Mechanical and dynamic characterization of prosthetic feet for high activity users during weighted and unweighted walking

Abstract

Many Service members and Veterans with lower-limb amputations have the potential for high function and the desire to resume physically demanding occupations that require them to carry heavy loads (e.g., military service, firefighters, farmers, ranchers, construction workers). However, it is currently unclear which prosthetic feet best accommodate heavy load carriage while also providing good overall function and mobility during unweighted activities. The main objective of this study was to investigate the ability of currently available prosthetic ankle-foot systems to accommodate weighted walking by examining the mechanical characteristics (i.e., forefoot stiffness) and dynamic function (i.e., rocker radius, effective foot length ratio, and late-stance energy return) of prosthetic feet designed for high activity users. Load versus deflection curves were obtained for nine prosthetic ankle-foot systems using a servohydraulic test frame and load cell. Effective roll-over shape characteristics and late-stance energy return measures were then obtained using quantitative gait analysis for three users with unilateral, transtibial amputation. Results from mechanical and dynamic testing showed that although forefoot stiffness varied across the nine feet investigated in this study, changes measured in roll-over shape radius and effective foot length ratio were relatively small in response to weighted walking. At the same time, prosthetic feet with more compliant forefoot keel structures appeared to provide more late-stance energy return compared to feet with stiffer forefoot keel structures. These results suggest that prosthetic ankle-foot systems with compliant forefoot keel structures may better accommodate weighted walking by reducing the metabolic cost of physically demanding activities. However, to more fully understand the biomechanical and functional implications of these results, other factors, such as the residual-limb strength of the user and the overall stiffness profile of the prosthetic foot, should also be considered.

Prosthetic forefoot and heel stiffness across consecutive foot stiffness categories and sizes

“…In addition to stiffness quantification, methods of mechanically testing prosthetic feet vary across studies. Although there have been numerous previous studies that have mechanically tested prosthetic feet, we identified three such studies that were similar to the current study in the types of prosthetic feet (size, stiffness category, and model (i.e., Rush HiPro,[ 18 ] AllPro,[ 18 ] Vari-Flex,[ 16 ] and Seattle Lightfoot2[ 16 , 17 ])) and pylon progression angles tested. In the current study we used an R2000 Rotopod for mechanical testing.…”

“…Several studies also have used a “functional” or instantaneous stiffness estimate, in which linear regression was fit to a smaller region of the force-displacement curve, at a particular load level (e.g., approximating near body weight). [ 6 , 16 , 18 ] However, distilling the information from each force-displacement curve to a single value of stiffness may be better suited for a foot with linear behavior, and is likely less representative of many contemporary prosthetic feet. Since many prosthetic feet have curvilinear behavior, one study described using a 2 nd order polynomial, in addition to calculating the linear stiffness of feet across the full curve, as it provided a better representation of the nonlinear force-displacement curve.…”

“…Womac et al used the same mechanical testing equipment,[ 16 ] whereas Major et al used a materials testing machine and sine plate (as described in ISO standard 10328),[ 17 ] and Koehler et al used a materials testing machine with the foot fastened to an aluminum bar. [ 18 ] While in the current study we tested feet at -15° and +20° sagittal pylon progression angles, Womac et al tested at 15 different sagittal pylon progression angles ranging from -15° to +30° (including the two pylon progression angles used in the current study),[ 16 ] Major et al tested at -15° and +20° (the two angles tested in the current study) as well as a level loading surface,[ 17 ] and Koehler et al tested only at a +20° pylon progression angle. [ 18 ] Target loading thresholds and the number of loading cycles also varied by study.…”

Effects of shear force reduction during mechanical testing and day-to-day variation on stiffness of commercial prosthetic feet: a technical note

“…3,6 To measure linear stiffness in a user-independent manner, investigators have used mechanical testing procedures. [7][8][9][10][11] Tests for linear stiffness typically generate force-displacement data curves from quasi-static loading conditions in which the prosthetic foot is positioned at a fixed angle relative to the loading platform. Previous mechanical testing methods have reduced tangential shear forces to avoid overconstraining the system during loading, often incorporating a roller plate (RoPl) with ball bearings into the loading platform.…”

“…3,6 To measure linear stiffness in a user-independent manner, investigators have used mechanical testing procedures. 7-11…”