Saskia Baltrusch

107 Chapter 4 While the exoskeleton did not have any effect on back muscle activity during walking, effects on abdominal muscle activity were found. When walking with the exoskeleton the activity of m. rectus abdominus and m. external oblique increased significantly. This is another indication of the impeding effect of the exoskeleton during walking. Participants increased their abdominal muscle activity to cope with the resistance against hip flexion. When flexing the hip, the abdominal muscles stabilize the pelvis in the sagittal plane and prevent it from anterior tilting by the downward pull of the hip flexor muscles [30,31]. In case of restricted hip flexion, participants probably also increased activity of their hip flexors. However, hip flexor activity is difficult to assess with surface EMG. In summary, walking with a passive trunk exoskeleton increases metabolic cost. Indicators for restricted hip flexion were found in both movement strategy and muscle activity. However, it is unclear whether the increase in metabolic costs can be solely explained by these indicators. Limitations Our protocol was performed in a laboratory setting and limited to a lifting time of 5 minutes, unlike real-life work settings, where lifting tasks are often much more variable in terms of technique and frequency. Thus, the outcome of this study cannot be directly generalized to a normal working environment, further studies are needed to assess the effect of a passive trunk exoskeleton on metabolic costs during a whole working day. Furthermore, due to data loss in the kinematic and EMG analysis, the statistical power for these parameters was lower than for our main outcome, metabolic cost, which explains why we found a significant effect in our main outcome, but only trends in the underlying mechanisms. Finally, due to the different designs of the various exoskeletons that are currently assessed in research, we cannot generalize our outcome to other assistive devices, since effects are dependent on the design of the exoskeleton. 5 Relevance and conclusion We have shown that wearing a passive trunk exoskeleton decreases metabolic costs during lifting and increases metabolic costs during walking. The remaining question is whether the observed effects have a meaningful effect on fatigue in daily practice. People tend to operate at ~36% of their maximal aerobic capacity 4