## 4 Discussion
The methods described to compare biomechanical and machine data in the sagittal plane for ankle moment and angles are promising, as demonstrated by the correlation seen for dorsiflexion angle data between the two test methods. The output measurements from the test machine used in conjunction with the camera provide a good surrogate for state-of-the-art biomechanical data generally obtained from gait analysis. The roll-over machine test allows researchers and prosthetic foot designers to evaluate device characteristics in a controlled and reproducible manner. Researchers could use this tool to compare different types of devices or adjust device properties before starting trials on users, ultimately reducing the burden of user testing. The limitations of the study presented are the small sample size for the user data collected and the use of a 2D camera to track the prosthetic foot angle on the test machine. Best-practices recommendations from [PERSON] were adopted to improve the video record reliability [29]. Future work should focus on the machine input parameters for heel strike during ramp-walking simulation to achieve an improved plantarflexion correlation. The VSA prototype foot was well accepted, and all users engaged in the study completed the treadmill trial. The VSA unit allows an assessment of different stiffness settings while the user is wearing the device. This reduces the time and effort to change to the prosthetic foot stiffness category or alignment. In addition, an adaptable prosthetic foot could be beneficial for different terrains and tasks, as previously reported by prosthetic users [30]. Our proposed method is not a fully comprehensive analysis that describes all the prosthetic foot characteristics. However, it allows for calculation and post-processing for different gait tasks, with multiple data sets, such as moments and forces, combined with the deformations of the ESR foot flexible elements. Machine testing combined with a variable stiffness unit can be beneficial for researchers and prosthetic foot designers to better understand prosthetic foot properties and possibly the specific user preferences. [PERSON] and [PERSON] showed the interaction between the CPO and the user when comparing ankle-foot stiffness preference [31]. We foresee the potential of these types of machine testing results to be optimized for user-specific input profiles. A user-specific gait could be used as input for the machine, and prosthetic foot stiffness can be changed while testing on the equipment to achieve a desired ankle range of motion for the user. [PERSON] et al. 's ankle elasticity model could be used in conjunction with a variable stiffness prosthetic foot to improve the economy of ankle- or hip-powered walking [32]. Following these two paths in future research, a more individualized prosthetic foot prescription depending on user gait could be developed. The high gait variability between users, as described by [PERSON] et al., during incline walking, may call for user-specific prostheticic foot adjustability [33]. [PERSON] et al. described the change in ankle quasi-stiffness, depending on the gait phase and speed of healthy subjects [34]. The ability for users to adjust their prostheses for increased dorsiflexion, e.g., during faster walking, may be considered a significant advantage when using a variable stiffness prosthetic foot.