Systematic variation of prosthetic foot spring affects center-of-mass mechanics and metabolic cost during walking

Abstract

Lower-limb amputees expend more energy to walk than non-amputees and have an elevated risk of secondary disabilities. Insufficient push-off by the prosthetic foot may be a contributing factor. We aimed to systematically study the effect of prosthetic foot mechanics on gait, to gain insight into fundamental prosthetic design principles. We varied a single parameter in isolation, the energy-storing spring in a prototype prosthetic foot, the controlled energy storage and return (CESR) foot, and observed the effect on gait. Subjects walked on the CESR foot with three different springs. We performed parallel studies on amputees and on non-amputees wearing prosthetic simulators. In both groups, spring characteristics similarly affected ankle and body center-of-mass (COM) mechanics and metabolic cost. Softer springs led to greater energy storage, energy return, and prosthetic limb COM push-off work. But metabolic energy expenditure was lowest with a spring of intermediate stiffness, suggesting biomechanical disadvantages to the softest spring despite its greater push-off. Disadvantages of the softest spring may include excessive heel displacements and COM collision losses. We also observed some differences in joint kinetics between amputees and non-amputees walking on the prototype foot. During prosthetic push-off, amputees exhibited reduced energy transfer from the prosthesis to the COM along with increased hip work, perhaps due to greater energy dissipation at the knee. Nevertheless, the results indicate that spring compliance can contribute to push-off, but with biomechanical trade-offs that limit the degree to which greater push-off might improve walking economy.

© 2011 IEEE

Figures

Figure 1

Controlled Energy Storage and Return (CESR) prosthesis prototype. The CESR foot stores energy in a compression spring after heelstrike, locks the spring energy into place with a clutch and then returns the energy during terminal stance in the form of plantarflexion push-off work. This energy recycling foot provides improved energy return as compared to conventional passive prostheses (Collins and Kuo, 2010).

Figure 2

Prosthesis power, energy storage and return. Average (A) ankle/foot power, (B) CESR spring energy storage and (C) prosthetic energy return are shown for Amputees (left column) and Non-Amputees wearing simulator boots (right column). The (A) ankle/foot power estimates all energy flow into and out of the foot, and the inset in (B) CESR spring power, represents energy storage contributions of just the spring. Power is plotted across stance phase from heelstrike to toe-off. Results from a Conventional passive prosthesis (dashed line) and typical Shod Non-Amputee gait (dotted line) are provided for reference. Prosthetic ankle/foot mechanics varied with spring stiffness in both Amputees and Non-Amputees: the softest, pre-compressed (Soft-PC) spring stored and returned the most energy, while the stiffest (Hard) spring stored and returned the least. In all figures, the results represent the means and standard deviations computed across all subjects.

Figure 3

COM work rate, prosthetic limb Push-off and intact limb Collision work. Average COM work rate for (A) prosthetic and (C) intact limbs are plotted across a full gait cycle (beginning with prosthetic heelstrike) for Amputees (left column) and Non-Amputees (right column). Phases of gait – Collision, Rebound, Preload, Push-off, Swing – are defined for each limb based on alternating regions of positive and negative COM work, and approximate regions are shown for the Medium spring by vertical lines. The (B) prosthetic limb Push-off work and (D) intact limb Collision work were integrated from the shaded phases shown, which were defined independently for each subject and condition. Softer spring stiffness led to increased prosthetic limb COM Push-off, but stiffness did not have a significant effect on intact limb Collision work. Also, amputees appeared to perform less prosthetic limb COM Push-off work and less intact limb COM Collision work than Non-Amputees. Reference lines are shown for walking on a Conventional prosthesis (dashed line) and in street shoes (dotted line) for Non-Amputees.

Figure 4

Average metabolic rate for Amputees (left) and Non-Amputees (right). Results for a Conventional passive prosthesis (dashed line) and shod Non-Amputee gait (dotted line) are shown for reference. We observed mixed metabolic results that were qualitatively similar in Amputee and Non-Amputee subjects across spring conditions. The Medium spring was significantly lower than Soft-PC spring in Amputees, and lower than the Hard spring in Non-Amputees subjects. Although metabolic energy expenditures were found to be of similar magnitudes in both Amputees and Non-Amputees, we consider this coincidental and attribute no special significance to the absolute comparison. There was no reason a priori to expect similar energetic magnitudes given differences between groups and experimental protocols. Qualitatively, Amputee subjects tended to expend more energy walking on the CESR foot than on the Conventional prosthesis, whereas Non-Amputees tended to expend less energy while walking on the CESR.

Figure 5

Prosthetic limb knee and hip powers, work during Push-off phase. Average prosthetic (A) knee and (C) hip are plotted across the full gait cycle (beginning with prosthetic heelstrike) for Amputees (left) and Non-Amputees (right). The prosthetic limb (B) knee work and (D) hip work during Push-off phase were defined separately for each subject and condition (see supplemental material, Fig. S1). Vertical lines show approximate Push-off phase for Medium spring. Knee and hip power showed little variation within a group due to varying CESR spring stiffness. Amputees, however, seemed to exhibit large negative knee work (i.e., absorption) near terminal stance, which was not observed in Non-Amputees subjects. Amputees also appeared to perform more hip work during Push-off as compared to Non-Amputees. Reference lines are shown for walking on a Conventional prosthesis (dashed line) and in street shoes (dotted line) for Non-Amputees.