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

The overall goal of this study is to evaluate the effect of a foot strengthening program (focused on the intrinsic foot muscles) on ankle and foot joint kinematics and kinetics (rearfoot, midfoot, and forefoot) in patients with symptomatic flexible flat feet. The investigators will also evaluate the effect on intrinsic muscle morphology, foot strength, pain and other symptoms. The investigators will compare to the standard conservative therapy, which is foot orthotic therapy.

A common condition is "symptomatic flexible flat foot". This clinical entity affects 1 in 5 adults and is characterized by misalignment of the foot joints, resulting in a flat medial longitudinal foot arch. As a result, the foot's load-bearing capacity is reduced, leading to overuse injuries such as back pain, patellofemoral pain, or medial tibial stress syndrome.

The literature shows that people with flexible flat feet have disturbed foot and ankle kinematics during locomotor tasks. However, the current literature lacks insight into the kinetics of the foot joints (i.e., forces that cause joint movement) during both relatively simple tasks such as walking and running and more challenging tasks such as landing from a step. The recent development of multisegmental kinetic foot models allows for detailed information to be obtained regarding (mal)adaptive joint loading patterns in this population and address shortcomings in the current literature.

In recent years, the small intrinsic foot muscles (IFM) have received more attention in the treatment of foot and ankle-related overuse injuries, such as plantar fasciitis, medial tibial stress syndrome, and Achilles tendinopathy. According to the "foot core" paradigm, the primary role of the intrinsic foot muscles is to support the medial foot arch. They contribute to the absorptive and generative capacity of the medial foot arch and work together with the plantar aponeurosis, ligaments, and extrinsic foot muscles. It is known that their strength and volume are reduced in people with flat feet.

A randomized controlled trial will be planned in which participants with symptomatic flexible flatfeet will be allocated to either receiving foot core strengthening exercises or custom made foot orthotics therapy. An asymptomatic group with flexible flatfeet will serve as a control group and will not undergo any intervention.

Primary objective 1: To evaluate changes in foot joint kinetics and kinematics and intrinsic foot muscle size following a foot core strengthening program in participants with symptomatic flexible flatfoot and whether this is different to the control therapy.

Secondary objective 1: To evaluate changes in foot posture, foot muscle strength, psychological scores, pain and symptoms following a foot core strengthening program and whether these changes are different in comparison to the control therapy.

Secondary objective 2: To evaluate changes in foot joint kinematics and kinetics and intrinsic foot muscle size between participants with asymptomatic and symptomatic flexible flatfoot.

Eligibility Criteria:
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)