Biomechanical evaluation of heel elevation on load transfer experimental measurement and finite element analysis

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1 Acta Mech. Sin. (2012) 28(1): DOI /s RESEARCH PAPER Biomechanical evaluation of heel elevation on load transfer experimental measurement and finite element analysis Yan Luximon Ameersing Luximon Jia Yu Ming Zhang Received: 24 February 2010 / Revised: 25 October 2010 / Accepted: 15 November 2010 The Chinese Society of Theoretical and Applied Mechanics and Springer-Verlag Berlin Heidelberg 2012 Abstract In spite of ill-effects of high heel shoes, they are widely used for women. Hence, it is essential to understand the load transfer biomechanics in order to design better fit and comfortable shoes. In this study, both experimental measurement and finite element analysis were used to evaluate the biomechanical effects of heel height on foot load transfer. A controlled experiment was conducted using custom-designed platforms. Under different weight-bearing conditions, peak plantar pressure, contact area and center of pressure were analyzed. A three-dimensional finite element foot model was used to simulate the high-heel support and to predict the internal stress distributions and deformations for different heel heights. Results from both experiment and model indicated that heel elevations had significant effects on all variables. When heel elevation increased, the center of pressure shifted from the midfoot region to the forefoot region, the contact area was reduced by 26% from 0 to 10.2 cm heel and the internal stress of foot bones increased. Prediction results also showed that the strain and total tension force The project was supported by the Research Grant Council of Hong Kong (PolyU5317/05E, PolyU5331/07E, PolyU5352/08E). Y. Luximon J. Yu M. Zhang ( ) Department of Health Technology and Informatics, The Hong Kong Polytechnic University, Hung Hom, Kowloon, Hong Kong, China ming.zhang@polyu.edu.hk Y. Luximon School of Design, The Hong Kong Polytechnic University, Hung Hom, Kowloon, Hong Kong, China A. Luximon Institute of Textiles and Clothing, The Hong Kong Polytechnic University, Hung Hom, Kowloon, Hong Kong, China of plantar fascia was minimum at 5.1 cm heel condition. This study helps to better understand the biomechanical behavior of foot, and to provide better suggestions for design parameters of high heeled shoes. Keywords Heel elevation Weight bearing Plantar pressure Numerical foot model 1 Introduction Many studies indicate a high prevalence of foot problems in women who often wear high-heeled shoes [1 4]. High-heeled shoes cause foot shape and load distribution change [5], which may lead to lower back pain, plantar foot pain, discomfort, muscle fatigue, and even foot deformities such as hammer digits and hallux valgus [3,6]. Walking on high-heeled shoes also alters the walking style, as described as high heel gait [7]. Load distribution between plantar foot and shoe (plantar foot pressure) is widely used to evaluate the effect of high heeled shoes [6,8 11]. Plantar pressure data are usually considered in the comparison of shoes and pressure pads, and evaluation of impairment associated with musculoskeletal and neurological disorders [12]. A number of studies indicated that the peak pressure at medial forefoot and central forefoot increased, but reduced under the heel and midfoot with heel elevation [9,11]. Approaches to alleviate the high pressure relied on heel cup, arch support, and total contact insert in order to reduce plantar pressure and improve the comfort [9]. In addition to the peak pressure, the impact force and perceived discomfort increase with heel elevation [9]. The center of pressure (COP) shifts forward and medially in high-heeled shoes [6,13]. Apart from the studies on commercially available shoes [9,10], very little has been done to understand highheeled shoe biomechanics and high-heeled foot shape. Com-

