Virtually walking : factors influencing walking and perception of walking in treadmill-mediated virtual reality to support rehabilitation

Psychomotor slowing, and in particular slow walking, is a common correlate of illness or injury, and often persists long after the precipitating condition has improved. Since slow walking has implications for long term physical and social wellbeing, it is important to find ways to address this issue. However, whilst it is well established that exercise programmes are good approaches to increase movement speed, adherence to therapy remains poor. The main reasons for this appear to be pain and lack of interest and enjoyment in the exercise. Virtual Rehabilitation combines physical therapy with Virtual Reality (VR). This is a rapidly growing area of health care, which seems to offer a potential solution to these issues, by offering the benefits of increased patient engagement and decreased perception of pain. However, the question of how to encourage patients to increase their walking speed whilst interacting with VR has remained unanswered. Moreover, to maximise the benefits of this type of therapy, there needs to be a greater understanding of how different factors in treadmill-mediated VR can facilitate (or hinder) optimal walking. Therefore this thesis investigated the factors influencing walking and perception of walking in treadmill-mediated VR, using a series of empirical investigations to determine the effect of a variety of factors in VR, which can then be applied in a clinical setting. A review of the literature identified that high contrast stereoscopic virtual environments, calibrated to real-world dimensions, with a wide field of view and peripheral visual cues, are likely to facilitate accurate self-motion perception. Empirical studies demonstrated that decreasing the visual gain (ratio of optic flow to walk speed) in VR can lead to a sustained increase in walk speed. However, these lower rates of visual gain are likely to be perceived as unrealistic, and may decrease immersion. Further investigation demonstrated that there is a range of visual gain which is perceived as acceptably normal, although even the lower bound of this acceptable gain is still higher than the optimum gain for facilitating faster movements. Thus there is a trade-off between visual gain for realistic perception, and visual gain for improved walking speeds. Therefore other components that can improve walking speed need to be identified, particularly for those applications where reduction of the visual gain is undesirable. Further empirical studies demonstrated that fast audio cues (125% of baseline cadence), in the form of a footstep sound, can increase the walk speed without disrupting the natural walk ratio. This effect was demonstrated in healthy populations, and also shown to be evident in a group of patients with chronic musculoskeletal pain. It was noted that in all the studies comparing a pain and non-pain group, the pain group walked more slowly across all conditions. Additional empirical studies demonstrated that the use of self-paced treadmills for interfacing with VR was found to be associated with somewhat lower baseline walk speeds than normal overground walking, although the self-paced treadmills preserved the normal walk ratio. This slowing of walking and preservation of walk ratio was seen in both healthy participants and also in participants with chronic musculoskeletal pain. Therefore, whilst self-paced treadmills can support natural walking, additional factors need to be considered if treadmill-mediated VR is to be used to facilitate the increase in walking speeds desirable for rehabilitation. Thus designing VR for rehabilitation is likely to involve consideration of a number of factors, and making individualised design decision based on specific therapeutic goals.