Influence of in-shoe heel lifts on plantar pressure and center of pressure in the medial–lateral direction during walking
Introduction
In-shoe heel inserts are used widely as a treatment option for some lower extremity problems such as Achilles tendon disorders [1], heel pain [2], low back pain [3] and limb length discrepancy [4]. Ordinary people use heel lifts with appropriate height in flat shoes in order to look taller. Since in contemporary culture wearing high heels is associated exclusively with females, in-shoe heel lifts seem to be the only option for males to increase their overall height as they can be hidden in ordinary men's shoes. The availability of off-the-shelf heel lifts along with fashion customs has led to an increase in their regular application. However, whether or not it is safe to wear thick heel lifts on a daily basis has yet to be examined.
It is widely believed that heel rise would induce plantar pressure redistribution, and the effects of high-heeled shoes on kinetics and kinematics have been reported extensively [5], [6], [7]. However, the effects of the heel rise caused by heel lifts might be different from that of high-heeled shoes due to their inherent differences in shape and material. The conventional heels are usually narrow shaped and made of materials lacking in elastic properties, while heel lifts tend to be wedge shaped and made of elastic materials. The plantar pressure redistribution caused by heel lifts have been reported in previous studies, but the insert height in their experiments was limited to 10 mm [8], [9], which is apparently not thick enough to significantly increase overall height. The commercially available thicknesses of heel lifts for the application of increasing height are usually ranging from 15 mm to 50 mm, so we think that further research on higher heel lifts is needed.
Elevation of the body's center of mass (COM) caused by thick heel lifts and inappropriate material may increase difficulty of maintaining balance during walking. Center of pressure (COP) describes the neuromuscular response to shifts of COM and measures of COP have been used as an indicator of dynamic stability [10], [11], [12]. While several studies have analyzed the effects of heel lifts on trunk muscle activation, plantar pressure, ankle and knee moments [1], [8], [13], few studies have devoted to understanding the characteristics of COP when people wear heel lifts. High-heeled shoes require increased demand on stability due to the base of support reduction and COM elevation. Although contact area on the shoe-ground interface remains unchanged in the wake of wearing in-shoe heel lifts, the impact of heel lifts on dynamic stability should not be neglected.
The purpose of this study was to investigate the effects of height and material of in-shoe heel lifts on plantar pressure and medial–lateral (ML) COP motion during walking, which may provide a basis for the in-shoe heel lifts designs that minimize adverse effects.
Section snippets
Participants
Seventeen healthy male adults gave informed consent and participated in this study. The average age of the subjects was 21.6 years (S.D. 1.2), average weight 59.0 kg (S.D. 4.7), average height 170.1 cm (S.D. 2.9). All participants had the same shoe size to avoid the effects of shoe size and for the convenience of the following COP analyses. Males were chosen as they are more likely to wear heel lifts to increase their height. All subjects had no history of lower extremity injuries in the
Results
Mean values of peak pressure (PP), pressure–time integral (PTI) and contact area (CA) are shown in Table 1. Both PP and PTI under the forefoot demonstrated an upward trend as the heel was elevated, while no differences were found between materials. Compared to the shoe only condition, the hard insert condition significantly increased the forefoot PP and the soft, medium (34 mm) and hard heel lifts significantly increased the forefoot PTI. For the midfoot region, PP, PTI and CA decreased as the
Discussion
The results in our study suggest that PP and PTI under the forefoot increased as the heel got higher, which is consistent with the previous studies [7]. The forefoot PP with the hard insert was higher than that of the soft and medium (34 mm) heel lifts, and this may because the considerable decrease of CA under the midfoot (72.39% decrease from the control condition) limits the hard lift's ability to distribute pressure. The relief on PP and PTI observed under the midfoot was associated with the
Conclusion
Thick heel lifts increase forefoot PP and PTI and induce larger range and velocity of ML-COP. Therefore, it is advisable to wear them with caution on a daily basis. Heel lifts with elastic properties provide immediate reduction in heel PP, while approaches to relieving heel PP by elevating the heel are not practical, at least within 34 mm heel rise. Both inadequate support by softer material and decreased compliance of harder material impair the dynamic balance control ability. Hence a heel lift
Acknowledgements
The authors would like to thank Zhipan Wu, Nan Zheng and Wei Zhu for their assistance with the preparation of this manuscript.
Conflict of interest statement: The authors declare that there are no known conflicts of interest related to this project that could have influenced this manuscript.
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