Automatic Prosthetic Foot Stiffness Modulation to Improve Balance in the Real World

Individuals with lower limb amputations are at higher risk of falling compared to able-bodied and other clinical populations and are more likely to sustain life-altering injuries. The higher fall risk is primarily due to the loss of muscles crossing the ankle, which are critical to maintaining balance control. Prosthetic devices are designed to provide appropriate stiffness for needed stability and support. While research has shown the optimal stiffness to maintain balance varies across ambulatory activities (e.g., straight walking versus turning), most clinically prescribed prosthetic devices are passive and only provide a fixed stiffness level. The one commercially available, powered prosthetic ankle-foot, has not been shown to restore balance control. Thus, a prosthetic device that actively adjusts ankle stiffness across different ambulatory activities is critically needed to advance the field and improve balance control for those with lower-limb amputations. The goal of this clinical trial is to identify prosthesis stiffness that optimizes balance control in individuals with below knee amputations as they perform typical ambulatory activities of daily living. By matching the ankle stiffness to the task requirements, the investigators believe balance control will significantly improve and fall risk will decrease for those with lower-limb amputations.

Each participant will be fit with a novel prosthesis that includes a low-profile prosthetic foot whose stiffness category will be determined by their body weight and activity level (standard clinical practice), a prosthetic foot whose stiffness category is two categories stiffer, and another whose stiffness is two categories less stiff. Participants will continue to use their existing prosthetic socket and suspension system, but their pylon length will be adjusted as needed. While wearing the different study prostheses in randomized order, nine ambulatory activities will be performed in a motion capture laboratory.

• Walking on level ground at self-selected speed.
• Walking on level ground at 15% slower than self-selected speed.
• Walking on level ground at 15% faster than self-selected speed.
• Walking up an 8% slope at self-selected speed.
• Walking down an 8% slope at self-selected speed.
• Walking on level ground at self-selected speed while carrying a 5 kg load in one hand on their prosthetic side.
• Walking on level but uneven terrain at self-selected speed.
• Walking around 2-meter diameter circle at self-selected speed with the prosthetic side on the inside of the circle.
• Walking around 2-meter diameter circle at self-selected speed with the prosthetic side on the outside of the circle.

Ambulatory activities 1 through 6 will be performed on an instrumented treadmill. The load to be carried in activity 6 will be configured as a grocery bag with handles. Activity 7 will be performed on a rocky terrain treadmill. Activities 8 and 9 will be performed overground across five force plates embedded in the laboratory floor while following the outline of a 2-meter diameter circle. Rest breaks during all activities will be provided as needed.

The data from this experiment will be used to calculate the peak-to-peak range of the frontal plane whole-body angular momentum (a measure of balance control). The investigators will use these calculations to determine a user- and activity-specific automatic stiffness modulation control scheme intended to improve balance control.

Each participant will then be randomized to wear their fixed stiffness prosthesis or the automatic stiffness prosthesis. Subjects will be fit with the IMU sensors on their ankles, wrists, and upper torso. The automatic stiffness prosthesis will be programmed to emulate the user-specific and activity-specific automatic stiffness modulation control scheme. Subjects will be provided at least 10 minutes to walk about the laboratory to learn how the study prosthesis performs. Subjects, wearing the study prosthesis (fixed or automatic stiffness), will then walk a defined course about the hospital facility (both indoors and outdoors) in which each of the laboratory-based ambulatory activities will be represented. A study investigator will accompany the subject at a following distance of ~5 meters while video recording their gait. After completing the defined course, the subject will return to the laboratory where they will complete the Prosthetic Limb Users Survey of Mobility and the Activities-Specific Balance Confidence Scale questionnaires to assess their perceived functional mobility and balance confidence. The subject will then don the other study prosthesis (fixed or automatic stiffness) and then repeat the walk about the defined course and complete the two questionnaires a second time.

The results of this experiment will determine if automatic stiffness modulation improves the balance control of those with lower-limb amputation as they perform typical ambulatory activities of daily living using both biomechanical analyses and self-report assessments of perceived functional mobility and balance confidence.

The investigators hypothesize that the automatic stiffness prosthesis will provide improved balance control in real-world environments compared to the user's fixed stiffness prosthesis that was clinically prescribed. The investigators further hypothesize that the Prosthetic Limb Users Survey of Mobility score reported after wearing the automatic stiffness prosthesis will be greater than the score reported after wearing the fixed stiffness prosthesis, and the Activities-Specific Balance Confidence Scale score reported after wearing the automatic stiffness prosthesis will be greater than the score reported after wearing the fixed stiffness prosthesis.

Inclusion criteria

• Unilateral transtibial (below-knee) amputation
• Been fit with a prosthesis and used it for at least 6 months
• Wear the prosthesis for 4 or more hours on average per day
• Be at least one-year post-amputation
• Able to walk on a treadmill

Exclusion criteria

• Presence of disorder, pain, or injury other than amputation that interferes with gait
• Current skin irritation or injury on residual limb
• Use of an assistive device for walking (cane, walker, etc.)