Patient-Preferred Prosthetic Ankle-Foot Alignment for Ramps and Level-Ground Walking

Abstract

Patient preference of lower limb prosthesis behavior informally guides clinical decision making, and may become increasingly important for tuning new robotic prostheses. However, the processes for quantifying preference are still being developed, and the strengths and weaknesses of preference are not adequately understood. The present study sought to characterize the reliability (consistency) of patient preference of alignment during level-ground walking, and determine the patient-preferred ankle angle for ascent and descent of a 10° ramp, with implications for the design and control of robotic prostheses. Seven subjects with transtibial amputation walked over level ground, and ascended and descended a 10° ramp on a semi-active prosthetic ankle capable of unweighted repositioning in dorsiflexion and plantarflexion. Preferred ankle angle was measured with an adaptive forced-choice psychophysics paradigm, in which subjects walked on a randomized static ankle angle and reported whether they would prefer the ankle to be dorsiflexed or plantarflexed. Subjects had reliable preferences for alignment during level-ground walking, with deviations of 1.5° from preference resulting in an 84% response rate preferring changes toward the preference. Relative to level walking, subjects preferred 7.8° (SD: 4.8°) of dorsiflexion during ramp ascent, and 5.3° (SD: 3.8°) plantarflexion during ramp descent. As the ankle angle better matched the ramp angle, socket pressures and tibial progression (shank pitch) both more closely mirrored those during level walking. These findings provide baseline behaviors for prosthetic ankles capable of adapting to slopes based on patient preference, and provide strong evidence that people with transtibial amputation can finely perceive ankle alignment.

Distribution of joint work during walking on slopes among persons with transfemoral amputation

“…for the able-bodied subjects (Alexander et al, 2017;Shepherd et al, 2020). The values of positive and negative joint power peaks increased while walking upslope and downslope, respectively.…”

Understanding patient preference in prosthetic ankle stiffness

“…The dial could be freely rotated beyond the minimum and maximum of VSPA stiffness; however, the VSPA Foot saturated at these extrema, such that any supramaximal changes to the dial were ignored by the controller. This method of adjustment converges faster to the user's preference than the two-alternative forced choice methods we have previously implemented [45,49], and produces slower and more predictable (and thus safer) stride-to-stride adjustment of mechanics. Although subjects were free to rotate the dial throughout the gait cycle, stiffness was only actively adjusted, to match the value indicated by the dial's position, during the swing phase of gait.…”

The role of user preference in the customized control of robotic exoskeletons

“…To use preference as an optimization criterion for lower limb exoskeletons, it must be robustly measured, and we need to understand how it changes as individuals adapt to the assistance. Recent studies have investigated users' preferences in assistance provided by lower limb prostheses (24)(25)(26)(27)(28)(29)(30) and exoskeletons (11,(31)(32)(33)(34). To measure preference, some studies used a forced choice paradigm, in which participants were presented with pairwise comparisons (A-B testing) (29,(31)(32)(33) or asked to compare a condition to their own internalized preference (24,25).…”

“…Recent studies have investigated users' preferences in assistance provided by lower limb prostheses (24)(25)(26)(27)(28)(29)(30) and exoskeletons (11,(31)(32)(33)(34). To measure preference, some studies used a forced choice paradigm, in which participants were presented with pairwise comparisons (A-B testing) (29,(31)(32)(33) or asked to compare a condition to their own internalized preference (24,25). Data obtained from such methods can be used to identify or learn a user's preference, but it may require extended experimental time to obtain enough comparisons.…”