## 2 Materials and Methods
### Experiment Procedures  
Both the augmented-reality-based training groups and the therapist-based training group participated in a program of 30 min per session, three sessions per week, for a period of four weeks. All the sessions were scheduled with the same interval of a one-day break. They were evaluated three times, before surgery, after surgery, and after treatment. Figure 1 shows the study flow diagram.  
Participants in the ART group received a 30 min exercise program. The program, including the training and measurements, was under the supervision and guidance of the therapist in charge or trained research assistant. The supervisors ensured that participants performed the exercises correctly and that no inappropriate movement susceptible to causing injury occurred. Additionally, they provided technical assistance when necessary to allow the participants to follow the exercise protocol. All the exercises of the first week after surgery were performed in the supine position and a pillow/towel was used to assist the participants while performing the exercises. The program consisted of the following exercises: (1) knee extension in a supine position with a high pillow/towel under the knee (quad set); (2) lifting a sandbag in a supine position with a pil  
Figure 1: Study flow diagram.  
low/towel under the knee (short arc quad); (3) knee flexion with a band in supine position; (4) knee flexion while pulling the heel in supine position (heel slides). All the exercises were performed ten times, two sets each, and with one and three minutes for rest, respectively, between each repetition and set. For the second week, the exercises were performed in the sitting and/or prone position and the participants were asked to perform the following exercises: (1) knee flexion in the prone position; (2) knee extension in the prone position with a towel under the knee; (3) knee extension in sitting position with high pillow/towel under the knee; (4) knee extension in sitting position. All the exercises were performed ten times, two sets each with one and three minutes for rest, respectively, between each repetition and set. During the third and fourth weeks, the exercises were performed in the standing position. It consisted of: (1) knee flexion while standing and leaning on a chair; (2) standing up to sit down while holding a chair; (3) standing up and lifting one leg forward while leaning on a chair; (4) stand up and lift one leg on the side while leaning on a chair; (5) one side knee flexion while holding a chair. All the exercises were performed within the range of motion of the joint, with two sets of 10 repetitions for each exercise. One minute of rest between each repetition and three minutes of rest between sets were provided. Figure 2 shows a participant during the AR-based training.  
Participants in the TBT group underwent a 30 min conventional training session. The conventional training program under the guidance of the therapist in charge was mainly set to increase the range of motion in the knee joint. The program consisted of a passive range of motion (PROM) performed by the therapist and 15 min of continuous passive motion (CPM) in each session.