## 3 Results  
The demographic data (Table 1) showed significant differences in body mass index (BMI) and navicular drop between non-flatfoot and flatfoot groups. BMI in flatfoot group was higher than in the non-flatfoot group. These results matched previous epidemiological study observations.  
After 6 min of fast walking, with the exception of the peroneus longus in the flatfoot group, MVIC of all the other tested muscles showed significant decrease, suggesting our 6-min fast-walking protocol required a significant amount of workload and caused muscle fatigue. Among all the tested muscles, MVIC of the tibialis anterior decreased the most, it dropped 32% and 35% for non-flatfoot and flatfoot groups, respectively (Figure 3).  
Figure 2: Body position during maximum voluntary isometric contraction (MVIC) test. (**a**) MVIC testing of the tibialis anterior. (**b**) MVIC testing of the gastrocnemius. (**c**) MVIC testing of the peroneus longus.  
The sEMG amplitude of both function tasks had significant differences within groups (pretest and posttest) but no significant differences between groups (non-flatfoot and flatfoot). Figure 4 showed the percentage of changes in sEMG amplitude between pretest and posttest during walking and single-leg standing tasks. The changes of muscle-activation level of the peroneus longus were significantly higher in the non-flatfoot group with an increase of 36% compared to the pretest (Figure 4a). For the flatfoot group, the changes of muscles-activation level of the gastrocnemius were higher with an increase of 62%. In the single-leg standing task, both non-flatfoot and flatfoot participants showed greater gastrocnemius muscle activation (44% and 48% for non-flatfoot and flatfoot) compared with pretest (Figure 4b). Interestingly, after 6 min of fast walking, the non-flatfoot group showed a tendency of greater abductor hallucis activation (43% increase), whereas the flatfoot group showed a tendency to decrease in abductor hallucis activation (21% decrease) in single-leg standing.  
The median frequency of the tibialis anterior in flatfoot group was significantly decreased (from 116 to 106 Hz) after 6 min of fast walking (\(p<0.001\)), further confirming that the tibialis anterior was likely most fatigued (Figure 5). Interestingly, the median frequency of the abductor hallucis in the flatfoot group increased (from 189 to 215 Hz) after 6 min of fast walking (\(p<0.001\)).  
\begin{table}
\begin{tabular}{c c c c} \hline \hline
**Variable** & \
\begin{tabular}{c} **Non-flatfoot** \\ **(Mean \(\pm\) SD)** \\ \end{tabular} &
\begin{tabular}{c} **Flatfoot** \\ **(Mean \(\pm\) SD)** \\ \end{tabular} & \(p\)-Value** \\ \hline Demographic measurements & & & \\ Age (year) & 24.47 \(\pm\) 3.74 & 24.74 \(\pm\) 2.68 & 0.81 \\ Height (m) & 1.68 \(\pm\) 0.07 & 1.69 \(\pm\) 0.08 & 0.61 \\ Weight (kg) & 58.87 \(\pm\) 9.78 & 66.5 \(\pm\) 12.89 & 0.07 \\ Body mass index (kg/m\({}^{2}\)) & 20.71 \(\pm\) 2.53 & 22.91 \(\pm\) 2.56 & 0.02 * \\ Navicular drop (cm) & 0.70 \(\pm\) 0.15 & 1.22 \(\pm\) 0.19 & \textless{}0.001 * \\ IPAQ & & & \\ Low activity & 1 & 1 & \\ Enough & 7 & 6 & \\ High activity & 7 & 12 & \\ Total (METs) & 3307.13 \(\pm\) 2434.97 & 4694.42 \(\pm\) 3735.24 & 0.22 \\ \hline \hline \end{tabular}  
* Significant differences between groups. IPAQ, International Physical Activity Questionnaires; METs, metabolic equivalents.  
\end{table}
Table 1: Baseline demographic data.  
Figure 3: Maximal muscle strength of (**a**) non-flatfoot and (**b**) flatfoot groups. * Significant differences between pretest and posttest. TA, tibialis anterior; GAS, gastrocnemius; PL, peroneus longus.