## 2 Materials and Methods
### MR Group  
Participants allocated to the MR group used a commercially available head-mounted Microsoft HoloLens 2 device (Microsoft Corporation, Redmond, Washington, DC, USA). The HoloLens was configured using the pseudo-weight algorithm developed by the Data Reality Corporation (Seoul, Republic of Korea) and a rehabilitation exercise program designed in the Department of Physical Therapy of Sun Moon University. The pseudo-weight training consisted of participants training using a virtual weight (dumbbell), lifting it as if it was physically real. The force needed to lift the dumbbell was the same as the force required for the real dumbbell, including the velocity and direction of the movement. Details of the pseudo-weight algorithm and its integration are published elsewhere [17]. The MR environment created through the HoloLens made it possible to present digital objects such as dumbbells and trainer avatars as if they were in the real world, mixing them with the surrounding environment. Moreover, the upper body of each participant was replaced by an avatar, allowing them to be present at the virtual training center event (they were physically in a laboratory room). The upper-body avatar was colored green when the exercise was correctly performed and turned red when incorrect movement was executed (e.g., flexion instead of abduction). The exercises were performed in this designed MR environment. Before performing the exercises, participants had 15 min to practice using the MR Glass while four (4) inertial measurement unit (IMU) sensors were attached to their upper limb joints (2 IMU sensors at 5 cm above the elbow and 2 others at the wrist). The IMU sensors were used to provide participants with feedback on the ongoing exercises (acceleration and deceleration of the movement) to ensure the correct performance of the movements (Figure 1).  
For this study, participants were asked to perform three different exercises (10 repetitions \(\times\) 3 sets for each exercise), always in the same order: (1) shoulder scap (standing position), (2) shoulder press (sitting position), and (3) shoulder external rotation (standing position) with 10 kg (1 RM) for males and 8 kg (1 RM) for females. The weight of 10 kg for males was chosen based on previous studies that used 10 kg as a weight to fully stimulate the muscle [22]. The 8 kg weight for females was adapted to our participants since they were unable to lift the 10 kg dumbbell for 10 repetitions. The selected exercises were designed to target the deltoids, the upper trapezius, the infraspinatus, and the supraspinatus muscles, as they involved a combination of shoulder flexion, abduction, and external rotation. Additionally, the chosen exercises are among the exercises recommended by [PERSON] et al. [23] for shoulder rehabilitation programs. Participants were instructed to perform the exercises by following the instructions given by a trainer avatar with a pre-recorded audio voice. Participants were asked to wear the HoloLens during exercise performance as well as during the electromyography (EMG) measurement. However, they were instructed to remove it during the resting periods. After completing the MR Glass exercise program, participants were asked to fill out a customized survey to evaluate their subjective feelings about using the MR Glass device. The survey consisted of ten questions with a simple \"yes or no\" answer. Figure 2 shows the screen of the MR-based pseudo-weight system and Figure 3 the exercise performed by the HoloLens device.