## 4 Discussion  
The purpose of this cross-sectional study was to compare functional outcomes measured as muscle mass, muscle power, and step test asymmetry, between patients undergoing MoM THA/HRA and MoP THA at mean 6.5 years follow-up. Our hypotheses were that MoM THA/HRA would demonstrate superior outcomes in all variables compared to MoP THA, due to the potential advantages of the large femoral heads, and the preoperative selection of younger and presumably more active patients. The first hypothesis was that muscle mass and muscle power was higher in patients undergoing MoM THA/HRA, compared to patients undergoing MoP THA. Our second hypothesis was that the inter-limb difference in muscle mass and muscle power was smaller in patients undergoing MoM THA/HRA, compared to patients undergoing MoP THA. Despite more muscle mass and power in both \begin{table}
\begin{tabular}{c c c c} \hline \hline
**IMU Parameters** & **MoM THA/HRA** & **MoP THA** & \(p\)**-Value a** \\ \hline Step test rotation asymmetry & & & \\ \hline Sagittal plane descending (\%) & 15.93 (14.86) & 19.24 (11.64) & 0.17 \\ Sagittal plane ascending (\%) & 19.98 (16.08) & 19.12 (14.74) & 0.95 \\ Frontal plane descending (\%) & 17.30 (13.37) & 14.68 (13.34) & 0.39 \\ Frontal plane ascending (\%) & 13.78 (11.29) & 15.24 (12.49) & 0.65 \\ \hline Step test time asymmetry & & & \\ \hline Ascending (\%) & 11.60 (8.53) & 20.60 (16.29) & 0.03 \\ Descending (\%) & 13.13 (10.61) & 18.81 (14.72) & 0.19 \\ \hline Step test acceleration asymmetry & & & \\ \hline Vertical plane ascending (\%) & 12.97 (11.38) & 16.64 (15.13) & 0.46 \\ Sagittal plane ascending (\%) & 17.29 (14.36) & 18.30 (20.39) & 0.56 \\ Frontal plane ascending (\%) & 18.07 (13.04) & 21.68 (12.91) & 0.32 \\ \hline \hline \end{tabular} \({}^{\text{a}}\) Two-sample Wilcoxon rank-sum (Mann–Whitney) test.  
\end{table} 
Table 4: Inter-limb difference in trunk Range of Motion in the sagittal and frontal plane measured from step test obtained in the MoM THA/HRA (\(n=45\)) and MoP THA (\(n=19\)) group, respectively. Values are mean (SD). legs in patients undergoing MoM THA/HRA, confirming the first hypothesis, the inter-limb differences in muscle mass and muscle power were generally similar between groups. The third hypothesis was that patients undergoing MoM THA/HRA experience less step test asymmetry compared to patients undergoing MoP THA. The step tests results only revealed better performance in ascending as asymmetry was less in the MoM THA/HRA group compared to the MoP THA group, whereas all the other parameters were similar. The hip muscle mass was similar in the prosthetic hip for both MoM THA/HRA and MoP THA, but the inter-limb difference in hip muscle mass was significantly higher in MoM THA/HRA, meaning that patients undergoing MoM THA/HRA generally had more muscle in the hip region compared with MoP THA. However, patients undergoing MoM THA/HRA in our study had significantly higher muscle power compared to MoP THA in both the implant side leg and the non-implant side leg. This finding was in accordance with DXA findings of more muscle mass in all areas in the MoM THA/HRA group compared to the MoP THA group. These findings have high consistency with previous literature that suggest general lower limb weakness due to reduction in muscle size (i.e., muscle atrophy) [41]. This was in line with our expectations, because the MoM THA/HRA group was generally younger and included more male patients than the MoP THA group, and an age- and sex-related reduction in muscle power has previously been reported [42; 43]. The sub analysis revealed that the inter-limb difference in mean muscle mass between MoM THA/HRA and MoP THA was not significant when comparing males to males or females to females. This might be due to type II error as a result of smaller samples. However, the significance of the other reported results had high consistency with the results already reported at overall MoM vs. MoP group levels in the results section.