Short communicationA new method to normalize plantar pressure measurements for foot size and foot progression angle
Introduction
Plantar pressure measurement provides useful information for clinicians and researchers regarding the structure and function of the foot, general mechanics of gait, and is a helpful tool to evaluate patients with foot complaints (Rosenbaum and Becker, 1997). However, plantar pressure pattern data are not easy to analyse and, therefore, difficult to interpret. In general, mean, peak, and time–pressure integrals of plantar pressure are used (Bus and de Lange, 2005). Thus far, the foot has been divided into anatomical regions (masking process) to calculate plantar pressure parameters. Generally, six to eleven of such regions are considered. Although the masking process extracts important information out of the plantar pressure and describes the overall foot loading, subtle differences within a region cannot be seen due to the masking process. Therefore, plantar pressure pattern scaled to a standard size and progression angle would be helpful to study the plantar pressure in more detail. Moreover, scaling the plantar pressure to a standard foot allows more sophisticated analysis techniques such as pattern recognition and machine learning. Tavares and Bastos (2005) presented an approach for matching objects in dynamic optical pedobarograph. Similarly, Pataky et al. (2008) used statistical parametric mapping (SPM) to match various plantar pressure images. However, both methods will influence the shape of the foot due to their matching algorithms.
The main purpose of this study was to develop and test a simple method that normalizes the plantar pressure pattern for foot size and foot progression angle. Subsequently, we will examine if various subjects have a similar footprint after normalization. Furthermore, the variability in plantar pressure pattern was analyzed.
Section snippets
Subjects and experimental setup
Twelve healthy subjects participated in this study and signed an informed consent. Subjects walked over the pressure plate at their preferred walking speed and at approximately half their preferred walking speed. Plantar pressure data were collected at 500 Hz using the footscan® 0.5 m plate (RSscan, Olen, Belgium) mounted on top of a force plate (Kistler, Winterthur, Switzerland), which were synchronized with the RSscan 3D-box. Contact of the foot on the plate was calculated by RSscan software
Results
The main purpose of this study was to test the normalization method. Fig. 2 shows an example of the normalization of foot size and progression angle for 6 ft. The left panel of Fig. 2 illustrates the variability in foot size and foot progression angle between subjects. After normalization, the contour lines of normalized feet of various subjects were almost identical (right panel Fig. 2).
To test the performance of the method for foot progression angle, subjects walked straight and at an angle of
Discussion
The results in this article demonstrate that the newly proposed technique is successful in scaling the plantar pressure pattern to a standard size and foot progression angle. The average (0.14 and 0.22 cm) and maximum (0.46 and 0.75 cm) difference of the contour lines between walking in a straight line and approaching at an angle are in the range of the accuracy of the pressure plate (2 sensors per cm in width and 1.3 sensors per cm in length). Moreover, the contour lines of plantar pressure
Conflict of interest statement
The authors Keijsers, Stolwijk, Nienhuis, and Duysens declare that they did not have any conflict of interest.
Acknowledgements
The authors would like to thank the Prothese Orthese Makerij and Ontwikkelings maatschappij Oost Nederland NV for their financial support.
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