The use of accelerometry to detect heel contact events for use as a sensor in FES assisted walking

Current sensors for the control of functional electrical stimulation (FES) assisted walking in hemiplegic individuals are not wholly satisfactory, as they are either not implantable or ineffectual in the detection of heel contact events. This study describes the use of an accelerometer placed on the trunk to detect heel contact events of both legs based on the examination of the anterior-posterior horizontal acceleration signal. Four subjects wore an accelerometer over their lumbar spine. Footswitches placed on the sole of one foot recorded the heel contact and heel off times for that foot. The acceleration signal was reduced to a series of pulses by studying the negative-positive changes in acceleration. It was found that there was approximately a 150 ms delay between heel contact and the negative-positive change in acceleration. This delay was consistent across different walking speeds, but was different between subjects and when hemiplegic gait was simulated. Therefore, accelerometers placed on the trunk are valid sensors for the detection of heel contact events during FES assisted walking.