Flexible flatfoot deformity, also known as flexible pes planus, is a common pathological condition characterized by decreased medial longitudinal arch height, eversion of the calcaneus and abduction of the forefoot, especially during loading. Conservative treatments are generally considered as the first step in treatment. In conservative treatments, exercise methods are one of the most frequently used interventions. It has been reported that exercise can improve functional outcomes such as navicular drop, plantar pressure distribution, foot posture, balance, and muscle strength in adults with flexible flatfoot. Although it is known that intrinsic muscle strengthening exercises are a frequently used intervention method in foot pathologies, there is a deficiency in the literature regarding studies investigating the effect of the combination of the most basic foot intrinsic muscle strengthening exercises with different exercise interventions on targeted clinical gains. However, the effect of three-dimensional foot-ankle exercise on flat feet is unknown. Additionally, there are a limited number of studies in the literature investigating the effects of hip abductor and external rotator muscle strength training on foot posture in individuals with flat feet. The aim of this thesis study, planned in the light of this information, is to determine the effects of three-dimensional foot-ankle extension exercises and hip abductor-external rotator muscle strengthening exercise primarily on navicular drop and MLA height in individuals with flexible flat feet; Secondly, it is to examine and compare the effects of static foot posture, hip abductor/external rotator, ankle circumference and intrinsic muscle strength on the functional performance of the lower extremity (endurance, dynamic balance and single leg forward jumping performance). To our knowledge, this study will be the first randomized controlled study to examine and compare the effects of two different exercise interventions that exercise the proximal and distal regions in flatfoot rehabilitation, and the results obtained will contribute to the development of better exercise and treatment programs in this population. In individuals with flat feet, when hip abductor/external rotator strengthening exercises or 3D foot-ankle extension exercises in the PNF pattern are added to short-foot exercise, MLA structure, muscle strength and lower extremity function will be improved more, and when these exercises are combined with short-foot exercise, they are comparable to intrinsic muscle strengthening exercises alone. The investigators hypothesized that it would produce superior clinical results.

This study was designed as an experimental, 3-arm, parallel group, single-blind randomized controlled study that included a 6-week intervention period. The study will be carried out in the university physiotherapy and exercise laboratory. Flat-footed individuals aged between 18-50 years will be included in this study. G-Power (Windows version 3.1.9.7) statistical program was used to determine the number of participants. Sample size calculation was made based on a similar study in the literature that evaluated the amount of navicular drop after foot intrinsic muscle strength training (the primary outcome measure of this study). Type 1 error probability 0.01; The power was accepted as 0.90 and the number of samples was determined as at least 15 people for each group. By adding the 20% loss rate to this number, it was decided that the required number of participants would be 20 people in total for each group. Participants who were confirmed to be included in the study were evaluated by the lead investigator, who was not involved in the evaluations, using Random Allocation Software (version 2.0) in a single block format, 3-dimensional foot-ankle exercise group (foot muscle exercise group, AEG), hip abductor and external rotator strengthening exercise. will be allocated to three different groups: group (hip muscles exercise group, KEG) and control group (CG). The investigator performing the assessments will be blinded to group allocation. Recording of groups' demographic information and assessment of outcome measures will be performed by a secondary investigative physical therapist who is blinded to group information. The principal investigator physiotherapist will be responsible for training the participants to perform the exercises assigned to them, communicating with the participants to monitor exercise performance and exercise compliance, allocating the groups and ensuring blinding. Outcome measurements will be repeated at the start of treatment and at the end of treatment.

Inclusion Criteria:

• Age range is 18-50 years
• Navicular drop amount is 10 mm or more
• Getting a value of +6 points or above from the Foot Posture Index

Exclusion Criteria:

• Orthopedic and rheumatological diseases of the spinal region and lower extremities
• Surgery history of the spine and lower extremities
• Neurological and systemic disorders
• Describing lower extremity and/or lower back pain in the last 6 months
• Disorders that may affect cognitive and balance (otitis media, visual impairment, etc.)
• Having received any physiotherapy and rehabilitation treatment for flat feet or hips in the last year
• Body mass index being above normal values (18-24.9 kg/m2)
• Having rigid flat feet and hallux rigidus-limitus deformity
• Doing regular sports or exercise

The evaluator researcher and participants will be blinded to group allocation.