Effect of heel height on in-shoe localized triaxial stresses
Introduction
The increasing popularity of high-heeled shoes can be attributed to modern fashion, job requirement and common belief in enhancing esthetic appeal. Wearing high-heeled shoes would change body alignment and muscle activities, redistribute the plantar pressure and ground reaction forces (GRFs), thus inducing clinical problems of lower extremity (de Lateur et al., 1991, Esenyel et al., 2003, Gefen et al., 2002, Hong et al., 2005, Linder and Saltzman, 1998). Forefoot pain, hallux valgus and calluses are among the common foot problems encountered by long-term wearers (Al-Abdulwahab and Al-Dosry, 2000, Dawson et al., 2002, Dunn et al., 2004, Kernozek et al., 2003, Menz and Morris, 2005), which are thought to be associated with excessive plantar mechanical stresses, including pressure and shear, whose distributions are influenced by shoe construction and its materials.
Heel height elevation is correlated with the prevalence of hallux valgus (Dawson et al., 2002, Menz and Morris, 2005). Hallux valgus deformity was thought to be caused by a disturbed flexor halluics longus balance at the first metatarsophalageal joint (Glasoe et al., 2010, Snijder et al., 1986). High-heeled shoes change the foot alignment and redistribute the plantar stresses, which might lead to an imbalance of muscle moment especially when external stresses force the hallux to abduct and thus triggering hallux deformity.
In high-heeled shoes, shear stress might be of particular importance with an inclined supporting surface, which would increase the risk of soft tissue problems. Excessive shear forces result in the thickening of the stratum corneum, leading to hyperkeratosis (MacKenzie, 1974). Shear stress together with pressure could occlude the blood circulation (Bennet et al., 1979, Dinsdale, 1974, Zhang and Roberts, 1993), which would reduce tissue tolerance or repair capability and make foot vulnerable to damages. Plantar pressure alone is not sufficient in predicting foot disorders related to the plantar tissue (Lavery et al., 2003, Veves et al., 1992). Combined effects of pressure and shear stresses should be considered, including magnitudes, directions and spatiotemporal properties.
Although the resultant applications of pressure and shear on the skin surface has been suggested as critical variables for skin contact mechanics (Zhang and Roberts, 1993, Zhang et al., 1994), there have been limited reports on in-shoe triaxial stresses measurement due to lack of the proper instrument. Some measurements in previous studies were conducted using uniaxial or biaxial force transducers (Akhlaghi and Pepper, 1996, Hosein and Lord, 2000, Lebar et al., 1996, Lord et al., 1992, Lord and Hosein, 2000, Tappin et al., 1980, Tappin and Robertson, 1991). Uniaxial shear transducers can only measure shear stress in one direction. Although biaxial shear transducers were used to measure shear in two directions, plantar pressure had to be obtained from a separate system. To avoid uncertainties and technical difficulties associated with multiple force measuring systems, it is important to have a single system for assessing localized pressure and shear stresses simultaneously. Recent studies used a platform with a 16-triaxial transducer array to measure the barefoot plantar pressures and shear stresses simultaneously (Perry et al., 2002, Yavuz et al., 2007). However, the force between the foot and ground cannot reflect the shod condition. Shoe design might cause different stress transfers and temporal responses on the foot–shoe interface from overground surface. In-shoe triaxial force transducer, along with force platform, could provide details on the differences.
In this study, we hypothesized that in-shoe plantar shear stresses are mechanical risk factors for hallux valgus deformity and plantar soft tissue problems for high-heeled shoe wearers, in additional to in-shoe plantar pressure. Plantar pressure and shear stresses under five major weightbearing regions were investigated using in-shoe triaxial force transducers. The temporal and spatial properties of these triaxial stresses were analyzed and reported.
Section snippets
Methods
Three pairs of high-heeled shoes (European size 37) with heel heights of 30, 50 and 70 mm were selected in this study (Fig. 1). The 30 mm heel height was the lowest available for this type of rigid and wooden shoe sole, which was selected for proper transducer installation. Ten adult female subjects experienced in wearing high-heeled shoes volunteered to participate in this study. All subjects were free from foot problems or diseases including but not limited to high or low arch foot, limb-length
Results
Increasing heel height influenced the distributions of plantar pressure and shear stresses, including the magnitudes, directions of shear and temporal responses. During balanced standing, the maximal PP was found at the heel (103 kPa) and PRS was the largest at the hallux (12.9 kPa) for the 30 mm control shoes (Fig. 3). As heel height increased, PP and PRS increased over the heel, MTH1 and MTH2, but decreased over MTH4. At the heel height of 70 mm, PRS occurred at the heel and increased by 116%,
Discussion
Simultaneous measurement of in-shoe plantar pressures and shear stresses over specific foot regions provide additional insights into the foot–shoe interface. In this study, the increase of heel height was found to redistribute plantar triaxial stresses, including the magnitudes, directions, spatial and temporal responses. As heel height increased, the forefoot pressure and shear stress shifted from MTH4 to MTH2, which supported the perspective that forefoot pronation increased with high-heeled
Conclusion
In-shoe triaxial force transducers allow the simultaneous measurement of plantar pressure and shear stresses at the foot-shoe interface. Results showed a medial shift of plantar pressures and shear stresses over the forefoot and a smaller peak anterior shear during push-off as heel height increased. These changes agree with the increase in forefoot pronation and ankle plantarflexion wearing high-heeled shoes. Results also showed characteristics of shear stresses related to foot problems. An
Conflict of interest statement
We confirm that there is no conflict of interest; the authors and the author's institution has no financial or other relationship with other people or organizations that may inappropriately influence the authors' work.
Acknowledgment
This study is supported by grants from the Research Grant Council of Hong Kong (Project nos. PolyU5331/07E and PolyU5352/08E) and a Research Studentship from The Hong Kong Polytechnic University.
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