Flatfoot causes abnormal biomechanics in the lower extremity, resulting in discomfort and excessive burden on lower extremity muscles during functional tasks, and it potentially leads to associated syndromes in the lower extremity. The aim of this study was to investigate how a demanding, repetitive task affects the muscle strength, activities, and fatigue of the lower extremities during function tasks. Nineteen individuals with flexible flatfoot (10M9F, age: 24.74 ± 2.68 years) and fifteen non-flatfoot participants (6M9F, age: 24.47 ± 3.74) took part in this study. All participants performed maximal voluntary isometric contraction and functional tasks, including walking and single-leg standing tests before and immediately after a 6-min fast-walking protocol. A surface electromyography system was used to collect muscle activation data. Our results showed that, after 6 min of fast walking, peroneus longus activity increased only in the non-flatfoot group, and gastrocnemius activity increased in the flexible flatfoot group. In the flexible flatfoot group, greater recruitment in abductor halluces and greater fatigue in the tibialis anterior was observed. Individuals with flexible flatfoot showed altered muscle activation pattern after 6-min fast walking. These findings can provide an evidence-based explanation of associated syndromes in flatfoot populations and lead to potential intervention strategies in the future.