The reality of myoelectric prostheses : how do EMG skill, unpredictability of prosthesis response, and delays impact on user functionality and everyday prosthesis use?
Myoelectric prostheses are designed to provide cosmesis and a degree of upper limb functionality for people with upper limb absence. However, self-reported rejection rates remain stubbornly high, with control of the prosthesis being commonly cited as one of the primary reasons. This observation may indicate that the significant engineering efforts aimed at improving prosthesis control may not have been addressing the most important issues. Surprisingly, there has been no empirical work outside of lab environments to understand the relative importance of key factors affecting prosthesis control. This thesis explores the impacts of three factors: (1) user skill in controlling an EMG signal, (2) unpredictability of prosthesis response introduced at the interface between the electrodes and the skin, and (3) the electromechanical delay in the prosthesis, on user performance, quantified in terms of: (1) functionality (kinematic and gaze), and (2), for the first time, everyday prosthesis use. Chapter 1 introduces the thesis, followed by Chapter 2, which contains a review of existing literature relating to the factors affecting control of myoelectric prostheses. Chapters 3 reports a protocol for the assessment of the impact of skill, unpredictability and delays on user functionality and real world use of a prosthesis. Chapter 4 introduces the first method for the visualisation of time series data from wrist worn accelerometers and presents the first time series data on everyday prosthesis use. Chapter 5 presents results of a study, which recruited 20 trans-radial myoelectric prosthesis users from 6 centres across the UK, drawing conclusions as to the relative impacts of each control factor on performance. Results suggest unpredictability introduced at the electrode-skin interface by the socket mounted electrodes may be the key factor affecting control. Additionally, the results show the delay to the onset of hand opening from a fully closed position to be approximately double the delay measured from any other starting hand aperture. Chapter 6 reports on upper limb activity in the 20 trans-radial prosthesis users and 20 anatomically intact participants. The results show that, by contrast to the anatomically intact participants, upper limb activity of prosthesis users is heavily biased towards the intact limb. Finally Chapter 7 summarises the main findings of the thesis, addressing limitations and suggesting future work.
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