Saskia Baltrusch

193 Chapter 7 4 Discussion The aim of this paper was to assess the effect of wearing a novel spinal exoskeleton (SPEXOR) on metabolic cost. We found that the exoskeleton reduces metabolic cost of repetitive lifting by 18 %. Using this exoskeleton during repetitive lifting tasks might therefore reduce the risk of fatigue-related injury in the low back. A reduction in metabolic cost can be caused by the exoskeleton taking over part of the mechanical work that has to be generated by the muscles to perform the lifting task. We therefore investigated the contribution of the exoskeleton and the muscles to the mechanical work generated around the hip and L5S1 joint during lifting. The results show that the exoskeleton generated a substantial part of the total mechanical joint work at the hip and L5S1 joint, taking over part of the mechanical work that otherwise had to be generated by the muscles around these joints. At the respective joints, the exoskeleton generated up to 18 % of the required work during the unloaded phase and up to 25 % during the loaded phase. This is equivalent in magnitude to the reduction of metabolic cost (18 %). Although the relation between mechanical work and metabolic work is not necessarily one-to-one, this suggests that metabolic cost is reduced by the exoskeleton taking over muscular work at the hip and the L5S1 joint. Despite the significant contribution to joint work of the exoskeleton, a significant reduction in muscle work was only found for the unloaded phases at the L5S1 joint. The loaded phases at the L5S1 joint and both phases at the hip joints showed the same trend of reduced muscle work, but this did not reach significance. This can be explained by the variation in individual participants’ behaviour as can be observed in figure 7. Individual participants distributed mechanical work differently over the knee, hip and L5S1 joints, between conditions. This individual variation in work distribution over the joints precludes the statistical significance of the mean reduction in muscle work, appearing around the separate joints. Summing up muscle work could be an option to avoid this problem. However, potential transport of muscle work between joints through bi-articular muscle, does not allow for such a simple summation [30]. Still, the trend towards lower muscle work around the joints and the significant work done by the exoskeleton indicates that a transfer from active muscle work to the passive work of the exoskeleton can explain the reduction in metabolic cost. 7