2 Biomechanical evaluation of heel elevation on load transfer experimental measurement and finite element analysis 233 mercially available shoes have many uncontrollable factors such as heel height, construction, material, toe box shape, shank curve and heel geometry [8]. These parameters may influence biomechanical measurements [1,14]. Results from these studies may be useful to obtain information about the given footwear design, however, do not provide much information on parametric understanding of footwear design. There is a need of the basis of understanding foot shape changes in high heels, plantar pressure in unrestricted foot in high-heeled condition, and other shoe parameters. Finite element methods can provide information on internal joint movements and load distributions, which are difficult to obtain experimentally. Several three-dimensional finite element foot models have been developed and used to understand the effects of shoe parameters on foot load transfer [15 17]. The variables such as plantar pressure, internal stress strain and joint movement could be predicted. It has been shown that FE modeling, if conducted properly, could potentially make significant contributions to the understanding of foot biomechanics and improvement of footwear design. We have developed FE foot model with high-heeled condition [17,18] and validated using experimental studies. The FE model will be helpful in understanding the heel height effects. Given that load transfer knowledge is essential and comprehensive studies have not been done, in this study, a controlled experimental method with custom-designed platforms and a three-dimensional FE foot model were used to evaluate the effect of heel elevation on biomechanical measures for the unrestricted female foot. The plantar pressure parameters such as peak pressure, contact area and center of pressure were measured in the experiment and compared with the model simulation. The internal stress distribution and deformation for different heel elevations are presented. 2 Experimental evaluation 2.1 Material and methods Participants Twenty four female volunteers participated in this experiment. Their feet did not have any foot deformities, systemic diseases, major skin lesions and neurological defects which might affect foot pressure and standing posture Experimental materials Three pairs of platform (foot support) with different heel heights of 0 cm, 5.1 cm and 10.2 cm, denoted as H0, H2, and H4, respectively, were used in the study (Fig. 1). The platforms were made of Pedilen R rigid foam 300 and machined using a computer numerical-controlled (CNC) miller. The surfaces of the platforms were extended from 2D profiles taken from the bottom profiles of three standard commercial lady shoe lasts which were based on the American last design [19]. All three pairs of shoe lasts were of size 38 but of different heel heights. F-Scan R in-shoe system (Tekscan, Inc.) was used to measure the plantar pressure distribution. A pair of F-Scan sensors was fixed on top of each pair of platform. a b c Fig. 1 Experimental setup. a 0 cm; b 5.1 cm; c 10.2 cm Experimental design A two-way (3 heel elevations 5 weight-bearings) factorial within subjects design was used. Four weightbearing conditions were 25%, 50%, 75% and 100% of full body weight. The three heel heights were randomly assigned to each participant. The dependent variables were the mean pressure, peak pressure, contact area, and center of pressure for both feet Procedure The individual data including foot dominance, height, weight and age were recorded. Participants standed on the first pair of foot supports and balanced the body for 5 minutes to be familar with the supports (Fig. 1). The metatarsophalangeal joint (MPJ) of feet were aligned on the MPJ curve of the foot support as close as possible. A weight measurement device was under the right foot support in order to monitor the weightbearing condition. After calibration of F-

