To date, myoelectric prostheses offer a limited replacement for the motor function that is missing in upper limb amputees (2). In the case of trans-radial amputation, most common myoelectric prostheses provide 1 or at most 2 controllable DoFs to compensate for the motor function of the hand and wrist joint. Although devices that provide a higher number of DoF have very recently become commercially available (e.g. i-Limb™ Ultra from Touch Bionics) (90), they are still controlled by at most two independent EMG signals.
In addition to the large number of DoFs in the anatomical hand, sensory feedback from different modalities including vision and proprioception plays an important role in the function of the anatomical hand. Despite research studies to enhance sensory feedback via tactile stimulation (162, 163), none of the commercially available prostheses provide feedback to the user, thus amputees have to rely on vision to monitor the movement (9).
The factors discussed above may provide some of the explanation for the high rate of myoelectric prosthesis rejection (23%) that has been reported recently in adult users (3); a figure that is similar to what was reported more than 20 years ago (3). Prosthesis rejection
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implies that unilateral amputees would have to rely heavily on the remaining non-amputated arm to perform manual tasks (25, 164).
The poor level of functional restoration offered by current prostheses may explain a number of problems experienced by upper limb amputees. For instance, apart from their amputation, most upper limb amputees are otherwise healthy and of working age. However, a recent study showed 39.7 % of a sample of 307 upper limb amputees were not able to work any longer (165), because of the decline in their functional ability after amputation and insufficient functional return from the prosthesis. Unsurprisingly, in general due to the functional limitations of current prostheses, most amputees who return to work after amputation take part in occupations that are not physically demanding (165-167).
Clinically, the increased work load on the remaining arm may leave unilateral amputees more vulnerable to overuse injuries in the non-amputated arm (168). In fact, a number of studies reported a high incidence of overuse-related injuries in unilateral amputees (169-172). Further, a study has suggested other benefits of wearing and using a myoelectric prosthesis wear, namely a reduction in phantom pain (157), implying that rejection of this type of prosthesis results in a secondary cost.
In order to evaluate upper limb prostheses, a number of tools have been developed over the years. Those can be broadly categorised into two groups: tools for measuring a user’s performance on particular functional tasks and questionnaire or interview-based tools to evaluate, for example, users’ perceptions of their prosthesis and the extent to which they make use of their prosthesis (4). Useful information regarding prosthetic hand performance and usage can be determined with such evaluation tools and they are well-suited to comparison studies. However, none provide detailed insight into the mechanisms by which a user learns to use their prosthesis and provide very little information with which to inform the design of new prostheses or new training regimes. For example, none of the clinical tools provides insight into the detailed motor control strategies or the attentional demands associated with prosthetic use.
As has demonstrated in this chapter, the motor control literature offers a solid platform from which to explore the process associated with learning to use a prosthesis and the differences in behaviours between healthy and prosthesis functional performance. Despite work in the area of upper limb motor control in prosthesis users carried out in the early 1980s (5, 6), there have
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been surprisingly few studies describing the characteristic changes in motor behaviour, and no previous work on visuomotor behaviour, associated with learning to use a prosthesis. This is despite the widespread agreement regarding the importance of vision in prosthetic use (6-10). Further, nothing is known about the relationships between visuomotor skill level and more clinically relevant measures, such as usage of the device in everyday life and acceptance of the prosthesis. Studies in these areas may lead to the development of improved outcome measures, improved designs and new training approaches.
This thesis, therefore, aims first to explore the changes to visuomotor behaviours when a myoelectric prosthesis is introduced and over the course of learning to use it. By studying this, the characteristics that change with practice and hence reflect skill acquisition (skill measures) may be identified. In this thesis, skill is defined as “the learned ability to bring about pre-determined results with maximum certainty, often with the minimum outlay of time, of energy, or of both” (173).
Due to the limited number of trans-radial amputees and to avoid burdening newly amputated individuals the core investigation of this thesis was in anatomically intact subjects. However, the results were subsequently validated in a small sample of trans-radial myoelectric prosthesis users. Moreover, in the later investigation in trans-radial myoelectric prosthesis users the relationship between current clinical evaluation tools and proposed measures of skill acquisition was examined.
However, prior to exploring visuomotor characteristics, the following chapter will discuss the approach to characterising gaze behaviour in a manual task. In this chapter, a clinically relevant manual task that will be used to explore visuomotor behaviour changes is identified and finally the development and validation investigation of a coding scheme for gaze data analysis are discussed.
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