A prosthetic socket is perhaps the most important part of a prosthesis because it allows force and energy to be transferred between the residual limb and prosthetic components, and influences successful rehabilitation following lower-limb amputation. People with lower-limb amputation are typically fit with a rigid prosthetic socket. However, the use of a prosthesis incorporating this type socket has led to negative secondary conditions such as osteoarthritis, back pain, residual limb skin breakdown and discomfort; 57% of prosthetic users are dissatisfied with the comfort of their prosthesis. People with unilateral transtibial or transfemoral amputation typically walk with asymmetric biomechanics, such as ground reaction forces, joint motion, and joint kinetics. People with unilateral amputation typically compensate for the functional loss of the muscles and tendons of the affected leg with greater forces borne by their unaffected leg. Asymmetric forces likely contribute to the higher prevalence of osteoarthritis and low back pain in people with a leg amputation. Pain and improper fit of the socket can result in loss of material, time, and money for patients, prosthetists, and manufacturers. An adjustable socket that accommodates residual limb volume fluctuations could allow prosthetists to optimize socket fit for each patient, which could reduce the risk of comorbidities such as osteoarthritis, pain, and the need to re-fit prosthetic sockets in the long term. Moreover, use of an adjustable socket design has the potential to improve function and comfort of Veterans with lower limb amputation. Quorum Prosthetics (Windsor, CO) has developed a new modular adjustable socket, the Quatro socket, that could decrease biomechanical asymmetry, decrease movement of the residual limb relative to the socket (socket pistoning), and improve comfort and satisfaction during walking due to its ability to accommodate residual limb volume changes of up to 12%. The Quatro socket is produced by 3D printing high-strength Nylon 12, which is stronger and lighter than typically used carbon fiber; 3D printing also decreases build time and allows customized changes down to 80 μm in thickness. Each socket can be custom printed from a scan of a patient's limb, cast, or existing socket, so it is form-fit to the individual user. The Quatro socket utilizes 3 Boa dials (Boa Technology, Steamboat Springs, CO) that ratchet to tighten or loosen cables that run through 4 padded panels to quickly and systematically adjust the volume and compression of the socket around the user's limb. This technology allows a user to don or doff the socket in ~20 sec and adjust for volume changes up to 12%. We will determine the effects of using a conventional rigid and an adjustable Quatro prosthetic socket with the same suspension and prosthetic components on the biomechanics, socket pistoning, and comfort/satisfaction of 10 female and 10 male Veterans with a unilateral transtibial amputation (Aim ...