# Reliability and Validity of the Orthotic Deformation Test
## 1 Introduction  
The windlass mechanism is essential in the development of bipedal gait [1]. This phenomenon begins when passive toes, specifically the hallux, approximate close to the foot dorsum during push off in the gait cycle. Consequently, chaining of the movements of the foot and lower limbs, based on the engineering concept of lifting weights via a pulley system, occurs naturally in the foot. The foot transforms itself into a more rigid structure to enable efficient and stable propulsion during walking and running [2]. The plantar aponeousrosis acts like a cable winding to the drum of a windlass (the first metatrasophalangeal joint), which is pulled by a handle (the proximal phalanx of the hallux). Therefore, plantar aponeurosis stretching plays an essential role in the establishment of the windlass mechanism [3]. However, some musculoskeletal disorders such as plantar fascitits are related to repetitive cycles of stretching and relaxing of the plantar aponeurosis [4]. Prefabricated and custom foot orthoses or insoles are devices commonly prescribed for the treatment of biomechanical pathologies related to excessive stress of plantar tissues [5-7]. Several studies have focused on determining the effectiveness of orthotic interventions [8]. Questionnaires or plantar pressure measurements are commonly used [9; 10]. However, these tools are time consuming, and devices to measure plantar pressure are costly and not often available. Wear and tear of the materials in orthoses also results in reduced effectiveness of these devices over time. A simple and rapid test to evaluate device effectiveness could reduce clinical examinations, which are associated with both monetary and time costs. The origin of biomechanical diseases is related to increasing foot plantar pressure during loading [11; 12]; the plantar pressure distribution depends on anatomical and physiological characteristics of the foot, as well as walking speed [13]. Commonly employed strategies for decreasing plantar pressure include changing the shape and material of insoles [14; 15]. It has been shown that addition of some elements (Figure 1A) at the orthoses, such as arch support, has a slight impact on plantar pressure [16]. However, custom foot orthoses fit the foot better than those that are prefabricated, and they have been shown to be more effective at relieving the pain of plantar fascitis over the medium term [17; 18]. Therefore, it is important to ensure geometric surface congruency between the foot arch and dorsal arch support to reduce pressure on the plantar foot [18; 19]. The foot in contact with the orthoses produces, attenuates, or re-orientates the motion of other non-contact joints of the lower extremity such the hips or knees [20]. Custom foot orthoses are composed of different elements. The medial longitudinal arch is molded to the foot shape and shifts plantar pressure from the rearfoot and forefoot toward the midfoot [21]; orthoses consequently reduce plantar fascia strain [6]. The medial longitudinal arch should resist arch depression by increasing contact and providing support. The addition of some elements, such as forefoot-rearfoot posts (Figure 1B), has been shown to increase medial longitudinal arch stiffness [22]. Therefore, arch stiffness has been shown to be the primary characteristic responsible for altering lower-limb biomechanics. Furthermore,  
Figure 1: Flat insoles and custom foot orthoses Panel (**A**) (**top**): Upper side of a flat insole with additional arch support. Panel (**B**) (**bottom**): Bottom side of custom orthoses with additional forefoot-rearfoot posts. polyppropylene is commonly used to manufacture foot orthoses due to its stiffness and durability in comparison with other materials [23; 24; 25]. Arch stiffness of polypropylene orthoses is commonly evaluated by podiatrists in clinical practice [26]. The correct stiffness is considered a sign of proper functioning of the orthosis [27].