Saskia Baltrusch

103 Chapter 4 4 Discussion The main goal of this study was to evaluate the effect of a passive trunk exoskeleton on metabolic load during lifting and walking. As hypothesized, we found a decrease in metabolic costs when wearing the exoskeleton during lifting and an increase in metabolic costs when wearing the exoskeleton during walking. Lifting Our results reveal that wearing the exoskeleton during lifting decreases metabolic costs. However, we only found a significant difference with respect to the control condition when using the high-cam exoskeleton, which yielded reduced metabolic costs of up to 17%. Wearing the low-cam exoskeleton metabolic costs showed a modest reduction of up to 8%, which did not reach significance. Our results are not in line with Whitfield et al. (2014) [16], who did not find a change in oxygen consumption when lifting a mass of 9 kg for 15 mins with a personal lift assistive device (PLAD). A possible explanation can be found in Sadler et al. (2011) [17], who tested the effect of the same device (PLAD) on lifting technique and found greater hip flexion and less lumbar flexion. This suggests a change from a stoop towards a more squat-like technique when using the device. Since this squat technique requires more metabolic energy than a stoop technique, due to higher muscle activity in the legs, these findings might explain the lack of effect on metabolic costs when wearing the PLAD system. This is in line with Whitfield et al. (2014) [16] who suggested that the lack of effect could be caused by the fact that some muscles may have been assisted, while other muscles had to work harder when wearing the device. In contrast to the results found for the PLAD system, in the present study participants changed their lifting technique to a stoop-like technique when using the exoskeleton, reducing their COM movement amplitude. The reduced downward motion of the COM requires less mechanical work to be generated against gravity. To compensate for the decrease in COM movement, participants possibly extended their arms more at the lowest point of the lift to pick up the box from the designated height. This change of movement strategy may contribute to the significant decrease in metabolic consumption when lifting from ankle height that we found in our study. These differences in response between 4