## 1 Introduction  
The foot is constituted of 26 bones connected by multiple joints, ligaments, and tendons. Foot problems may cause inflammation and pain, which do have a negative impact on quality of life [1]. Flatfoot, also termed pes planus, is a postural deformity in which the arches of the foot collapse. Common symptoms include attenuation, insufficiency, or rupture of the supporting structures of the medial longitudinal arch (MLA) [2]. Statistics show that the prevalence of flexible flatfoot was 13.6% in adulthood [3], and surgery might be required for severe cases [4].  
The disorder gives rise to abnormal biomechanics, including hindfoot valgus, forefoot abduction, abnormal internal rotation of the talus and calcaneus, and increased internal rotation of the tibia [2, 5, 6]. Therefore, individuals with flatfoot require more muscle action than non-flatfooted people to support and propel the weight of their bodies due to instability in foot structure [7]. During walking, changes in muscle activation occur in different phases and different muscles; this includes a higher percentage of peak surface electromyography (sEMG) amplitude by the tibialis anterior in the contact phase and the tibialis posterior during propulsion, but a lower percentage on the peroneus longus throughout the stance phase [8]. Individuals with flatfoot also show higher tibialis anterior activity and lower activity in the peronei, soleus, and gastrocnemius in early stance relative to the mean stance phase [9].  
Less than 1% of individuals with flexible flatfoot are symptomatic [10]. A flattened arch results in overpronation, leading to changes in foot structure. Therefore, problems may extend to the proximal joint. Some studies have revealed that a higher prevalence of anterior knee pain, intermittent lower back pain, medial tibial stress syndrome, and knee cartilage damage is associated with having flat feet [11, 12, 13]. The abnormal biomechanics of flatfoot causes ankle instability, which can, in turn, lead to knee, hip, or even trunk problems.  
A previous electromyographic study revealed that the activity of the abductor hallucis decreased corresponded with an increase in navicular drop, signifying that this muscle plays an important role in supporting the MLA [14]. Another study involved specifically causing fatigue in this muscle; that study revealed increases in navicular drop that resulted in foot pronation [15]. Muscle fatigue increasing the amount of biomechanical changes of flatfoot might cause associated syndromes and other injuries.  
Most previous studies have compared biomechanical differences between non-flatfoot and flatfoot [5, 9, 16], but few studies have focused on the contribution of muscle overuse to biomechanical changes related to flatfoot. Although some fatigue protocols have been used in studies as interventions on both non-flatfooted and flatfooted individuals, those protocols do not typically represent daily activities.  
Pathological mechanisms of flatfoot-associated syndromes remain unclear, as do the alterations of lower-extremity biomechanics for performing functional tasks. In order to understand the mechanisms of fatigue on lower-limb muscles, the purpose of our study was to assess changes in tibialis anterior, gastrocnemius, peroneus longus, and abductor hallucis muscle strength, activities, and the phenomenon of fatigue during functional tasks before and immediately after a 6-min fast-walk protocol in non-flatfoot individuals versus those with flatfoot. Our hypotheses were that the protocol would decrease muscle strength, increase muscle activation, and decrease sEMG median frequency after a 6-min fast-walk protocol.