## 4 Discussion  
This research examined the effects of tennis shoe outside patterns on the frictional force, lower extremity joint angle, and lower extremity moment during tennis-specific movements. To this end, four tennis shoes were manufactured with different medial and lateral tread widths on the outside. Furthermore, a mechanical test measured the frictional force of the tennis shoes, and a biomechanical analysis of forward and side braking motions was conducted for a biomechanical test. Mechanical tests can find significant differences in the frictional forces between the outside patterns by controlling all parameters, such as velocity, vertical load, and the location of the force, but cannot embody natural movements [9; 21]. It is a fact that the biomechanical test mimicking actual body movement is limited by difficulties in controlling all factors [21; 40]. Likewise, researchers have attempted to perform mechanical and biomechanical analyses simultaneously to supplement the two measuring methods [2; 41]. Therefore, this research carried out a mechanical and biomechanical analysis of natural movements. Differences between Mechanical Frictional Force and Frictional Force upon Braking Motion between Outsole Patterns  
As a result of the mechanical frictional force test, the maximum CoF showed statistically significant differences among all the tennis shoes. The rotational friction force showed statistically significant differences between the tennis shoes in a clockwise rotation; however, no statistical difference was revealed in a counterclockwise rotation. More specifically, the mechanical CoF depending on the forward motion was greater in the tennis shoes (A and B) consisting of a single pattern compared with the combined pattern (C, D), whereas the tennis shoes (B > A) with a thicker tread showed a high CoF. Depending on the side, the mechanical CoF was more significant in the tennis shoes (A and C) with thinner outer tread outsles than thicker outer tread outsoles (B and D). This finding seems consistent with previous studies asserting that differences in tennis shoes' frictional force exist depending on outside patterns [6; 18; 19; 42]. A difference of 0.6 mm in outside tread width can have a considerable effect based on the mechanical frictional force test. From this aspect, outside pattern and tread width are considered essential in professional sports footwear development. In the case of comparing two different tread widths of herringbone design by contrasting them to the outside's medial and lateral parts, there was difficulty in clearly comparing the size, direction, and pattern of frictional force. Further research on changing outside patterns according to systematic criteria is needed. In the mechanical frictional force test, differences in the frictional force were revealed between tennis shoes based on the direction of movements; however, frictional force in the forward and side braking motions did not show a statistical difference between tennis shoes. Generally, a frictional force is expressed as resistance against the horizontal motion on the vertical pressing (vertical load) between the ground and footwear outside. In the mechanical frictional force test, the vertical load mass, center of pressure (CoP) position,  
\begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline \multirow{2}{*}{**Maximum Moment**} & \multicolumn{4}{c}{**Mean \(\pm\) SD**} & \multirow{2}{*}{\(F\) (\(p\))} & \multirow{2}{*}{\(Post Hoc\)} & \multirow{2}{*}{\(Effect Size\)} & \multirow{2}{*}{\(Statistical Power\)} \\ \cline{3-4} \cline{6-7}  & & & & & & & & \\ \hline \multirow{8}{*}{Ankle} & Dorsiflexion & \multirow{2}{*}{\(0.01\pm 0.01\)} & \multirow{2}{*}{\(0.00\pm 0.01\)} & \multirow{2}{*}{\(0.01\pm 0.03\)} & \multirow{2}{*}{\(0.01\pm 0.01\)} & \multirow{2}{*}{\(1.81\) (\(0.21\))} & \multirow{2}{*}{\(0.153\)} & \multirow{2}{  
and horizontal speed were constantly controlled. However, those variables are dynamically changed during a tennis game in forward and side braking motions.