# Estimation of Stride Length, Foot Clearance, and Foot Progression Angle Using UWB Sensors
## 4 Discussion  
Table 3 compares the SL, FC, and FPA data obtained in this study (expressed as KIST) with data acquired in previous studies that used an IMU. The SL estimation error was 45.84 mm in this study, compared to 54 mm [43] and (\(-\)) 27 mm [44] in previous studies. However, two previous studies applied IMU sensors to the shank for gait experiments. Therefore, the measurement error for the SL value acquired from this research was considered to be in a range similar to that of the previous research that used an IMU. \begin{table}
\begin{tabular}{c c c} \hline
**Gait Parameter** & **Researcher** & **Difference of Motion Capture** \\ \hline \multirow{3}{*}{Stride length} & KIST & 45.84 mm \\  & [PERSON] et al. [43] & 54 mm \\  & [PERSON] et al. [44] & \(-\)27 mm \\ \hline \multirow{3}{*}{Foot clearance} & KIST & 7.60 mm \\  & [PERSON] et al. [45] & 4.2 mm (Heel)/13.1 mm (Toe) \\  & [PERSON] et al. [46] & 7.0 mm/2.7 mm (Toe) \\ \hline \multirow{3}{*}{Foot progression angle} & KIST & 2.82\({}^{\circ}\) \\  & [PERSON] et al. [31] & 2.6\({}^{\circ}\) \\ \cline{1-1}  & [PERSON] et al. [29] & 2.5\({}^{\circ}\) \\ \hline \end{tabular}
\end{table} 
Table 3: Mean gait parameter difference of motion capture compared with other sensors. \begin{table}
\begin{tabular}{c c c c c c c c c c} \hline \multirow{2}{*}{**Subject**} & \multicolumn{3}{c}{**Stride Length (mm)**} & \multicolumn{3}{c}{**Foot Clearance (mm)**} & \multicolumn{3}{c}{**Foot Progression Angle (\({}^{\circ}\))**} \\ \cline{2-10}  & **2.0 km/h** & **3. FC estimation error was 7.60 mm in this study, compared to 4.2 mm/2.7 mm [45], and 8.6 mm [46] in previous studies. In a study by [PERSON] et al. [45], the FS estimation error varied according to the attachment location of the IMU. The heel clearance and toe clearance measurement errors were 7.0 mm and 2.7 mm, respectively, when the IMU was located at the toes, and 4.2 mm and 13.1 mm, respectively, when located at the heel. This phenomenon was similar to that observed by [PERSON] et al. [46]. These results were attributed to the identical foot experiencing different accelerations according to the part within the three-dimensional movement of the foot. Therefore, the detected foot trajectory was considered to differ according to the installation location of the IMU on the foot. This was also a significant error factor when tracking the foot trajectory using the IMU. In addition, the FC measurement error in this study was within a range similar to that used in previous studies that used an IMU. The FPA estimation error was 2.82\({}^{\circ}\) in this study, compared to 2.6\({}^{\circ}\)[31] and 2.5\({}^{\circ}\)[29] in previous studies. In a study by [PERSON] et al. [31], the FPA measurement error was observed to increase to 5.22\({}^{\circ}\) when the sensor location was misarranged during the positioning of the coordinate system of the IMU attached to the foot. In summary, the error that occurred during the process of applying ZUPT and arranging the sensor coordinate system in the gait experiment using the IMU sensors was considered to have a significant impact. The measurement error of the FPA acquired from this study differed from the results of previous studies, which used an IMU of approximately 0.22\({}^{\circ}\)-0.32\({}^{\circ}\). Overall, the SL, FC, and FPA values estimated using UWB were similar to those estimated using IMU or displayed marginal differences. There are two likely sources of error in the methodology with UWB sensors used in this research:  
First, in the case of offset removal using SVR, the signal strength information is used to indirectly assess the variation in the relative angle of the anchor and tag attached to different feet. In this case, the relative angle and signal strength of the antenna do not display a linear relationship [40], and the signal strength can vary from the external impact of gadgets, such as Bluetooth devices.