3 234 Luximon Yan, et al. Scan mats for both feet, participants were required to keep in standing position while maintaining a percentage of body weight (25%, 50%, 75% and 100%) on the right foot and the remaining weight on the other foot. All procedures were monitored by the experimenter. The participant stayed in this position for one minute for each loading. Ten seconds real time plantar pressure data were recorded at a sampling frequency of 50 Hz. After each pair of foot supports was tested, participants were given around 5 minutes to rest before the next expeiment started Data analysis There were sensors for each F-Scan sensor mat and 500 frames of pressures were recorded at the sampling frequecy of 50 Hz for 10 s. Pressure data was then analysed. p i jk represent pressure in row i (where i = 1, 2,, 60), column j ( j = 1, 2,, 21) and frame k (where k = 500 1, 2,, 500). The average pressure (P i j = p i jk /500) of the 500 frames were calculated. From the average pressure P i j, peak pressure and mean contact area were calculated for whole foot and for different foot regions separately. In this study, the foot was divided into three regions, namely forefoot region (toes and 1st 5th metatarsal heads, representing 40% of foot length), midfoot region (30% of foot length) and heel region (30% of foot length). The center of pressure k=1 along heel to toe direction for whole foot was analyzed based on the distribution of the average pressure P i j and was standardized in term of participant s foot length. The heel end was represented by 0% while the toe end was represented by 100%. 2.2 Results Demographic information The age of the participants was years on average with range from 19 to 36 years. The body height was cm on average with standard deviation of 3.63 cm, and the average body mass was kg with standard deviation of 7.16 kg Plantar pressure One-way ANOVA results showed that the contact area (p = ) and COP (p = ) were not significantly different between the left and the right feet. Therefore, left and right foot data were analyzed together in the following analysis. Two-way ANOVA of heel elevation and weightbearing was conducted for peak pressure, contact area and COP. The main effects of heel elevation and weightbearing on all the variables were significant (p < 0.05). The peak pressure was plotted in Fig. 2. The Student- Newman-Keuls (SNK) Test indicated that peak pressure for Fig. 2 Peak pressure for different regions

4 Biomechanical evaluation of heel elevation on load transfer experimental measurement and finite element analysis 235 the whole foot for H4 were significantly higher than H0 and H2 (p = 0.05). In addition, results for H0 and H2 were significantly different for peak pressure (p = 0.05). The highest peak pressure for H0 was in the heel regions, whereas for H2 and H4 it was in the forefoot region. Figure 3 displays the relationship between the weightbearing and the contact area for different foot regions. The effect of heel elevation on the contact area was significant. The SNK test showed that the contact area when using H4 was significantly less (p < 0.05) than that when using H0 and H2. However, the contact areas when using H0 and H2 were not significantly different (p = 0.05). Fig. 3 Contact area for different regions Figure 4 indicates that the COP shifted towards the forefoot when heel elevation increased. When using H0, the COP for the whole foot was located in the midfoot area closer to the heel (COP =33.29%), while for the H4 it was located in the forefoot area (COP = 68.35%). For H2, COP was somewhere (COP = 41.64%) between the COP of H0 Fig. 4 Center of pressure (COP) along heel to toe direction and H4. Given that the highest peak pressures for H0, H2 and H4 were located at the heel, forefoot and forefoot regions, respectively, the average COP for H0, H2 and H4 in these regions were calcuated (COP for H0 in heel = 12.19%; COP for H2 in forefoot = 76.04%; COP for H4 in forefoot = 76.59%). Further analyses were performed for different regions of the foot. When considering heel elevations, the SNK test indicated that the peak pressure at the forefoot region was significantly higher for H4 than H2, and then higher for H2 than that for H0 (p = 0.05). Furthermore, the peak pressure at the heel region reduced with increasing heel elevation. The SNK test showed that the peak pressure was significantly lower for H4 than H2 and significantly lower for H2 than H0 (p = 0.05). However, the peak pressure at the midfoot region were not following the increment or decrement with heel elevation. The peak pressure was highest for H2, which is significantly higher than that for H4 and then significantly higher for H4 than that for H0 (p = 0.05). In addition, the forefoot contact area did not have significant

