This randomized controlled study will examine the acute effects of a medial arch support and a prefabricated insole on lower-limb muscle mechanical properties and pain in young adults with pes planus. Ninety participants will be allocated in a 1:1:1 ratio to a medial arch support group, an insole group, or a control group. Muscle mechanical and viscoelastic properties will be assessed with MyotonPRO, and pain will be assessed with a visual analog scale.