Thin pressure-measuring insoles have enabled new clinical studies of in-shoe distribution of plantar pressure. One of these new devices, based on force-sensitive resistor (FSR) technology, exhibits time-varying behaviour due to the mechanical properties of the sensor materials. This has been variously attributed to initial bedding in, wear or time-dependency behaviour. Cyclic and sustained loading were used in laboratory conditions to investigate this phenomenon. The results show that the sensitivity of the sensor varies rapidly over the first minute of loading. There is also a slower long-term effect which reduces over time. From this information, procedures for optimal calibration and use are suggested.