5 236 Luximon Yan, et al. difference between H0 and H4, but significantly higher than H2 (p = 0.05). Midfoot contact area for the H2 was significantly higher than that for H0 and H4. Hindfoot contact area for H4 was significantly lower than that for H2, and then it was significantly lower for H2 than that for H0. In general, the peak pressure and the contact area at the forefoot, midfoot, heel and whole foot all increased with weightbearing. However, the increment was non-linear (Figs. 2 and 3). The peak pressure at the forefoot region increased more rapidly with weightbearing when using H4 rather than H2 and H0. However, the peak pressure increased most rapidly on H2 at the midfoot region and on H0 at the heel region with weightbearing. In the case of COP, Fig. 5 indicated it shifted slightly toward the heel when weightbearing was increased for H4 condition, but not for H0 and H2. Fig. 5 The 3D surfaces and FE model. a Foot bones; b Soft tissue; c The attachment points of the plantar fascia and all major ligaments 3 Finite element foot model with high-heeled suport 3.1 Model development A comprehensive FE model of a female foot was developed [17] and used for comparing to experimental results and evaluating internal stress effects of heel elevation [17,18]. The subject was a healthy female of age 28, height 165 cm and weight 54 kg and the subject was free from lower limb disease. The 3D geometries of foot bones and soft tissue were built from coronal magnetic resonance (MR) images, which were obtained at 1 mm interval using a 3.0-T MR scanner (Seimens, Erlangen, Germany). The foot was scanned in a neutral, non-weightbearing condition. The foot model consisted of 28 distinct bony segments. A total of 78 ligaments and the plantar fascia were included and defined by connecting the corresponding attachment points on the bones [20]. The FE software ABAQUS v6.7 was used for the creation of FE mesh and subsequent analysis (Fig. 5). The 4-noded tetrahedral elements were chosen for meshing foot bones and encapsulated soft tissue. Axial connector elements were used to apply extrinsic muscle forces at the insertion sites. The details of the model development can be found in Ref. [17]. Due to the time consuming experimental procedure, only three settings of heel heights (0 cm, 5.1 cm and 10.2 cm) were selected to represent the heel range from flat heel shoes to extreme high heels. In order to investigate the details of variation of stresses, four 3D foot support platforms (0 cm, 2.5 cm, 5.1 cm and 7.6 cm denoted as H0, H1, H2, and H3, respectively), whose H0 and H2 were same as the supports in the experimental evaluation, were employed. The 8-noded hexahedral elements were used for meshing the high-heeled foot supports. The foot bones, cartilages and ligaments were idealized as homogenous, isotropic and linearly elastic. The Young s modulus and Poisson s ratio of the bony structures were defined as MPa and 0.3, respectively [21]. The mechanical properties of the cartilage, plantar fascia and ligaments were assigned according to pervious study [17]. The Young s modulus and Poisson s ratio of the foot supports were set as MPa and 0.1, respectively. The encapsulated bulk soft tissue was defined as nonlinearly elastic based on the in vivo uniaxial stress strain data on heel pad [22] and the second-order hyperelastic polynomial form was used to represent soft tissue. 3.2 Simulations and results Balanced standing on foot supports with different heel elevations were simulated using the numerical model. A vertical ground reaction force corresponding to about half body weight (270 N) was applied at the center of force location on the bottom of foot supports of which only vertical movement were allowed. The superior surfaces of soft tissue, distal tibia and distal fibula were fixed throughout the analysis to serve as the boundary conditions. The example of boundary condition of 5.1 cm high-heeled condition is demonstrated in Fig. 6. Different musculotendon forces are added on the foot model in order to simulate the conditions from nonweightbearing to standing situation when on heel support.

