Spatial frequency content of plantar pressure and shear profiles for diabetic and non-diabetic subjects

How high does pressure and shear stress sensor resolution need to be in order to reliably measure the plantar pressure and shear profiles (PPSPs) under normal and diabetic feet? In this study, pressure and shear stress data were collected from 26 total diabetic and control subjects using new instrumentation that measures vertical and horizontal force vectors of the plantar contact surface during multiple instances in the gait cycle. The custom built shear-and-pressure-evaluating-camera-system (SPECS) performs simultaneous recordings of pressure and both components of the horizontal force vector (medio-lateral and antero-posterior) at distinctive regions under one׳s foot, at a spatial resolution for each sensor equal to 1.6 mm by 1.6 mm. A linear interpolation method was used to simulate the effect of increasing sensor size on PPSPs. Ten square-shaped sensors were included in the analysis, having edge lengths of: (1.6 mm, 3.2 mm, 4.8 mm, 6.4 mm, 8 mm, 9.6 mm, 11.2 mm, 12.8 mm, 14.4 mm, and 16 mm). A two-dimensional Discrete Fourier Transform was performed on each data set, for each of the ten sensor sizes. To quantify the difference between sensor sizes, a comparison was made using the maximum pressure and shear stress data over the entire plantar contact surface, equivalent to the peak of the spatial frequency spectrum. A reduction of 5% of any component of the stress vector (i.e., pressure, or medio-lateral shear stress, or anter-posterior shear stress) due to an increase in sensor size was deemed significant. The results showed that a sensor measuring 9.6 mm by 9.6 mm caused meaningful reductions in all three stress components (p<0.001), whereas sensors measuring 1.6 mm by 1.6 mm, up to 4.8 mm by 4.8 mm, can capture the full range of spatial frequencies in both pressure and shear stress data.