The Effect of Foot Core Exercises or Foot Orthotics in Symptomatic Flexible Flatfoot

Inclusion Criteria: For both asymptomatic and symptomatic flexible flatfoot: * Adults between 18 and 65 years of age, able to run at a self- selected low speed. * Physically active: min. 1h30 and max 6 h/week * Foot Posture Index \> 6 AND Navicular drop \> 5 mm * Informed consent (ICF) obtained For asymptomatic flexible flatfoot: cfr. supra For symptomatic flexible flatfoot: * Diagnosis made by a physician * ICF obtained * Symptomatic = * Foot pain ( \> 4 on Visual Analog Pain Scale score) OR * Medial arch pain OR * Metatarsalgia (general or diffuse hyperkeratosis accepted) OR * Lateral impingement pain OR * Sinus tarsi pain OR * Tibialis posterior dysfunction stage I and II (without indication of rupture) and painful single heel raise test Exclusion Criteria: For both groups: * \< 18 years, \> 65 years * Any medical contraindication to physical exertion * Systemic diseases * Recent lower limb surgery (\< 6 months) * Lower limb osteosynthesis material * Pregnancy * Pacemaker * Leg length discrepancy \> 3 cm * Body Mass Index (BMI) \> 30kg/m² * Constant ankle pain * Ankle fractures * Single hyperkeratotic lesion with nucleus under metatarsal head * Recent participation in a physical rehabilitation program (\< 3 months) * Recent orthotics (\< 3 months)