locally, growth inhibitory factors decrease to restore this and growth promoting factors increase, promoting muscle satellite cell proliferation [35]. In addition, m-TOR signaling pathway activity enhances protein synthesis initiation [36], increases the activity of many motor units, including fast-twitch fibers [37], and causes metastatic muscle hypertrophy [38]. This theoretical background supports the muscle strength-enhancing effect of BFR exercises.