Accuracy and precision of two in-shoe pressure measurement systems

The recent rapid adoption of insole pressure measurement systems for clinical and ergonomic evaluations of human gait has necessitated a comprehensive understanding of the accuracy and precision of such systems. Five bench experiments were performed to examine the Pedar and F-Scan in-shoe pressure measurement systems. The insoles examined were the Pedar Y-sized right insole and the F-scan insole trimmed to the size and shape of a Pedar Y-sized insole. Data were sampled at 50 Hz at different levels of applied pressure, calibration procedure, duration of pressure application, insole age of use and experiment day or week. The system accuracy was determined by the per cent error of measurement, the system precision by the 95% tolerance interval of the per cent error. The results show that system accuracy and precision varied among levels of applied pressure, calibration procedure, duration of pressure application and insole age of use. The Pedar system showed the greatest accuracy and precision when the insole was new and measurements were taken (1) after a system calibration as specified by the manufacturer, (2) in the 50 - 500 kPa pressure range and (3) within a few seconds after pressure was applied. Under this condition, the measurement error was in the range -0.6 to 2.7%, and the magnitude (upper bound minus lower bound) of the 95% tolerance intervals was from 13.5 to 18.7%. Measuring less than 35 kPa with the Pedar system is not recommended. To ensure the accuracy and precision of the F-Scan system, users are recommended to estimate the range of the applied pressure and then choose a similar pressure level for calibration. Under this condition, the measurement error was in the range 1.3 - 5.8% and the magnitude (upper bound minus lower bound) of the 95% tolerance intervals was estimated to be in the range 1.1 - 14.8%. When the calibration pressure was outside this range of applied pressure, the per cent errors were considerably higher, ranging from -26.3 to 33.9%.