Football is an increasingly popular exercise and common practice among adolescent and elite athletes. However, these athletes tend to have injuries involving the lower extremity and foot. Recent literature has proposed a promising concept of core stability on the arch of the foot and proximal hip control exercise for a knee injury. They have highlighted that foot core training begins with targeting the plantar intrinsic muscles through exercise intervention, which may enhance the capacity and control of the foot core system. Moreover, it is hypothesized that the 'complexity algorithm' of exercise intervention for proximal hip control may provide more sufficient effects on musculoskeletal pain in the lower extremity. However, some conflicting issues such as assessment and training of foot intrinsic muscle in functional positions still largely lack devices and research to elucidate the underlying mechanism of its development and integrated exercise interventions proximally and distally on these athletes.

Firstly, the investigators aim to design and develop of novel intrinsic foot muscle assessment and training device for sporting tasks and to examine the feasibility and reliability of muscle stiffness in foot and hip joints before and after exercise intervention using shear wave ultrasound elastography (SWUE) in athletes without and with foot and ankle overuse injuries; second, investigators will investigate whether immediate and persistent alteration after the integrated therapeutic exercise on motor control and muscle stiffness.

The foot often plays a crucial role in sensorimotor control and movement performance in standing, walking, and running. Anatomically, the intrinsic foot muscles (IFM) are the primary local stabilizer to provide static and dynamic stability in the foot, which are part of the active and neural subsystems to constitute the foot core system. The intrinsic foot muscles (IFMs) may play a key role in supporting foot arches (e.g. the medial longitudinal arch, MLA), providing flexibility, stability, shock absorption to the foot, and partially controlling foot pronation. Due to the difficulties in teaching and learning the plantar intrinsic foot muscle (IFM) exercise, the accuracy and follow-up after learning this exercise could be questioned following this exercise program; Physiologically, the effects of exercise intervention may be achieved following more than 4-week intensive exercise intervention at least. How to learn and activating this kind of exercise efficiently and effectively is key for employing this exercise in young athletes with and without flexible flat feet.

This project consists of two main parts - first, we aim to design and develop of novel intrinsic foot muscle strengthening device using 3-D printing techniques and to examine the feasibility and reliability of the morphology and neuromotor control features in intrinsic and extrinsic foot muscles before and after exercise intervention using Transcranial Magnetic Stimulation (TMS) and sonographic imaging in football /basketball athletes with without flexible flat feet (FFF); second, we will investigate whether immediate and persistent alterations in the morphology and motor control of IFMs and dynamic postural control after this therapeutic exercise with novel 3-D printing foot core exerciser. More importantly, we also elucidate important clinical evidence-based information on long-term novel therapeutic exercise intervention for coaches, clinicians, and health policymakers.

Inclusion Criteria:

Athletes with plantar heel pain:

1. Diagnosis of painful heel syndrome by clinical examination, with the following positive clinical signs:

   1. Pain in the morning or after sitting a long time
   2. Local pain where the fascia attaches to the heel
   3. Increasing pain with extended walking or standing for more than 15 minutes

2. Was associated with inflammatory symptoms (pain, swelling, etc.)

3. Pain in the area of the insertion of the plantar aponeurosis on the medial tubercle of the calcaneus.

4. Not perform ankle stretching exercises as treatment of the plantar fasciitis.

Healthy individuals:

1. A neutral foot alignment: determined by measurement of the resting calcaneal stance position (RCSP: between 2°of inversion and 2°of eversion) and scores on the navicular drop (ND: between 5 and 9 mm) test.
2. Foot Posture Index Score is between 0 and 5 .
3. No pain in the lower limbs
4. No history of lower limb injury or surgery that has affected function or caused the Individual to seek previous medical or therapeutic intervention.

Exclusion Criteria:

1. Traumatic injury to lower limbs which impacted joint integrity and function (i.e., fractures) resulting in at least 1 interrupted day of desired physical activity
2. History of spinal, pelvic or lower limb surgery
3. Major neurological, cardiorespiratory or circulatory disorders
4. Past history of traumatic head injury with or without loss of consciousness
5. Have been taking non-steroidal anti-inflammatory or corticosteroid medication in the past month
6. Recent intervention/management within the last 6 months

a single-blinded randomized controlled trial will be conducted, in which patients will be randomized into groups by sealed envelope method.