## 1 Introduction  
Leisure activities are increasing among modern people, and they are trying to experience various cultures. Among them, participation in sports is increasing not only for physical health but also for adventure sports that exceed one's limits [1,2]. In terms of public health, an increase in the population participating in sports is a very important and necessary activity for health, but sports injuries may also occur due to excessive competition, incorrect skills, and immaturity of skill. As such, sports injuries, which were previously limited to athletes, occur frequently in the public due to the increase in sports participation [3].  
Sports injuries are typically brought on by musculoskeletal conditions that affect the bones, muscles, ligaments, tendons and nerves [4]. These conditions have a variety of side effects, including decreased muscle strength and endurance, muscle atrophy, joint degeneration, a reduction in cardiopulmonary endurance, harmed proprioception and confidence loss. As a result, it is becoming more crucial to prevent, treat and recover from sports injuries [5; 6].  
The knee joint is the most active joint in sports, and injuries frequently occur during exercise [7]. In particular, the anterior cruciate ligament (ACL) is the most important structure of the knee joint and is responsible for maintaining the stability of the knee joint [8]. Injuries to the ACL are most commonly caused by internal rotational forces acting on the tibia relative to the femur, but they are frequently caused by extrinsic mechanisms and intrinsic mechanisms [9; 10].  
After surgical treatment for an ACL injury, the joint range of motion (ROM) and extension and flexion movements are limited while the injured area is fixed for a long period of time, resulting in weakening of the quadriceps and hamstrings. In this way, after ACL reconstruction, it is important to restore the function of the previous level of competitiveness through a rehabilitation exercise program so that the patient can safely return to daily life and sports as soon as possible [11; 12; 13].  
The rehabilitation exercise program for an ACL injury progresses step by step, including ROM exercise, weight-bearing, isokinetic muscle exercise, proprioception exercise and functional exercise, in parallel with physical therapy [10; 12; 14]. However, weakened quadriceps and muscle function may increase the risk of re-injury and arthritis when using high-intensity loads or performing strenuous exercises for rapid recovery. Therefore, there is a need for effective treatments, even at low intensities.  
Recently, blood flow restriction (BFR) training developed by Dr. [PERSON] has been recommended, and BFR limits blood flow to muscles with the Kaatsu Master device, inducing a hypoxic environment in muscles and resulting in effective muscle hypertrophy and muscle strength improvement [15; 16]. Previous research found that combining BFR and resistance exercise could improve muscle function and hypertrophy while having the same effect as high-intensity resistance exercise treatment despite low intensity [17].  
Most of the exercise programs related to BFR are being conducted for athletes, the elderly or the general public, and there is a lack of studies that have combined BFR treatment with the rehabilitation exercise program after an ACL reconstruction. Therefore, the purpose of this study was to examine the effect on the improvement in quadriceps muscle function, muscle hypertrophy and functional ability of the knee through complex exercise using BFR added to the existing rehabilitation process after ACL reconstruction of the knee.