## 4 Discussion
Thus, it can be seen that implicit positive simulation results with actually clear implications that could possibly be derived are not verified by the captured data, as for the M. biceps brachii of subject 1 or M. deltoideus of subject 3. The reasons for these effects could be assigned to multiple error sources, which could be due to the not really error-safe MVC-EMG measurement method or also due to errors in the co-simulation method or the single elements of the model, which cannot be definitively identified at this stage and has to be investigated in further evaluations. Nevertheless, some key findings like the detectable reduction of the M. deltoideus load due to the exoskeleton support, the non-loading of the M. latissimus or even the identification of special aspects like the increase of M. biceps brachii load, which is verified by the sEMG measurement, show the potential of the co-simulation model that it could be used as a tool to adjust parameters of the technical support systems in the virtual worlds in order to derive recommendations for design alterations of the technical support system regarding the user's needs. A validation of the co-simulation is, indeed, still pending. Nevertheless, the simulated results mainly replicate the reality for the investigated use case, implying a good representation of the reality by the co-simulation model. With this verification, alterations of the exo support can be done virtually, which can then be evaluated with co-simulation model. When the desired effects occur for the musculoskeletalhuman model, the new exo support can be realized to result in an improved comfort for the user. The same can also be done with alterations at the power tool in the virtual, e.g., mass alteration or mass redistribution, resulting in a different mass center and inertia, which can then also be transferred to reality. Despite the promising results, limitations must also be pointed out. For an initial evaluation of the co-simulation model, a study with three subjects was conducted. A higher number of subjects would further increase the significance of the evaluation with regard to the variance in the anthropometry of the subjects. In addition, some abnormalities in the simulation results might be rather linked to other reasons with an increased number of subjects. Also, only four muscles were respected when analyzing the effects of the exoskeleton and power tool. There are many muscles around the shoulder joint responsible for the complex motion of the shoulder, which are influenced by the load of the power tool and the assistance of the exoskeleton. However, in this first verification step, the effects for the main muscles, which were also even able to be captured due to the presence of the exoskeleton, should have been analyzed and used for the evaluation of the co-simulation model. The other muscles will be analyzed in future investigations, which also includes the analysis of activation of agonist and antagonist muscles. Furthermore, measurement errors can occur when conducting subject studies. As the selected application involves a very dynamic movement, measurement uncertainties can occur due to the movement. Examples of this could be masking of markers during movement recording and slippage or poor skin contact of sEMG electrodes. Lastly, the strength of the musculoskeletal human models of the subjects was not adjusted due to the high effort and time required for modelling and capturing relevant data, but rather the common scaling process of OpenSim was used. Thus, a comparison of muscle activations in total between simulated and captured data was not conducted. For the evaluation of the co-simulation model, the curves of the simulated muscle forces are qualitatively compared with the measured muscle activation, which only allows for an evaluation of basic effects occurring in the co-simulation method. In the future, the muscle forces of the human model could be adapted for specific subjects to improve the validity of the results through a quantitative comparison.