## 6 Conclusions  
The proposed flexible-rigid design consists of three compression elements (\"pelvis\", \"femur\", and \"tibia\") connected by two flexural joints (\"hip\" and \"knee\"). We simulated the human leg-inspired kinematic behavior of our proposed design within the OpenSim 3.0 \begin{table}
\begin{tabular}{c c c c c} \hline Human vs. Prototype Joint & Stage 1 & Stage 2 & Stage 3 & Stage 4 \\ \hline Hip & 3.1 & 1.3 & 1.9 & 0.97 \\ \hline Knee & 9.4 & 4.5 & 8.8 & 4.7 \\ \hline Human vs. OpenSim 3.0 Model & Stage 1 & Stage 2 & Stage 3 & Stage 4 \\ \hline Hip vs. Prototype Joint & 1.03 & 0.54 & 0.75 & 0.41 \\ \hline Knee vs. Prototype Joint & 8.3 & 7.5 & 7.2 & 6.7 \\ \hline \end{tabular}
\end{table} 
Table 2: Maximum absolute error (%) for each stage of gait. Figure 9: The bottom retro–reflective markers on the human and prototype are the motion–tracked end–effectors. Both end–effectors demonstrate similar behavior, validating the kinematics of the proposed design. During the gait motion, ‘o’ is the start, and the “*” is the end. simulation environment. The purpose of OpenSim 3.0 is to act as a human biomechanics solver, and we bridged and validated the ability to simulate custom musculoskeletal-inspired designs in the same environment. The designs had a similar flexibility and range of motion to the human body for a specific gait-inspired experiment, as demonstrated by the kinematic results for joint angles (Figure 8). For the hip joint, the vectors stretch from the pelvic region to the bottom of the tibia element, and the knee's range of motion used the lowest point of the femur to the tibia for the vectors. The structure was capable of extending the hip joint \(\theta_{hip}\) flexion to approximately 40\({}^{\circ}\), where human hip flexors are able to produce 35\({}^{\circ}\) across the sagittal plane throughout gait motions [41]. In the same plane, the knee joint, \(\theta_{knee}\) was capable of flexing to 110\({}^{\circ}\), which is comparable to a human with 130\({}^{\circ}\) of knee flexion [41]. To further our kinematic analysis, we found the position of the end-effectors (Figure 9) to result in a maximum error of 3.3% between the final positions of the simulations and the prototypes. The behavior of the proposed manipulator was comparable to a human leg; however, this proof of concept was not intended to carry a load where future research could lead to a load-bearing self-standing flexible-rigid structure. The proposed methodology (Figure 4) for designing custom musculoskeletal-inspired models within the OpenSim 3.0 simulation environment demonstrated the potential for accurate kinematic and dynamic relationships between both human and assistive robotic devices. E.J. : project administration, writing, conceptualization; V.L. : visualization, data curation, resources, writing--review and editing; C.C. : software, validation, investigation, writing, visualization; N.C.: software, validation, simulation, formal analysis, data curation, visualization; M.L.N. : software, validation, investigation, formal analysis, visualization, writing; D.C.: electronics, design, validation, investigation; M.T. : supervision, project administration, writing--review and editing. All authors have read and agreed to the published version of the manuscript. This research was supported by a grant made to the Braingeneers research group by Schmidt Family Futures. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript. Under an UCSC IRB Protocol #HS3588, we have been able to collect motion captured data of a person. We collected data on ourselves within this study, and the subject signed written informed consent as part of the UCSC IRB Protocol. Data Availability Statement: Data availability will be given upon request to the corresponding author. The authors would like to acknowledge the technical support of the Braingeneers research group as well as a donation made possible by [PERSON] and [PERSON] by recommendation of the Schmidt Futures program.