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Title: Three-dimensional insole design for relieving plantar pressures in high-heeled shoes
Authors: Wan, Ka Wing Frances
Degree: Ph.D.
Issue Date: 2018
Abstract: High-heeled shoes (HHS) have long become part of the female attire in different contexts. Wearing HHSs, however, damages foot health in the long run. HHSs keep wearers' feet in plantar-flexed positions, change the morphology and weight bearing conditions of the feet, causing high forefoot plantar pressure and impact force during gait. These excessive plantar pressures over the forefoot regions could lead to foot problems such as forefoot pain, corns and calluses and deformities. The primary goal of this research was to design, with reference to experimental and numerical analysis, an insole with different mechanical stress-strain properties in different foot regions that could lower peak plantar pressure whilst provide adequate foot support to maintain foot stability. To achieve the project goal, a 3D foot and ankle imaging system was developed to study female foot anthropometry in heel-elevated postures. The high resolution scans acquired by the system allowed fine details to be recorded. The short capture time (within 1 second) minimized measurement errors introduced by unintentional body sways. After validating the accuracy and reliability of the system, foot anthropometric data of fifty young female subjects were collected by using the system. To the best of our knowledge, this foot anthropometric study was the first to focus on forefoot measurements while standing at different heel heights. The geometric characteristics of the forefoot, as illustrated by the various linear and angular measurements, were shown to change with different heel elevations. The most obvious observation is a wider toe spread in the horizontal plane, indicating that slight adjustments might be required in the toe box design when fabricating shoes of different heel heights, especially in the metatarsal-phalangeal joint regions of the fourth and fifth toes and the distal interphalangeal region of the hallux and second toes. The anthropometric data depicted how the foot morphology changes with increasing heel elevations and provided a scientific basis for the geometric design parameters of the new insole. To identify specific high plantar pressure regions and determine the functional requirements of the insole for different foot plantar regions, a thorough evaluation on plantar pressure distribution, foot stability, and activities of selected lower extremity muscles during the use of HHSs was carried out. Twenty young female subjects participated in the study. Four heel height conditions (1cm, 5cm, 8 cm and 10 cm) covering the most common heel range were tested. Muscles activities of rectus femoris (RF), vastus lateralis (VL), vastus medialis (VM), peroneous longus (PL), tibialis anterior (TA) and medial gastrocnemius (MG) were found to increase with heel height in both balanced stance and single-leg stance, suggesting that extra efforts were required to maintain body balance in HHS. From the pressure measurements taken from the PedarR-X insole measuring system, the plantar pressure distribution during quiet standing and normal walking were found to change significantly in HHS in comparison to flat-heeled shoes. During quiet standing, the toes and metatarsal region was found to bear nearly 60% of the body weight at 10 cm heels, which was almost a double when compared to the flat-heeled condition. The peak and mean plantar pressures in the medial and centre metatarsal regions were found to increase significantly (>100% increase) in high-heeled conditions. When walking in 10 cm high-heeled shoes, the pressure contact area decreased significantly in the mid and rear-foot. These findings suggested that highly pressure relieving materials are necessary to cushion the forefoot area, while an arch cushion and a heel cup could help increasing the contact area in the rearfoot and thus maximize the pressure redistribution ability of the new insoles.
Through the examination of trajectories of centre of pressure (COP), the use of HHS was found to worsen foot stability in quiet standing as the variations of COP in both antero-posterior (AP) and medio-lateral (ML) directions increased with heel height. The adverse effects starts to accentuate when heel height reaches 8cm and become even worse at 10cm. High heel experience, however, appears to help maintaining one's postural control in high heeled conditions. The foot stability analysis also presented a method to quantify and analyse walking stability. By constructing a mean curve of the centre of pressure in ML direction (COPx) during the stance phase, the pattern of COPx progression, the intra-subject COPx variability among strides, and thus the walking stability could be determined. Findings and observations in the experiments served as a reference to determine the appropriate range of geometric and functional design parameters of the new insole, including the form and shape, thickness, dimensions and locations of the cushions, and materials properties requirements. In order to evaluate the effects of these design parameters on peak plantar pressure reduction, eight insole design factors including insole thickness, arch cushion thickness, mid-foot cushion thickness, height of heel cup rim, insole stiffness, stiffness of pads for metatarsal and heel, stiffness of medial arch support, and stiffness of cushion underneath the proximal metatarsal head, were defined and three levels were assigned to each design factors. A set of 16 designs was generated with combinations of different levels of each design parameters through applying the techniques of D-Optimal design. FE simulations were then carried out for each of these 16 designs. Through assessing the magnitude of the peak plantar pressures of these simulations, the mean effect of each level of the design factor on pressure relief could be computed. An optimal design on plantar relief was then achieved through combining the "best" level (greatest effect on plantar relief) of each design factor. Results of the study showed that insole thickness was the most critical design factor in forefoot pressure reduction, followed by the stiffness of materials to be used at the metatarsal region. In general, softer (Young's modulus < 2 MPa) and thicker materials (4 mm to 6 mm) could effectively reduce forefoot peak pressures. The result revealed that the design requirements for insoles to be used in HHS were different from those of insole to be used in flat-heeled condition. The research enhanced our knowledge on female foot morphology in heel-elevated postures and provided useful information for the selection of suitable materials for insoles to be used in HHS. Having demonstrated satisfactory repeatability and accuracy, the newly developed 3D imaging system could be adopted in other anthropometric studies. The combined use of fractional factorial designs and FE simulation model provided efficient and reliable numerical solutions to evaluate the effects of different insole parameters on the pressure distribution in high-heeled condition, optimising the effectiveness of the new insole design. The output of the study could extend to the development of other customized insoles and inserts.
Subjects: Hong Kong Polytechnic University -- Dissertations
Foot -- Care and hygiene
Women's shoes
Pages: xiv, 190 pages : color illustrations
Appears in Collections:Thesis

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