Ensuring a well fitting prosthetic socket is a major challenge in the care of people with amputation. Poor fit causes pain, skin issues, reduced mobility, and lower satisfaction, affecting daily activity and quality of life. Current prosthetic socket fit assessment relies on prosthesis user feedback, prosthetist experience or intrusive equipment. The investigators have developed a non-invasive prototype socket fit testing system that attaches externally to the end of any prosthetic socket and uses two air cylinders to apply resistance and track motion. During testing, socket motion is mapped to a cursor on a screen. Participants perform a test where they move their leg and socket to randomly presented targets. Movement duration and accuracy provide measures of how well the socket and leg move together. This pilot study will assess feasibility and validity of the socket fit testing system performance for above-the-knee prosthesis users.

One of the biggest challenges in prosthetic care is making sure the prosthetic socket fits well with the residual limb. This "socket fit" is critical for both comfort and function. When the socket doesn't fit properly, it can cause pain, skin problems, and reduced performance. Poor socket fit can also lower a person's satisfaction with their prosthesis, limit their ability to perform daily activities, and negatively impact quality of life.

Current methods of assessing socket fit are qualitative-based on patient feedback or comfort surveys-or they use expensive and/or invasive equipment. These methods don't provide precise measurements of how well the socket and residual limb are working together, and some require altering the socket to insert instrumentation inside.

To address these limitations, the investigators are testing a non-invasive prototype device for measuring socket fit. This device attaches externally to the end of any prosthetic socket and uses two long air cylinders connected to sensors (potentiometers and load cells) and a TV monitor. With it, prosthesis users can perform different leg movements against resistance while wearing their socket.

The standardized test protocol requires participants to perform hip movements in all directions (flexion, extension, abduction, and adduction). The movement range is scaled to each person's height and hip motion, so the test has a consistent level of difficulty across individuals. The main hypothesis is that a better socket fit will allow more accurate movements, measured by how long the movements take and how much error is present. The overarching hypothesis is that better socket fit will result in more accurate movements, as measured by movement duration and error.

In this study, the investigators will evaluate the prototype device in adults with unilateral above-the-knee (transfemoral amputation, TFA). The investigators plan to recruit 18 participants with TFA:

• Eight participants with TFA will complete 2 testing visits to examine whether practice influences results and to confirm that the protocol is feasible.
• Ten participants with TFA will be purposively sampled to ensure that their current socket includes features that can make the fit easy to adjust. Participants will complete 2 testing visits so the investigators can determine whether the system can measure differences in socket-limb coupling, using metrics such as movement duration and error.

The investigators will also compare these movement metrics with participants' self-reported Socket Comfort Scores. Showing that the test protocol is feasible to implement, the device operates as intended, and differences in coupling can be detected will give the investigators pilot data to support larger clinical trials.

Specific Aims

1. Test the feasibility, repeatability and practice effects of the prototype socket fit testing protocol in people with transfemoral amputation.
2. Show that the system can detect differences in socket fit in people with transfemoral amputation, using movement metrics such as duration and error.
3. Validate the movement metrics by comparing them to patient-reported Socket Comfort Scores.

Inclusion Criteria:

• be between 18 and 70 years old
• have a unilateral transfemoral amputation
• be at least 12 months post-amputation
• have a residual limb that is free of open wounds or infections
• wear a prosthesis for a minimum of four hours per day
• be able to ambulate short distances with a prosthesis
• be able to stand on the intact leg for brief periods
• be able to hold onto parallel bars with the arms for balance
• have pain-free range of motion at the hip on the amputated side
• have no back problems that make standing or hip movement difficult
• have corrected vision that is sufficient to see the cursors on the screen
• be able to follow simple instructions in English

Exclusion Criteria:

• residual limb issues (e.g. sensitive or frail skin) that may be made worse by testing
• a prosthetic socket that cannot be disconnected from the distal prosthetic components
• unable to read and speak English