## 4 Discussion  
The Kinect Azure system was compared to two well-known motion capture systems during single-leg squat tests. Kinect automatically detects the'spine base' anatomical point, while OptiTrack and Xsens monitor markers on the sacrum. At the lowest point of the squat, some participants bent forward, which resulted in a discrepancy among the \begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{**Participants**} & \multicolumn{2}{c}{**Right Knee Valgus at**} & \multicolumn{2}{c}{**Left Knee Valgus at**} & \multicolumn{2}{c}{**Right Knee Valgus at**} & \multicolumn{2}{c}{**Left Knee Valgus at**} \\  & **15\% Squat Depth (\%)** & **15\% Squat Depth (\%)** & **30\% Squat Depth (\%)** & **30\% Squat Depth (\%)** \\ \hline
1 & \(-\)0.28 & 1.31 & & \\
2 & 5.43 & \(-\)0.35 & 6.96 & 0.66 \\
3 & 4.85 & 2.30 & 9.31 & 6.32 \\
4 & 9.02 & \(-\)0.55 & 13.74 & \(-\)0.62 \\
5 & 3.15 & \(-\)0.10 & 7.44 & \(-\)0.14 \\
6 & 1.85 & \(-\)2.05 & 1.91 & \(-\)1.11 \\
7 & 2.12 & 0.24 & 2.83 & 1.50 \\
8 & \(-\)1.50 & 0.18 & 0.51 & 2.22 \\
9 & 3.37 & 1.11 & 2.88 & 5.07 \\
10 & 8.83 & 6.01 & & 6.41 \\
11 & 6.12 & 4.12 & 9.81 & 8.00 \\
12 & 4.77 & 3.11 & 6.57 & 4.04 \\
13 & 2.11 & 1.95 & 6.38 & 7.49 \\
14 & 6.05 & 1.55 & & \\
15 & 6.37 & 1.89 & & \\
16 & 6.32 & 4.90 & & 6.85 \\
17 & 3.85 & 1.52 & & \\
18 & 3.41 & 1.95 & & \\
19 & 2.98 & 0.99 & & \\
20 & 0.60 & & & \\
21 & 2.73 & \(-\)0.79 & 4.80 & 2.19 \\
22 & 1.51 & 0.06 & 2.21 & 1.75 \\ Average & & 2.63 & & 4.50 & \\ SD & & 2.63 & & 3.59 & \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Knee valgus values of subjects. Figure 4: Representative recordings of single-leg squats vs. squat depth in six subjects. Panels (**A**–**F**) show three male and three female subjects at exactly 15% squat depth; note that although the knee is only slightly bent, the valgus tendency is evident in those who are prone to this deviation (**B**,**C**,**F**). Panel (**G**) shows the valgus shift vs. squat depth plot of these individuals; letters on the curves correspond to the image shown. Squat depth is measured on the horizontal axis, and knee deviation on the vertical; the lines that are not continuous indicate that the subject didn’t reach the maximum squat depth at 30%. Vertical dotted lines show the 15% depth mark, where the valgus or stable tendencies can already be observed. Panel (**H**) shows the distribution of valgus shift at 15% and 30 squat depths in 44 knees. The line represents the median. Spine Base point of the Kinect and the Xsens and OptiTrack markers that were fixed on the sacrum. Therefore, the three systems were technically very accurate as compared to one another, and the observed numerical difference is the result of the non-simultaneous anatomical points that were followed by each system, i.e., dorsal sacrum versus spine base. A similar phenomenon was observed in the knee: the Xsens sensor followed the medial tibia and extrapolated the 'knee' point from these data. The OptiTrack monitored the lateral epicondyle of the femur. In contrast, the Kinect system used the contours and the depth dimension of the knee area and used artificial intelligence to calculate a knee midpoint, arguably providing a better approximation of the mid-pstellar point than the other two systems. Moreover, during the single-leg squat test, the calculated midpoint of knee may be a better choice than the lateral epicondyle of the femur. Nonetheless, although further refinements of the motion tracking systems may be possible in the future, the accuracy of the current Kinect Azure system is adequate for studying lateral knee and vertical spine base shifts during a single-leg squat. The major aim of this study was to establish a novel Kinect Azure-based method to evaluate the knee medial-lateral movement under a single-leg squat (SLS) test, in order to create a simple and reliable method for evaluating excessive knee valgus, a predisposing factor for ACL rupture and osteoarthritis [8; 11; 13; 15; 16; 32; 33].