Correction for sensor creep in the evaluation of long-term plantar pressure data

Plantar pressure measurements are frequently applied in the biomechanical assessment of lower extremity injury risk. Different conditions such as footwear, orthopedic insoles or movement tasks are usually compared by separate trials of the individual conditions. However, injury risk may be related to fatigue from long-term exercise and no actual measurements of plantar pressure during fatigue treatment have been published. A simple method is presented for determining sensor creep over a 3 h walking trial carrying a heavy load (49% of bodyweight). Plantar pressure measurements were conducted using Pedar insoles with capacitative sensors. Repeated standing trials were conducted and the total force underneath both feet measured under the assumption that this value should remain constant over time. The percentage fluctuation from the first such static measurement can be used to correct measured parameters of interest for the influence of sensor creep. The pressure sensor values increased by up to 17% after 3 h and the method presented permitted the correction of measured plantar pressure parameters to account for this sensor creep behaviour. Such correction appears necessary for correct interpretation of the fatigue effects on plantar loading. Creep correction as described here should be performed individually and separately in each long-term trial.