## 4 Discussion  
The evaluation results of the physical strain and human-machine interaction showed that an adaption of the support level to the motion phases was expected for the two selected tasks. The screw phase was the most strenuous and required the highest support level compared to the arm lifting and lowering phases, while the maximum support level was  
Figure 15: An overview of the interaction force (\(y\)-axis in N, \(x\)-axis in%-Cycle) between the arm and armrest. Two red dashed lines enclose the arm lifting phase, while two blue dashed lines bound the arm lowering phase. The red circles highlight the peaks of the interaction forces, which were observed at the beginning of the arm lowering phase. not always necessary in the other two phases and was even perceived as resistance by some participants, especially when arm lowering. It was interesting to note that aside from the arm elevation angle, the electric current of the screwdriver was also a promising means of determining the switching time between the motion phases to adjust the support level. Both the subjective perception and the EMG activities showed the overall exertion and the exertion of the dom. shoulder during the two overhead tasks. The screw phase was the most strenuous in both tasks. There were different push force levels of the participants and a general higher push force level in task 2 due to the higher torque compared to task T1. The mean push force was generally not affected by the support level of the exoskeleton for participants P1, P4, and P5 during the screw phase in both tasks. Only participant P3 showed a lower mean push force with increasing support levels. This indicated that the participants experienced the same load for all three support levels in both tasks, which was a precondition for the comparability analysis. The relative reductions in the shoulder muscle activities as well as the reduction in the perceived exertion under the supported conditions indicated a positive effect of the shoulder exoskeleton Lucy on the user, which is consistent with previous studies of similar overhead tasks [10; 16]. The perceived physical strain reported by the participants revealed a stronger relief in S100 than in 550 in both tasks. However, support level S100 led to a stronger reduction in shoulder muscle activity than support level S50 only in T1, and not in T2. This may be related to the different physical loads of the two tasks and their required support. Task T2 was more physically demanding than T1 because of the higher torque needed and the longer duration of the screw phase. The perceived support during the screw phase indicated that improving the support level from S50 to S100 was sufficient for T1 but still not enough for T2. Some participants rated the support level S100 as too high when lowering the arms. This was also observed in previous studies for other manual activities [16]. An increase in the activation of the non-dominant LD was particularly noticeable for condition S100 when lowering the arms. Furthermore, an increase in the interaction force between the dominant arm and the armrest of Lucy was observed in most cases. According to the results, some potential optimization can be made for motion phases of screw and arm lowering to extend the use cases of Lucy. An additional or an enhanced support force to supplement the push force during the screw phase was expected for T2. An additional mechanical structure for coupling the exoskeleton and the screwdriver or a stronger cylinder are possible approaches. The mechanical coupling could allow for further application-oriented optimization of the support performance of the exoskeleton. However, it must be investigated how a stronger support force affects the load on the wrist. First, simulative studies on new support concepts already exist and are so far quite promising [11]. The mechanical structure should apply additional support forces directly to the power tool during the screw phase.