Circumferential compression of the lower limb and implications for soft exoskeleton design

A variety of pathologies impair human gait by reducing the ability to control the lower limbs, which causes the need for gait-assistive devices. The increase in interest and the advances in wearable-robot technologies have given rise to exoskeletons for locomotion assistance and gait rehabilitation, or strength and endurance augmentation. Recently, a whole new generation of soft exoskeletons has emerged that addresses technological and usability challenges of traditional exoskeletons. The growing use of soft wearable robotics among vulnerable user populations is increasing the need to determine user centred design standards and ergonomics aspects of the physical contact between soft exoskeletons and humans. Excessive mechanical loading of tissues can cause discomfort and pressure-related soft tissue injuries. The risk for tissue damage depends on the nature of mechanical loading and the nature of the soft tissues affected. Existing approaches to assessing potential risks to tissue viability were reviewed, with the focus on pressure-related pain perception (Reviews 1 and 2) and deep tissue oxygenation (Review 3). The relationship between circumferential compression magnitude, duration, frequency, anatomical and mechanical properties of compressed tissues, muscle oxygenation, and discomfort/pain perception was investigated experimentally at the lower limb using a computerised cuff inflation system with pneumatic cuffs to simulate soft exoskeletons (Studies 1-3). During compression, discomfort was continuously rated on an electronic Visual Analogue Scale, and deep tissue oxygenation was continuously monitored using Near-infrared spectroscopy. Assessments were performed at different assessment sites in static (standing) and dynamic (walking) conditions. Compression was best tolerated at the calf, using narrower cuffs and intermittent inflation pattern. Pressures that caused discomfort and pain were lower during walking than standing still. Intermittent cuff inflation caused an increase in muscle oxygenation. Finally, the relationship between pneumatic cuff inflation pressure and interface pressure was established for different assessment sites and cuff widths (Study 4). Using regression equations, interface pressures can be predicted for specific assessment sites and cuff widths. This thesis has contributed to informing user-centred design and ergonomics evaluation of soft exoskeletons, specifically in relation to contact pressure. On the basis of these findings, further research can now be performed involving vulnerable groups, such as older age adults and patients with neuromuscular disorders, with minimised risk of soft tissue injury.