# Hardware-in-the-Loop Test of a Prosthetic Foot
## 4 Experimental Results
### Discussion  
The results revealed that the gait characteristics of the HiL test qualitatively resembled those of the VPP model. This implies that the dynamic interaction between the prosthetic foot and the amputee can be replicated by a HiL setup. Nevertheless, there were some limitations in the current implementation that prevented accurate mimicking of human gait characteristics.  
Figure 10: Total hip torque normalized over body weight and leg length during the stance phase of the right leg.  
Figure 9: Measurement data during the HiL test. (**a**) The GRFs in the \(x\) and \(y\) direction during the stance phase of the right leg. The black dashed line indicates the body weight. (**b**) The desired and real displacement of the Stewart Platform’s Leg 1 are shown. Leg 1 undergoes the largest motion command of all six legs.  
The biomechanical parameters used in this study had to be selected based on several hardware limitations of the Stewart Platform and the suspension frame. These parameters represent a light person (\(m_{\text{VPP}}=30\,\text{kg}\)) with long legs (\(l_{\text{0}j}=1\,\text{m}\)) and a relatively steep angle of attack (\(\alpha_{0}=75^{\circ}\)), which is biomechanically not meaningful. Neither, this test was able to replicate the gait dynamics of amputees observed in gait laboratories (asymmetry, energy consumption, etc.). Nevertheless, this study did not claim to quantitatively emulate a walking amputee, but rather to investigate whether a HiL test of a human wearing a prosthetic foot was feasible.  
The VPP model was a good basis to model the amputee for this first proof-of-concept study. In the future, making further adaptations to the implemented VPP model is desirable. For example, the initial conditions should be tuned. Furthermore, the trajectory of how the CoP shifts from the heel to the toes could be implemented based on observations in humans. Furthermore, using more complex models that more realistically model human gait is targeted. A suitable model could be the neuromuscular-skeletal model by [PERSON] and [PERSON] [29], which includes the foot geometry and models the legs more realistically.  
Even though this first HiL test worked in the sense that basic gait characteristics could be replicated, no final conclusion can be drawn whether HiL is a suitable future testing method for prosthetic feet. As next steps, the VPP model can be further tuned (e.g., mass, stiffness, initial conditions) or changes to the prosthetic foot made (mounting, e.g., with different rotations or shifting it forward/backward). With these tests, we wanted to investigate whether changes in the gait dynamics of the modeled amputee were visible. If this were the case, HiL tests could be a meaningful means to understand the dynamic interaction of humans and prosthetic feet better and improve foot prostheses. From these first results, the following requirements for prostheses testing with HiL can be inferred: Powerful hardware is required in order to replicate the dynamic motion. This includes, i.a., a stiff actuator, high computation power, and a stiff suspension frame. Furthermore, the accuracy of the gait characteristics was just as good as the underlying model of the amputee.