Abstract
Functional Electrical Stimulation (FES) is the application of electrical stimulation to neural pathways or muscles in order to achieve an effective muscle contraction with the aim of restoring lost or impaired function. In 1961 Liberson introduced the use of electrical stimulation for foot drop correction, a common condition following a cerebrovascular accident or stroke. Despite growing evidence on the beneficial use of FES for foot drop, and more than 40 years on from Liberson’s work, FES systems for foot drop have not gained wide-spread use, and the basic design remains unchanged. It was the aim of this work to investigate the use of alternative sensors and the development of a sensor system that will improve the reliability, ease of use, and cosmetic aspects of a FES foot drop correction system. The proposed method is a novel approach of using a single gyroscope placed at the anterior aspect of the shank to obtain feedback for a FES foot drop correction system. Previous work carried out in the Centre for Biomedical Engineering had demonstrated the potential of the angular velocity gyroscope (Gyro) as an alternative sensor to foot switches. It is believed that the replacement of the heel switch with the gyroscopic sensor would offer several advantages, which could improve system reliability and function. The Gyro is a small and lightweight sensor - with potential for further miniaturisation and implantation - which can be easily donned and doffed - positioning is not very critical -with minimal encumbrance to the patient. The nature of the Gyro contributes to its high reliability and long lifetime during which there is little or no deterioration in its performance. The first part of the project involved the development of an automated reference method for gait event detection that can be used to assess the accuracy of the new gyroscope-based sensor. A kinematic-based approach was adopted and the new method was validated using data from 12 subjects. The new algorithm based method was compared to times given by visual inspection and force platforms. Ninety percent of all timings given by the algorithm were within one frame (16. 7 ms) when compared to visual inspection. The new method for gait event detection required a thorough understanding of 3D co- ordinate data processing. As part of this, an investigation was undertaken to analyse the frequency content of gait kinematic data and the means for noise reduction in the first and higher derivatives of motion data. Hardware and software were then developed in order to perform gait event detection using the gyroscope signal. The sensor was housed within a small and easy to don and doff package. Software was implemented on a portable microcontroller based unit that can be worn by the patient at the waist. The sensor system was evaluated by two groups: able-bodied subjects (n=5) and patients with foot drop (n=3). Data collected from these two studies were used to evaluate and compare the performance of the new sensor to that of the commonly used foot switch using the reference kinematic system. The overall accuracy of the gyroscope sensor system was 96 % in the able-bodied trials and 94 % in the patient trials, where accuracy is the percentage of time where the sensor detects the correct phase as determined by the reference system. The results suggest the practicability of the new sensor system to control the timing of the stimulation. Further testing of the new sensor system is needed to establish its reliability when walking outside the laboratory, and over different terrains. Additional testing by a patient group with a larger size and more varied pathological causes for foot drop would be necessary prior to clinical use of a system based on the Gyro sensor. The design or modification of an existing stimulator to integrate the control unit is also suggested as a follow-up from this study. The shank location of the sensor in the proximity of the stimulation electrodes over the peroneal nerve would allow the design of a self-contained unit that would integrate the stimulator, electrodes, sensor, and control unit. It is believed that this would offer a significant advancement to the current technology.