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.

A unified perspective on ankle push-off in human walking

“…In clinical populations with decreased ankle push-off capabilities (e.g. individuals with lower-limb amputation wearing conventional passive prosthetic feet), COM push-off has also been observed to decrease Caputo and Collins, 2014;Collins and Kuo, 2010;Herr and Grabowski, 2012;Houdijk et al, 2009;Zelik et al, 2011), further supporting the contention that the burst of ankle push-off work contributes to energy changes of the COM. When ankle push-off was increased in a prosthesis (e.g.…”

“…with a bionic foot), then COM push-off also increased, in rough proportion (Herr and Grabowski, 2012;Zelik et al, 2011). For example, on average, a 7.8 J increase in prosthetic ankle push-off work resulted in an 8.0 J increase in COM push-off work for amputees wearing an energy-recycling versus conventional prosthetic foot (Zelik et al, 2011). Likewise, when ankle push-off work was reduced using a restrictive ankle-foot orthosis, COM push-off work decreased in proportion (linear regression slope ±confidence interval of 0.75±0.3, R 2 =0.59; Huang et al, 2015).…”

“…When ankle push-off was increased in a prosthesis (e.g. with a bionic foot), then COM push-off also increased, in rough proportion (Herr and Grabowski, 2012;Zelik et al, 2011). For example, on average, a 7.8 J increase in prosthetic ankle push-off work resulted in an 8.0 J increase in COM push-off work for amputees wearing an energy-recycling versus conventional prosthetic foot (Zelik et al, 2011).…”

Mechanical and energetic consequences of reduced ankle plantarflexion in human walking

“…Despite these limitations, our findings may have implications for patient groups with reduced push-off. Both the amount and timing of push-off appear important for energy economy, as also suggested by studies of ankle fusion (Doets et al, 2009;van Engelen et al, 2010), ankle exoskeletons (Malcolm et al, 2013;Sawicki and Ferris, 2008), ankle orthoses (Bregman et al, 2011) and lower limb prosthetics (Collins and Kuo, 2010;Zelik et al, 2011). If push-off cannot be restored, an alternative is to reduce the collision loss, for example with arc-shaped foot bottoms (Adamczyk and Kuo, 2013;Adamczyk et al, 2006;Vanderpool et al, 2008;van Engelen et al, 2010).…”

Impact testing of the residual limb: System response to changes in prosthetic stiffness

“…Interpretation of results from previous studies has often been hindered by confounding variables, as changes in the stiffness of a particular prosthetic component are often accompanied by changes in other notable considerations such as mass, material, or alignment [16]. Additionally, the purpose of these compliant prosthetic components is to protect the residual limb's musculoskeletal system from the forces transmitted along the limb during walking.…”