The Difference Threshold of Ankle-Foot Prosthesis Stiffness for Persons with Transtibial Amputation

Most prosthetic feet behave like springs, and their stiffness affects many important facets of amputee gait. Despite the importance of prosthesis stiffness, the ability of amputees to sense stiffness changes-that is, distinguish between more or less stiff feet-is unknown. This perceptual resolution has implications for the methodology and overall significance of selecting the optimal foot stiffness during prescription. In this experiment, we used a custom, variable-stiffness ankle prosthesis to make small adjustments to stiffness in between steps, and below-knee amputees were asked to identify whether the ankle became more or less stiff. We determined that the average difference threshold of stiffness was 8%, meaning that subjects could correctly identify an 8% change in stiffness 75% of the time. This high sensitivity underscores the importance of optimizing prosthesis stiffness on an individual basis, and suggests a shift is needed in the characterization of commercial feet and the use of stiffness variation during the prescription process.