The primary aim of this pilot study is to characterize the relationships between prosthetic ankle-foot stiffness, user reported comfort, and residuum-socket interface pressure in Veterans with transtibial amputation, and use these relationships to guide prosthesis optimization to maximize daily comfort. User comfort is of paramount importance to leg prosthesis users and has a direct impact on satisfaction with a prostheses, use or rejection of a prosthesis, and ultimately mobility and independence. Nearly 90% of prosthetic clinical encounters in the US are related to the lower limb, representing the vast majority of delivered prosthetic device interventions. However, surveys suggest that between 33% and 57% of leg prosthesis users report dissatisfaction with comfort while wearing their prosthesis, and 39% indicated that comfort and fit were their biggest concerns with a new prosthesis and a primary reason for changing prosthetists. Moreover, 51% and 37% of Vietnam Veterans and OIF/OEF Service Members reported prosthesis related pain. Importantly, regular use of a prosthesis and satisfaction with its comfort can increase the likelihood of returning to work following lower limb loss and this is a critical factor to Veterans’ community reintegration. Self-perceived comfort is a multifactorial, dynamic, psychophysical construct, but evidence suggests it is influenced by residuum-socket interface pressures. These interface pressures are affected by socket design, but also by prosthesis alignment given its influence on transfer of ground reaction forces through the socket to proximal anatomy. By the same mechanism, prosthesis stiffness should also theoretically affect interface pressures, but this relationship has not been quantified. To this end, prosthesis properties could be tuned to manipulate interface pressures for maximizing comfort. Therefore, the Specific Aims of this study are: 1) Define the maps connecting prosthetic foot stiffness, residuum-socket interface pressures, and user-perceived comfort, and 2) Assess the efficacy of human-in-the-loop optimization to tune prosthetic foot stiffness for minimizing interface pressure independent and in addition to the clinically optimized socket. We will address the study aims through use of novel robotic prosthesis emulator platform that includes a high- performance mechatronic system for rapid, controlled, and high-resolution keel stiffness modulations of a tethered prosthetic ankle-foot end effector. Both aims will involve ten participants with unilateral transtibial amputation. For Aim 1, participants will first undergo fitting and accommodation to the emulator system and protocol, and then walk at steady state under different prosthesis ankle-foot stiffness settings stratified by a certain percentage from a reference value specific to their body mass that reflects a common dynamic elastic response foot. Peak interface pressures and socket comfort will be measured at each stiffness setting to quant...