## 4 Discussion  
The aim of the provided publication was the setup of a co-simulation model from a musculoskeletal human model and models of technical support systems (in this case an exoskeleton and a power tool) in order to enable the derivation of design options for the technical support systems. The potential of the presented co-simulation model is highlighted when analyzing the simulation results and by comparing the simulation data from the co-simulation model with measurement data collected in a subject study. Figure 9: Comparison of simulated muscle forces (solid lines) and measured muscle activation via MVC-sEMG (dashed lines) for the three support levels at each subject. Comparison of the courses of the simulated muscle forces with the measured muscle activations of the selected muscles (**a**–**d**) for all subjects considering all three support levels (no support, 50% and 100% support) and the absolute support forces of the exoskeleton (**e**). A comparison of the simulated muscle forces at all three support levels for all three subjects showed that the results are plausible. For muscles such as the M. deltoideus shoulder muscle (see Figure 8), a reduction in the user load due to the exoskeleton support is to be expected, as the exoskeleton used is a shoulder support system [1]. This is also reflected in the simulation. The fact that other muscles, such as the M. biceps brachii, sometimes experience increased muscle forces due to the support can also be considered plausible. The use of exoskeletons can sometimes cause higher strain on muscle areas that are not the focus of the relief. Another explanation could also be that the human neuromuscular control strategy alters. With varying boundary conditions, the use of other muscles for the same motion can be the effect. According to the use case of overhead drilling, subjects may rely in the non-supported situation more on one muscle and with increasing exoskeleton support, another muscle can be recruited more. Thus, not only a reduction in load is accomplished by an exoskeleton, but also a re-structuring of muscle recruitment patterns can occur, which has to be aware during design of the exoskeleton. There are, however, some exceptional simulation results differing from other results like the results for the M. deltoideus in the supported situations (50% and 100% support level) for subject 2 compared to subject 1 and 3. At this stage, the co-simulation model is mainly able to reproduce the reality into the virtual world. In a next step, the co-simulation model can be used to derive design alternatives for the technical systems from the simulation into the reality. Thus, the co-simulation model should ideally replicate the curves of muscle loads in the simulation, i.e., the real occurrences in the simulation by the co-simulation model. In many cases, the curves are in good agreement, especially regarding the principal shape of the curves. Special aspects, such as a strong increase in the simulated force of the M. biceps brachii with a support level of 100% for subject 1 and for subject 3 at 50% and 100% support level (see Figure 9) are usually also shown in the measured data. Other implications like the poor influence on the M. latissimus by the use case itself and further by the assistance of the exoskeleton are also shown in the simulation. The overhead drilling depicts a use case, which does not really cause load for the M. latissimus, which is shown by the sEMG. Indeed, it is comparatively easy to capture the M. latissimus via sEMG, respecting the use case with exoskeleton, and, thus, an interesting information, whether the co-simulation model simulates any non-existing loads in the musculoskeletal human models. Due to the correct simulated representation of low loads on M. latissimus, in the further evaluation more loaded muscles in the back at the investigated use case will be analyzed, like the M. levator scapula. However, in some comparisons there are also some deviations noticeable between simulation and measurements.