## 4 Discussion  
We investigated the effect of the HADF, HABF, and HNEF on changes in the CoP and quadriceps muscle activity. Our results are in line with those of previous studies, in that the quadriceps activity and the change in the CoP significantly increased for the HADF and HABF compared to the HNEF. Based on the position and sum distance results, the CoP of the HABF moved laterally in the frontal plane, that of the HADF moved radially in the frontal plane, and that in the frontal plane for the HNEF moved minimally to zero. An alternative explanation could be a biomechanical one; specifically, when the hip joint was adducted, the lower limb moved medially, and the resulting position was reflected medially. Similarly, when the hip joint was abducted, the lower limb moved laterally, and the resulting position was reflected laterally [30]. Because the forward lunge is a closed kinetic chain movement, the foot is immobilized when the hip is internally adducted. This causes the knee to move medially, creating a variable amount of pressure at the lateral aspect of the patellofemoral joint. At the same time, such a movement increases the risk of developing knee valgus [31,32]. Similarly to this is the danger of knee inversion that occurs when the hip is abducted. Compared to the HABF and the HADF, evidence implies that the HNEF is more beneficial for the patellofemoral joint. According to the results of assessments of the CoP, the HNEF showed a slight lateral movement in the coronal plane, which may be explained by the fact that the CoP was in the middle of the legs during the onset of the movement. After the forward lunge, the CoP shifted forward toward the flexed leg. Because all of our subjects were right-leg dominant, the CoP of the HNEF would have shown a slight lateral movement in the coronal plane. Compared to the other two exercises, the HNEF added little significant knee valgus pressure and thus was less likely to produce valgus and varus pressures. The results of the range and max distance assessments showed that the CoP range of displacement and the maximum distance of movement in the HADF and HABF were significantly larger than those of the HNEF on the medial and lateral sides of the frontal plane. The HADF and HABF may produce a noticeable medial and lateral expansion of the knee joint so that there is a greater distance compared to the HNEF [30]. At the same time, the range values were larger than the max distances. This suggests that the movement trajectory of the CoP does not change continuously in one direction. There was  
Figure 5: Comparison of muscle activity of VMO and VL between three positions: (**a**) muscle activity of VMO; (**b**) muscle activity of VL; (**c**) ratio of muscle activity of VMO/VL. * Significant difference between groups (_p_ < 0.05). \begin{table}
\begin{tabular}{l c c c c c c} \hline \hline  & **HADF 1** & **HNEF 2** & **HABF 3** & **F** & \(p\) & **ES 4** \\ \hline VMO (\%MVIC) [5] & 58.30 \(\pm\) 14.42 & 34.88 \(\pm\) 9.49 & 45.28 \(\pm\) 16.23 & 14.721 & 0.000 * & 1.71 \\ VL (\%MVIC) [6] & 40.98 \(\pm\) 12.64 & 33.79 \(\pm\) 11.57 & 48.44 \(\pm\) 16.19 & 5.791 & 0.005 * & 0.53 \\ Ratio [7] & 1.48 \(\pm\) 0.31 & 1.11 \(\pm\) 0.33 & 0.96 \(\pm\) 0.23 & 16.745 & 0.000 * & 1.27 \\ \hline \hline \end{tabular}  
* Hip adduction forward lunge. 2 Hip neutral forward lunge. 3 Hip abduction forward lunge. 4 Effect size. 5 Vastus midialis obliques. 6 Vastus lateralis. 7 Muscle activity of vastus midialis obliques/muscle activity of vastus lateralis.
* Significant difference between groups (_p_ < 0.05).  
\end{table} 
Table 3: Comparison of muscle activity of VMO and VL between three positions. a process of reverse movement for all three positions that was larger in the HNEF than in both the HADF and HABE. The smaller reverse movement in the latter two may have been due to wobbling of the ankle joint from the adaptation process in response to the increased stress at the beginning of the forward lunge [33].