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

Abstract

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.

Introduction

Since the 1960s, functional electrical stimulation (FES) has been used to correct foot drop in stroke patients [1]. It works by delivering an electrical stimulus to the tibialis anterior during the swing phase of gait to allow proper foot clearance and prevent the need for hip circumduction or other unstable gait compensations. While FES systems have proved successful in improving walking ability in stroke patients by improving walking speed, functional mobility, step length on the affected side, ankle flexion, muscle strength, motor coordination and balance [2], [3], [4], [5], [6], [7], [8], there are some fundamental problems associated with their use. Baker [9] has identified two such problems; the complexity of the gait cycle and the need for implantation of the system, and both problems put demands on the sensor and control system used.

For any FES system to work in an effective manner, it requires a sensor to detect when the stimulus should be delivered. At first, the system can be initiated manually by an external switch. However, during the gait cycle itself, stimulation should be automatic. Therefore, a sensor is required to detect the phases of the gait cycle, so that the stimulation can be delivered during the swing phase of gait on the affected side. In order to do this effectively, the sensor must be able to detect heel off of the affected limb to trigger stimulation just prior to the swing phase and either heel contact or heel off of the contra-lateral leg to stop stimulation.

Originally, footswitches, usually based on force sensitive resistors, were placed on the sole of the foot and used as sensors for FES [2]. This seems like the natural choice of sensor; heel loading and unloading can be detected with a footswitch. However, the footswitch as a sensor for FES assisted walking correction has many problems associated with it, most notably, reliability and the lack of implantation potential. The reliability of footswitches is often diminished due to their tendency to detect heel contact events during the swing phase as small forces are exerted on the heel during swing. Similarly, the reliability of the footswitch is reduced in patients who shuffle their feet when they walk [10], [11]. The implantability of the stimulator is important as patients often reject systems that require a lot of donning time or are not aesthetic. This problem may be addressed through the use of implanted cuff electrodes placed on the sural nerve to use the patient’s own sensory nerves to detect heel contact events [12]. However, such sensors are still ineffective in patients who shuffle their feet when they walk [10], [11].

Hand switches have also been used to trigger stimulation, which may be preferable in the event of encountering an obstacle or uneven terrain [13]. However, hand switches increase the attentional demands of walking, which may reduce the patient’s capacity to respond to perturbations to their balance [14], leading to an increased risk of a fall event.

Accelerometers have the potential to be used as sensors for FES control systems. Accelerometers have been used previously to detect phases of the gait cycle [15], although these authors have placed the accelerometer on the leg, which requires more involved data processing. Implantation of the accelerometer could be carried out in the abdominal cavity of the patient, which would allow for implantation of both stimulator and sensor at the same site. Placement of the accelerometer on the trunk facilitates detection of phases of the gait cycle using changes in measured acceleration patterns and a trunk-mounted accelerometer has also been shown to measure instantaneous walking speed [16], which could provide another control input for stimulus intensity.

From the placement of the accelerometer on the trunk, it may be possible to detect phases of the gait cycle by changes in acceleration patterns. It was the purpose of this study to determine the potential of accelerometry to detect gait events for their prospective use in a FES stimulator for the correction of drop foot in hemiplegic patients.