Saskia Baltrusch

106 Chapter 4 is due to a hard stop that is provided by the device. These findings reveal that the high-cam Laevo provided the user with more support over the range of motion relevant to our task than the low-cam Laevo at both lifting heights, and explains the significant reduction in metabolic costs for the high-cam exoskeleton as opposed to the low-cam exoskeleton. Another important characteristic of both Laevo exoskeletons is the hysteresis effect. The moments provided by the exoskeleton are higher for the flexion phase than for the extension phase, thus support and stored mechanical energy get lost in the system during the movement. This explains the larger effects on back muscle activity found in the static task by Bosch et al. (2016) [28] compared to dynamic task in the present study. The lack of statistical significance in outcome parameters of muscle activation and movement strategy, which underlie the observed reduction in energy cost, shows that individual participants responded differently to the exoskeleton when lifting. Individual participants changed their lifting strategy from squat to stoop to different extents. This resulted in inter-individual differences in COM movement changes, and different changes in muscle activation patterns to arrive at the consistently reduced metabolic costs. Statistical power of this study, however, was not sufficient to perform subgroup analysis. Determining which of the underlying factors accounts most for the observed reduction of metabolic costs requires further research. Walking Wearing the exoskeleton during walking increased metabolic costs by 12% at PWS and 17% at PWSX, confirming our second hypothesis. The significantly slower preferred walking speed with the exoskeleton compared to walking without the exoskeleton indicates that the exoskeleton hinders walking. The changes in movement strategy underline that. Participants shortened their steps when walking with the exoskeleton at the faster speed, suggesting that they probably had to cope with the resistance of the device against hip flexion. This is in line with a previous study [22] which tested the effect of the same exoskeleton on functional performance. The results yield increased perceived difficulty of tasks that involve hip flexion, underlining the importance of the possibility to disengage the device to allow unrestricted hip flexion in walking and similar tasks.