# From Knowledge to Leverage: How to Use Musculoskeletal Simulation to Design Exoskeleton Concepts
## 3 Simulation-Based Design
### Knee and Spine Exoskeleton  
At this point of the design process, it would be obvious to repeat the methodology for the spine, i.e., equip the three-dimensional rotations between the pelvis and the thorax with idealized motors, examine the relationship between angles and required support moments,  
Figure 4: Patella–femoral and L5–S1 reaction forces with idealized knee extension motors.  
Figure 5: Patella–femoral and L5–S1 reaction forces with a passive spring-loaded exoskeleton on the knees.  
Figure 3: Idealized knee motor extension moments as functions of the knee angles; trend line. The arrows indicate the direction of movement.  
and design a passive, spring-loaded upper body exoskeleton based on these specifications. However, designing a three-degree-of-freedom exoskeleton for the spine is significantly more complicated than a single degree-of-freedom spring for the knee. Furthermore, we can see from Figure 2 that the patella-femoral forces and the spinal compression forces peak simultaneously in the beginning of the movement, where Figure 1 shows that the knees and the spine are simultaneously flexed. From this knowledge, we conceived the idea that a multiarticular exoskeleton spanning the knees and lumbar spine simultaneously may be a simpler solution for both body parts.  
This can be obtained by bilateral rods spanning points on the shank and thorax, as depicted in Figure 6. The distal parts of the shanks have relatively little soft tissue and are well-suited for anchoring such rods, for instance, to special-made boots, while harness-like straps can affix the proximal ends of the rods on the thorax.  
In the formalism of the multibody simulation system, these rods represent measurable degrees-of-freedom to which reaction forces, i.e., forces of category 2, can be assigned. This simultaneously offloads the spine and knees, as shown in Figure 7. We can see that the potential for knee offloading is significant, albeit smaller than with the designated knee exoskeleton. We can also see that the spinal compression is drastically reduced and now peaks in the last third of the movement.  
Figure 8 shows the relationship between idealized rod forces and rod length. We see that the relationship is close to linear, except for a sharp increase towards the end of the movement. However, the latter applies to the upright standing posture for which Figure 2 reveals little need for support, and we can consequently disregard this part and create a common trend line only for the descending parts of the two curves in Figure 8. This trend line has a slope of \(-604\) N/m, which is the ideal spring constant, and it crosses the first axis at rod length 1.57 m, which is therefore the ideal slack length of the spring. Replacing the idealized motor (category 2) in the rods with springs (category 1) of these characteristics yielded the joint reaction forces of Figure 9, which in comparison with Figure 2 shows reduction in all the reaction forces of interest.  
Figure 6: Addition of bilateral rods spanning knees and the lumbar spine.