## 1 Introduction  
The human foot is a complex mechanical structure containing twenty-six bones, thirty-three joints, and more than a hundred muscles, tendons, and ligaments [1]. By comparison, while their aim is to replace lost function of the biological foot, prosthetic feet are simpler and less adaptive. Energy-storing-and-returning (ESR) prosthetic feet are regularly fitted for medium to high active users. Since their first introduction in the 1980s, the design of ESR prosthetic feet has evolved and more complex devices have been engineered, fine-tuning their benefits [2, 3]. During gait, the same basic principle can be applied to ESR prosthetic feet, where the composite leaf springs in the device store energy during load application and subsequently return the energy [4]. The stiffness of the elastic structures in the prosthetic foot depends on the geometry and materials used, so each prosthetic foot design has its own stiffness characteristics dictated by its design. During our research, we counted more than a hundred different ESR prosthetic feet available for lower limb amputees [4; 5]. The large number of devices makes it difficult for the certified prostheticist and orthotist (CPO) to select the most appropriate prosthetic foot for each user. Every prosthetic foot type has its own stiffness category selection chart, which complicates the decision-making process. The CPO needs to select the \"right\" stiffness category by considering the user's weight, mobility level, and impact level. Next, the CPO's work continues with the alignment process, which needs to be adapted for each user based on visual evaluation of their gait and feedback. Advanced measurement tools are available to quantify the changes in alignment [6; 7]; however, the success of the alignment depends highly on the dialogue between the prostheticist and the user to find the optimal adjustment settings. Previous studies have measured the effect of ESR prosthetic foot stiffness on amputee gait [8; 9]. However, the mechanical evaluation of prosthetic feet can be challenging as stiffness is design dependent [10]. Advanced test equipment for prosthetic foot testing could help during the design process and reduce the burden of user testing. Data collection on a machine representing values closer to biomechanical data during different gait tasks would help the communication between design engineers and biomechanical engineers. The ESR prosthetic feet have typically no evident center of rotation or articulation compared with the anatomical ankle joint, making it difficult to relate the investigation of stiffness characteristics to the mechanical deformations during the stance phase [11]. The functional joint center location provides a visual indication of the virtual ankle joint and depends on prosthetic foot design [12]. It is not clear how sagittal stiffness and functional joint center position are related. In this study, our objective was to assess two methods of collecting data on the same prosthetic foot. State-of-the-art biomechanical analysis of amputee gait for level-ground and ascending and descending ramp walking was compared to a mechanical test machine simulating each gait task. A quality machine assessment can be beneficial for researchers to compare different prosthetic foot designs. Additionally, the user testing could be reduced during the design phase or stiffness characterization and therefore provide an additional tool for prosthetic foot designers. Our specific aims were to (1) propose load and angle profile data for the machine roll-over test for level-ground and ramp walking, (2) collect sagittal ankle moment and ankle angle data on the test machine and on users and contrast the results with each test method, and (3) compare functional joint center positions for three different prosthetic foot stiffnesses between the two test methods for level-ground walking.