## 4 Discussion  
Prosthetic foot stiffness category significantly impacts biomechanical variables; however after some accommodation time, the user's perception of stiffness becomes inconsistent [21]. While a few studies have evaluated prosthetic foot functions using quasi-static machine testing methods, limited work has been done on dynamic testing [22]. In addition, studies focusing on mechanical testing have been limited to heel and heel loading in separate motions [23; 24]. [PERSON] et al. showed changes in roll-over characteristics between different prosthetic feet for level-ground ambulation using a mechanical testing apparatus [25]. Similar studies have investigated the impact of prosthetic foot sagittal stiffness on trans-tibial amputees by measuring oxygen consumption and ankle push-off power [26; 27]. This study presents a novel approach to evaluating prosthetic feet for ramp ascent and descent using a mechanical roll-over test. In the comparison of biomechanical test results and test machine simulation for level-ground walking, we found a strong correlation between the ankle angles during dorsiflexion. However, the plantarflexion angle was larger in the machine test than in the biomechanical test, which may be because the machine test was performed without shoes, while the biomechanical test was performed with shoes. The cushioning provided by shoes might have affected the correlation between the two test methods. For ramp ascent, the maximum dorsiflexion prosthetic foot angles obtained during machine and biomechanical testing were comparable between the three stiffness conditions. However, the plantarflexion angles were lower in biomechanical test data versus the machine test data. This result could be explained by the participants' gait strategy for ramp ascent to ensure flexion angles and moments up the movement chain of the lower limb during weight acceptance [28]. This may be achieved by, for example, initiating a stance with the foot flat to ease the moment on the knee. Figure 10: Functional joint center location for three stiffness settings, condition 1 (softest), condition 2 (mid), and condition 3 (stiffest), calculated for machine test and biomechanical tests during level-ground walking. During ramp descent, a variability was seen in plantarflexion between the two test methods. The shoe heel cushion likely affected this result. Additionally, the input on the tilt-table angle may have a significant impact on the increased sagittal moment on this terrain. A lower tilt-table angle at heel strike would generate a decreased sagittal reaction moment, corresponding more to the biomechanical test data. The functional joint center is an additional parameter to help interpret the prosthetic foot response during roll-over. Our calculation method of the functional joint center can describe its change of position, depending on the prosthetic foot stiffness. However, the FJC position calculated using the machine was on average, 4.5 cm more posterior and 1 cm more proximal than the FJC position calculated using level-ground biomechanical gait measurements. The FJC position moved proximally and posteriorly with decreasing stiffness of the VSA. This change in position can be explained by the decreasing stiffness constant of the VSA ankle unit from stiffest to softest, as a result of which, the FJC moves closer to the VSA pivot point. Our calculation of the FJC position is sensitive to the marker position collected and is limited to the sagittal plane. The novelty of this study is the use of a machine-based roll-over test for a prosthetic foot and evaluation for three types of terrains, level, ramp ascent, and ramp descent, for three different prosthetic foot stiffnesses. To the best of our knowledge, this is the first study providing a machine test input for common functional tasks such as incline/decline walking.