Saskia Baltrusch

195 Chapter 7 A reduction in metabolic cost can also be a result of a changed lifting behaviour as shown in previous studies [20, 21]. Using the SPEXOR exoskeleton in the present study, participants did not systematically change movement behaviour when wearing the exoskeleton. Neither individual joint angles nor the movement of CoM, representing the summation of effects of the different joint movements, was different between the conditions. The COM displacement is an important determinant of work performed in lifting and hence of metabolic cost. Changes in kinematics can therefore not explain the change in metabolic cost. Low interference with tasks and minor hindrance by the SPEXOR exoskeleton when performing work-related tasks has been shown in a previous study [34] and might explain why the movement behaviour without and with the exoskeleton remained the same. Despite the relatively high mass of the exoskeleton (6.7kg), metabolic cost still decreased. This can be explained by the fact that the mass of the exoskeleton is close to the users’ COM and therefore has less impact on the demand of lifting. The exoskeleton’s mass may, however, affect comfort. Baltrusch et al. (2020) [34] recommended that the mass and the dimension of the same exoskeleton should be reduced to prevent from pressure points on the hip. This, however, was mainly a problem for walking. Squatting and lifting showed lower discomfort values. The reduction in metabolic cost is in line with a previous study of Baltrusch et al. (2019) [20], in which it was found that the Laevo exoskeleton reduced metabolic cost during repetitive lifting by 17 %. The Laevo exoskeleton generated a torque of up to 23 Nm around the lumbar joint [14], whereas the SPEXOR exoskeleton generates a torque of up to 50 Nm around the L5S1 joint and up to 25 Nm around the hip joint. Given the higher support level, we expected a bigger effect on metabolic cost when wearing the SPEXOR exoskeleton, compared to wearing the Laevo. Still, the reduction of metabolic cost was about the same for both exoskeletons. This might be related to the fact that participants changed to a less demanding lifting strategy when wearing the Laevo and did not change movement behaviour with the SPEXOR. However, more research is needed to assess the difference between these two systems. The results of this study should be interpreted in the light of some limitations. The lifting task was performed in a laboratory and its duration was chosen to reach steady state. In a real work environment, lifting tasks can last longer and 7