Saskia Baltrusch

104 Chapter 4 studies may derive from the different designs of the exoskeleton. While the design of the PLAD provides a connection of the pelvis part to the legs through elastic latex bands, running to the lower legs, the Laevo consists of leg pads on the anterior sides of the thighs. As during lifting the exoskeleton transfers forces from the low back to these leg pads, resistance occurs when squatting, favouring less pronounced squatting with Laevo. A tendency to a decrease in back muscle activity was observed when wearing the Laevo exoskeleton, especially when lifting from ankle height (Figure 7b). This indicates that the required torque of the trunk extensor muscles is partly supported by the torque generated by the Laevo, although this effect was smaller than expected. In addition, we found a small but significant increase in the activation of abdominal muscles when lifting with the exoskeleton, which was especially pronounced in the low-cam condition. This indicates that participants increased abdominal activity to overcome the resistance of the exoskeleton during trunk flexion. It is arguable whether this change in muscle activity influenced metabolic costs, since the increase is rather small. However, the effect on abdominal activation may partly account for reduced beneficial effects of the low-cam exoskeleton on metabolic costs of lifting compared to the high- cam exoskeleton. Bosch et al. (2016) [28] evaluated the effect of the Laevo on muscle activity during an assembly task in a forward bent position. They found that muscle activity in the lower and upper back decreased by 38% and 44%, respectively, when the participants used the exoskeleton. Abdominal muscle activity did not change. The participants in their study had to perform a pick and place task, requiring work in a static position with the trunk bent forward to 40 degrees flexion. Due to the fact that their participants performed a static task, continuous support was provided by the Laevo exoskeleton, which was not the case in the present study due to the dynamic behaviour of the lifting task. This explains the larger effects on back muscle activity found by Bosch et al. (2016) [28] compared to the present study. Finally, Bosch et al. (2016) [28] and Whitfield et al. (2014) [16] found decreased activity in the M. biceps femoris when wearing an assistive device. We did not find any changes in the leg muscles, which may be due to differences between tasks investigated and techniques to perform these tasks.