Electronic measurement of plantar contact area during walking using an adaptive thresholding method for Medilogic ® pressure-measuring insoles.

BACKGROUND: Pressure-measuring insoles have the potential to measure plantar contact area (PA) during walking. However, they are not widely used for this purpose because of the need for a reliable process that can convert the insole output into PA. The purposes of this study were to: (1) develop an adaptive-threshold method for pressure-measuring insoles that can improve the accuracy of the PA measurements during walking, and (2) experimentally assess the accuracy and generalizability of this method.

METHODS: A sample of 42 healthy, ambulatory, young adults (age=24.3±3.2years, mass=67.2±16.9kg, height=1.63±0.08m) completed 10 trials walking on an elevated walkway while wearing Medilogic® pressure-measuring insoles (sizes 35-45). A total of six insole sizes were considered. Insole data were converted to PA using three unique adaptive-thresholds that were based on percentages of the maximum sum of digital values (MSDV) during an analyzed step. Three values were considered: 0.1%, 0.2%, and 0.3% of the MSDV. Additionally, a fixed-threshold, which is typically used to estimate PA, was assessed. These two techniques, applied to the insole worn on the left foot, were compared with PA obtained from high-resolution reference footprints obtained from optical pedography of the right foot and processed using digital image processing algorithms. An assumption of PA symmetry between the left (insole) and right (barefoot) feet was made and comparisons were conducted over the entire stance phase of walking. The generalizability of the algorithm was assessed by comparing PA errors from insoles with respect to the optical pedography results based on insole size criteria.

RESULTS: The adaptive-thresholds of 0.1%, 0.2%, and 0.3% of MSDV produced mean errors of 7.31±17.44%, -8.62±15.01%, and -20.45±14.18%, respectively. Using the 2-digital value fixed-threshold produced a mean error of 20.88±22.44%. The best performing adaptive-threshold varied among insole sizes.

CONCLUSION: It was observed that the fixed-threshold technique produced large magnitudes of errors. The proposed adaptive-thresholds of 0.1% and 0.2% of the MSDV reduced PA error to ±10% during walking. The adaptive-threshold method consistently reduced PA error vs. the fixed-threshold for each insole size.