2.2.2. Experimental Setup and Procedure

The Tai Chi exergame was developed with Kinect V2 sensor in Unity3D platform (Unity 5.5.2f1) by C# for the real-world application. The main scene of the exergame is implemented with two avatars: virtual trainer and trainee (Figure 3). The avatar-based rendering of motion preserves the privacy of the user, which is critical for the healthcare systems. The motion of the trainee avatar is updated by the real motion of the user. User's motion is captured by Kinect V2 (Microsoft Corp, Redmond, WA, USA) and mapped to the trainee avatar using the Kinect V2 asset for Unity3D [36]. The motion of the trainer avatar is retargeted by the pre-recorded standard motion from a certified Tai Chi instructor (Master Level).

The whole experimental setting is illustrated in Figure 4. Both Kinect V2 and Xsens motion capture system were used to capture each subject's motion. The Kinect sensor was placed at a height of 0.8 m. Since subjects should stretch their arms frequently because of the characteristics of Tai Chi motion, subjects were instructed to stand around 3.0–3.5 m away from the Kinect sensor. Considering that some Tai Chi motion with body rotation may not be well captured by the Kinect due to the self-occlusion [37], a wearable inertial sensor-based motion capture system-Xsens MVN BIOMECH (Xsens Technologies B.V., Enschede, The Netherlands) was also utilized to obtain high-quality motion data for the algorithm validation [38,39].

**Figure 3.** Main scene of the Tai Chi exergame (left: trainer avatar, right: trainee avatar).

**Figure 4.** Experimental setup for Tai Chi motion evaluation.

A smart phone, supported by a tripod, was used to record each subject's motion when he/she was playing the Tai Chi exergame. The motion videos were distributed to three Tai Chi experts for independent performance evaluation. Each expert was provided a 10-cm visual analog scale (VAS) [40] and asked to place a vertical mark on the scale to indicate the performance level of motion for each subject. The anchor statements for VAS in this study are "cannot follow Tai Chi at all" (score of 0) on the left and "master level with standard Tai Chi motion" (score of 100) on the right. The raw scale score is then converted to a 0–100 scale.
