# From Knowledge to Leverage: How to Use Musculoskeletal Simulation to Design Exoskeleton Concepts
## 4 Discussion  
The example demonstrates how a systematic approach to musculoskeletal simulation can spark ideas for exoskeleton design and allow the designer to arrive at optimal design parameters in a single or a few iterations performed in silico. The general nature of the approach indicates that it has potential to be used for design scenarios beyond the chosen example. The presented methodology tends to minimize muscle exertion rather than joint loads because the objective function (1) is formulated in terms of forces. However, because more of the joint load in the human body is typically generated by muscle actions than by external forces, design choices that minimize muscle effort also tend to reduce joint reaction forces, as we can see in the example. It should be noted, though, that the offloading of some body parts by an exoskeleton may lead to increased loads on other body parts, so a practical design process should consider body loads more comprehensibly than what  
Figure 8: Idealized rod reaction forces and a selective trend line covering only the descending part of the two curves. The arrows indicate the direction of movement. Figure 7: Patella–femoral and L5–S1 reaction forces with idealized bilateral linear actuators. Figure 9: Patella–femoral and L5–S1 reaction forces when supporting the body with bilateral, spring-loaded rods. was demonstrated here, and a method to systematically evaluate all simulated tissue loads relative to their normative loads should be devised before applying this method to practical exoskeleton design. Other examples may reveal situations where no approximately linear relationship between a degree of freedom and the required force can be identified, in which case the design task would include creation of nonlinear forces and/or active actuator strategies. This obviously leads to more complicated designs, but the insight would enable the designer to quickly move on from the search for linear spring solutions to more advanced options. In Figure 3, the linear regression is not a very close representation of the data, so it is surprising that the result of using the approximated spring presented in Figure 5 is almost as good as the result of using the idealized joint motors in Figure 4. To understand this, we should bear in mind that the idealized moments shown in Figure 3 are costless for the algorithm and therefore prone to large fluctuations with small benefits. It is possible that a hybrid formulation with a small activation cost on the idealized joint motors could produce a clearer guideline for the designer. Investigation of this idea is left to future work. The assumption of a passive exoskeleton can be made when the results of Figures 3 and 8 are subjected to linear regression. Without linear regression, the results could indicate baseline actuation strategies for active exoskeletons, but this would require advanced dynamic control systems, the regulation of which cannot be inferred from these simulation results. The reliance on inverse dynamics simulation causes the inherent assumption that the movement is independent of the presence of the exoskeleton; i.e., after donning the exoskeleton, the user will complete the work task exactly as it was measured in the first place. This is likely false to some extent and is a limitation of the method that is hard to mitigate until a physical prototype has been created much later in the process. Only practical experience with the method in real design processes can reveal whether this limitation is problematic. Whether the exemplified exoskeleton concept will work in practice can only be investigated by further design iterations and physical tests. In a practical design case, simulation should be performed in a variety of movements, including normal walking, to investigate the influence of the exoskeleton beyond the single movement for which it was designed. In particular, it may turn out that the exoskeleton prevents certain important postures like deep knee squat, in which case it must be modified.