Foreign object held in recessed areas of shoe outsole as an acquired characteristic in footwear examination: A preliminary study

Abstract

Foreign objects as acquired characteristics in footwear examination have a great value in declaring a proposed identification. However, they do not have the same significance in excluding any footwear because they may disappear for many reasons as time passes by. The aim of this study was to establish the value of foreign object held (FOH) in the recessed area of the outsole, based on its reproducibility and specificity. To investigate the reproducibility of FOH, we asked 37 volunteers to wear Chinese police uniform shoes and to walk and run on both PVC soft glass and blacktop. Before each progression, there was an exposure of shoe outsoles on a soccer field full of rubber and quartz particles to allow access of FOHs to the shoe outsole. The numbers of FOHs in each shoe outsole were recorded both at the beginning and end of each progression. The variation rate of FOH number in each outsole was then calculated. To evaluate the specificity of FOH, we first invited 160 volunteers to wear their uniform shoes and to walk and jump on the soccer field. FOHs in two recessed lines of their outsoles were observed. Next, 25 of the subjects were invited to record their plantar pressure distribution using Novel’s EMED system. We also collected 113 shoes with FOHs in their outsoles, from all over China. The FOHs in different sub-areas of each outsole were observed and counted, followed by the calculation of their density in each sub-area. FOH reproducibility analysis showed that there were more losses of FOH after running than walking on both substrates; walking on blacktop tended to lead more losses of FOH than on soft glass. Thus, both the substrate condition and progression pattern affect the reproducibility of FOH. FOH specificity analysis showed that the FOH tended to be concentrated in the middle of the recessed line at heel part and in the middle and lateral half of the recessed line at arch part. The pressures where FOHs presented tended to be concentrated in 61.2%–88.2% (interquartile range) of corresponding maximum pressures. At the same time, FOHs tended to be found in areas with moderate wear and rarely in areas that seldom contacted the ground. All these results indicated that FOHs were not acquired randomly. Rather, they were related to the distribution of pressure.