## 4 Discussion  
This study aimed to assess the changes in lower-limb muscle activities during functional tasks before and immediately after a 6-min fast-walk protocol in individuals with flatfoot and non-flatfoot. The key finding of this study is that, in the flatfoot group, sEMG median frequency decreased for the tibialis anterior and increased for the abductor hallucis after a 6-min fast-walk task, suggesting fatigue in the tibialis anterior and recruitment in the abductor hallucis after the protocol. Both groups showed increased gastrocnemius activity during the single-leg standing test. In the walking test, peroneus longus activation increased in the non-flatfoot group, whereas gastrocnemius activity increased in the flatfoot group. Previous studies measured the biomechanical parameters in individuals of flatfoot under static or dynamic conditions without fatigue protocol as an intervention [7, 9, 19]. To our knowledge, this is the first study to use such a demanding functional protocol in an attempt to emulate the high-loading conditions of daily life; however, some studies assessed the effect of foot intrinsic muscle fatigue by different protocols [15, 20]. In this study, we used a 6-min fast-walk protocol to investigate changes in muscle activation of individuals with flatfoot and compared them with those of non-flatfoot adults. Following the protocol, both groups showed considerably decreased muscle strength and significant decreases in MVIC, demonstrating that this high-load protocol was sufficiently challenging for both groups. By comparing muscle activation during functional tasks between individuals with and without flatfoot, this study demonstrated that demanding repetitive motor tasks could  
Figure 4: Changes of muscle-activation level during (**a**) walking and (**b**) single-leg standing for non-flatfoot and flatfoot groups. * Significant differences between pretest and posttest. TA, tibialis anterior; GAS, gastrocnemius; PL, peroneus longus; AH, abductor hallucis. Figure 5: Median frequency for (**a**) non-flatfoot and (**b**) flatfoot groups. * Significant differences between pretest and posttest. TA, tibialis anterior; GAS, gastrocnemius; PL, peroneus longus; AH, abductor hallucis. increase muscle recruitment and alter muscle-activation level to compensate and provide support to the MLA in individuals with flatfoot. We observed that, after the fast-walk protocol, the tibialis anterior is fatigued (decreased EMG median frequency), and its strength is decreased (MVIC). As an invertor, the tibialis anterior contracts to prevent version and provide joint stability during walking. Loss of arch support and greater extrinsic muscle activation for postural control occurred in flatfoot individuals. Hence, individuals with flatfoot required greater gastrocnemius activation for generating propulsion force during walking. As part of the triceps surae complex, gastrocnemius with higher activation during walking could result in powerful promotion force during the heel-rise phase, which increases tension in the Achilles tendon by 30% and causes excessive tension in the triceps surae [19; 21], as well as causing early heel rise during walking [7]. In addition, we also found that the flatfoot group had increased peroneus longus activation after 6 min of fast walking. Altered triceps surae activation [22], early heel rise during walking [23], and unbalanced ankle inversion/eversion strength [24] have been associated with medial tibial stress syndrome. Our results indicated that a demanding repetitive walking task could cause altered lower-extremity muscle activation in individuals with flatfoot, and this could lead to the development of associated syndromes, such as medial tibial stress syndrome. Although the AH muscle did not show significant changes in EMG amplitude (activation level) during walking or single leg stance after the 6-min fast-walking protocol, the AH muscle did show an increase in EMG median frequency.