## 4 Experiments and Analysis
### Sensing Location Determination of the IMU Sensor  
As it is an IMU sensor, the MPU6050 (TDK Electronics, TDK Worldwide, Tokyo, Japan) [38; 39] can read gravity acceleration values along x, y, and z-axes, as well as three-axis gyroscope values. Figure 7a shows the three-dimensional axes and rotation of MPU6050. As shown in Figure 7b, the MPU6050 sensor is cost-effective, very lightweight, and small, making it a great fit for a wearable sensor in daily life. Among the sensing values provided by MPU6050--using the x, y, and z acceleration values--the current forward tilted angle (\(\theta\)) of the sensor can be obtained through Equation (1), as shown in Figure 7c [40]. In Equation (1), x, y, and z are the gravity acceleration values along each of the axes of the MPU6050. \[forward\ tilted\ angle\ (\theta)=arctan(\frac{x}{\sqrt{y^{2}+z^{2}}}). \tag{1}\]  
To verify the accuracy of the sensor, we tilted it close to certain angles (0\({}^{\circ}\), 30\({}^{\circ}\), 45\({}^{\circ}\), 60\({}^{\circ}\), and 90\({}^{\circ}\)), as shown in Figure 8, and we checked if the forward tilt angle (\(\theta\)) obtained from the sensor was similar to the angle of the sensor that is measured when taking a photo from the side. As shown in Table 1, the small difference (<0.5\({}^{\circ}\)) between the forward tilted angle (\(\theta\)) and actual angle sensing data can be useful for our approach. Figure 8: MPU6050 sensor accuracy measurement. Figure 7: MPU6050 (inertial measurement unit (IMU) sensor). (**a**) Three axes. (**b**) MPU6050 sensor. (**c**) Relation between gravity acceleration and the forward tilt angle (\(\theta\)). First, the IMU sensor was attached to several parts of the upper body (as shown in Figure 9), while the waist and neck were kept straight, which is the standard for good posture (as shown in Figure 1b). Subsequently, the participant was instructed to gradually extend their neck forward to reach FHP. The CVA of the standard postures (standard CVA) and FHP states differed by up to 20\({}^{\circ}\), wherein the optimal position was determined by testing various attachment positions to verify if the forward tilt angle (\(\theta\)) could be measured to represent the CVA value. All 10 male participants were asked to adopt the same posture in the same experimental environment, and their CVAs were measured by taking a photo from the side. The information regarding the 10 male participants is shown in Table 2. In accordance with the guidelines detailed in the Helsinki Declaration [41], the experiment was conducted after obtaining the written consent of all participants. All participants were normal people with no history of FHP, and their use time of cellphones and computers is shown in Table 2. \(\theta\). The CVA values shown in Table 3 are the average values obtained from adopting different sensor attachment positions for the 10 participants, and the mean and standard deviations of these values are also provided. \begin{table}
\begin{tabular}{l l l l l l} \hline  & **Figure 8a** & **Figure 8b** & **Figure 8c** & **Figure 8d** & **Figure 8e** \\ \hline Forward tilted angle (\(\theta\)) & 87.8\({}^{\circ}\) & 60.2\({}^{\circ}\) & 44.3\({}^{\circ}\) & 28.3\({}^{\circ}\) & 3.0\({}^{\circ}\) \\ Angle measured from the side & 87.2\({}^{\circ}\) & 60.6\({}^{\circ}\) & 44.7\({}^{\circ}\) & 28.5\({}^{\circ}\) & 3.1\({}^{\circ}\) \\ \hline \end{tabular}
\end{table} 
Table 1: Comparison of the forward tilted angle (\(\theta\)) with the actual angle. Figure 9: IMU sensor location: [1] forehead, [2] chest, [3] back of neck, and [4] cervical spine. When sensors are placed at the [3] back of the neck and [4] cervical spine, the difference in \(\theta\) between the FHP and the standard posture is \(\sim\)20\({}^{\circ}\), which is approximately the same as the difference in CVA between them. However, when the sensor is placed on the [2] chest, the difference in \(\theta\) is only 10\({}^{\circ}\), while the CVA changes by 20\({}^{\circ}\).