6 Biomechanical evaluation of heel elevation on load transfer experimental measurement and finite element analysis The surface interaction between the plantar foot and supporting surface was assigned with a coefficient of friction of 0.6 [23]. The foot deformations during balanced standing on four foot supports are displayed in Fig. 7. Plantar pressure distribution and internal stresses strains within bony and soft tissue structures under various supporting conditions for balance standing were predicted by the model. The plantar pressure pattern for 0 cm is shown in Fig. 8a. The model predicted peak plantar pressure of 0.07 MPa, 0.08 MPa and 0.23 MPa at the forefoot, midfoot and heel region, respectively, and the values showed high contact pressure was located at the central heel region. The contact area of whole foot from FE prediction was 61.0 cm2. The predicted COP was located at about 27% along the heel to toe direction. 237 Fig. 6 The example of loading and boundary condition for standing on 5.1 cm high-heeled support Fig. 7 Foot deformations during balanced standing on foot supports with different heel elevations. a 0 cm; b 2.5 cm; c 5.1 cm; d 7.6 cm Figure 8b depicts the von Mises stress of the foot bones during balanced standing with 0 cm support. The von Mises stress is used to predict yielding of material under multiaxial loading conditions using results from simple uniaxial tensile tests. From the FE prediction, peak von Mises stress of MPa was predicted at the calcaneus, followed by the second metatarsal (6.83 MPa), fourth metatarsal (6.78 MPa), third metatarsal (6.51 MPa), navicular (4.58 MPa), first metatarsal (4.12 MPa), lateral cuneiform (3.79 MPa) and talus (3.09 MPa). The plantar junction of calcaneal-cuboid joint sustained the highest stress. The insertion sites of plantar fascia at the inferior calcaneus and the metatarsal heads experienced high stress as a result of tension in the plantar fascia. The insertion site of Achilles tendon at the posterior calcaneus sustained high stress because of applied muscle forces loading. The plantar pressure distributions during balanced standing on three different high-heeled foot supports (H1, H2 and H3) from FE predictions are presented in Fig. 9. The peak plantar pressure region shifted from the central heel region to the central forefoot region at 7.6 cm sup-

7 238 Luximon Yan, et al. port condition. The peak pressure in forefoot, midfoot and heel region were 0.10 MPa, 0.06 MPa, 0.14 MPa for H1, 0.09 MPa, 0.09 MPa, 0.16 MPa for H2 and 0.20 MPa, 0.11 MPa, 0.09 MPa for H3. The contact area from FE predictions showed slightly reduction of 8.77%, 9.25%, 6.90% corresponding to H1, H2 and H3, compared to that with H0 (61.0 cm 2 ). The predicted COPs for H1, H2 and H3 moved forword along heel to toe direction and matched experimental results within 3 mm deviation. There was a general increase in maximum von Mises stress of foot bones with increasing heel height of foot supports from 0 cm to 7.6 cm, especially for 7.6 cm. The peak von Mises stress in major bones with different heel elevations are compared in Fig. 10. Peak von Mises stress appeared at plantar junction of calcaneal-cuboid joint. In the forefoot region, relatively high von Mises stresses concentrated at the second to the fourth metatarsal shafts as well. The FE predicted peak strain and tension of plantar fascia during balanced standing were 1.3%, 0.8%, 0.6% and 2.8%; 151 N, 90 N, 60 N and 278 N for H0, H1, H2 and H3, respectively. At 5.1 cm high-heeled foot support condition, the strain and total tension force of plantar fascia was minimum. a b Fig. 8 FE prediction for 0 cm support under half body weight. a Plantar pressure distribution; b von Mises stresses of the foot bones a b c Fig. 9 Plantar pressure distributions from FE prediction for different heel elevations. a 2.5 cm; b 5.1 cm; c 7.6 cm 4 Discussion This study investigated the effects of heel elevation on foot load transfer using both experimental method and biomechanical foot model. The experiments provided the validation for the model and biomechanical computer simulations calculated the internal joint movements and load distributions. Fig. 10 FE predicted peak von Mises stress of the foot bones during balanced standing on different foot supports The results from this study are partially consistent with those of previous studies. The results on separate foot regions demonstrated that the pressure shifted from heel region

8 Biomechanical evaluation of heel elevation on load transfer experimental measurement and finite element analysis 239 to forefoot region when heel elevation changed from 0 cm to 10.2 cm which agreed with those of previous research [6,8 10]. The predicted plantar pressure pattern from the result of the biomechnical model agreed with the experimental data (Figs. 2, 8 and 9). Both of the measured and predicted values showed high contact pressures at the central heel region for 0 cm and then moved to forefoot region with heel elevation increasing. Since the metatarsal phalangeal joint (MPJ) of the participants were forced to bend by the high heels, participants had to stand nearly on forefoot only in 10.2 cm heel situation which led to very low pressure on heel but extremely high pressure on forefoot. In this situation, the COP moved from midfoot region to forefoot region with the heel elevation increasing as expected [6,7,13]. The COP was shifted from 33.3% for 0 cm to 68.3% for 10.2 cm in heel to toe direction. The FE prediction also proved that von Mises stress of foot bones increased relatively when heel elevation increased, especially in the second, third and fourth metatarsals, which indicated that metatarsal area had high risk for foot problems in high heeled shoes. Experimental results showed that the pressure on midfoot region did not simply follow the decreasing trend with increasing heel elevation. It seems that the midfoot pressure increased from 0 cm to 5.1 cm heel elevation first and then dropped from 5.1 cm to 10.2 cm heel elevation condition, but still higher than 0 cm. Previous study showed that the midfoot pressure reduced with increasing heel elevation and the authors explained those based on the high cavus-type of arch when wearing high heeled shoes [9]. In fact the pressure caused by cavus-type of arch was measured in flat platform and it did not consider the shank curve for high heeled shoes situation. In commercial shoes, shank curves are varying with heel elevation and the standards for shank curves are not consistent in different manufacturers. Therefore, the reduce of midfoot pressure might be caused by the design of shank curves [9]. In this study, the pressure loaded on midfoot for 5.1 cm heel support effectively reduced the pressure on forefoot and heel region, hence balanced the pressure distribution on whole foot. The FE prediction results also showed that the strain and total tension force of plantar fascia was minimum at 5.1 cm foot support condition. These indicated that low heel elevation, but not zero heel height, might provide better solution for heel design. It is also suggested that the shank curve design is important to distribute the pressure. When shank curve fits foot arch well, it can shift the pressure and reduce the forefoot pressure. Future studies are needed to find out best fiting curve for each heel elevation. The results on weightbearing agreed with previous studies [24] that pressures and contact area increased nonlinearly with weighbearing. In addition, the shift of the center of pressure for 10.2 cm heel elevation seemed to slightly shift towards the heel when weightbearing increases (Fig. 5). Participants might try to lean the body backwards to maintain the stability when they had to stand on one foot for very high heeled situation, indicating low stability in high heeled shoes. This study demonstrated the static standing situation for various heel elevation in both experimental and simulation method. The future research could look forward to dynamic condition when wearing high heeled shoes. Therefore, custom design shoes with known parameters should be used to investigate dynamic foot plantar pressure. The model has to be validated in more complex environment and it still needs to be improved in order to simulate very large deformation in 10.2 cm heel elevation. In addition, present study mainly focused on heel elevation. There are many other factors which can be investigated in future studies, such as shank curve, toe shape, heel shape and inserts which could affect high heeled shoes biomechanics. 5 Conclusion Foot problems caused by high heeled shoes have been studied for many years, however, commercial shoes used in the most previous research may be confounded due to the factors other than heel elevation. In this study, the custom made platforms with different heel elevations were used to reduce these confounding effects and to study the effects of heel elevations and weightbearing conditions on peak plantar pressures, contact area and center of pressure. Both experimental method and numerical modeling method were applied. Results showed that numerical simulation could provide more biomechanical information of footwear effect when combined with physical experiment. Both experiment and model indicated that there is no linear relationship from flat support (0 cm) to very high heeled support (10.2 cm). Heel elevations had significant effects on peak pressure, contact area, center of pressure and internal stress. It is suggested that low heel shoes was not necessarily more harmful than flat shoes as far as plantar pressure and internal stress strain are concerned. In addition, the forefoot has high risk for disease especially for those who wear high heeled shoes for a long time daily. References 1 Broch, N.L., Wyller, T., Steen, H.: Effect of heel height and shoe shape on the compressive load between foot and base. Journal American Podiatric Medical Association 94(5), (2004) 2 Dawson, J., Thorogood, M., Marks, S.A., et al.: The prevalence of foot problems in older women: a cause for concern. 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