Conceived and designed the experiments: SHC ADK. Performed the experiments: SHC. Analyzed the data: SHC. Wrote the paper: SHC ADK. Designed and built artificial foot: SHC.
S.H.C. is part-owner of Intelligent Prosthetic Systems L.L.C., which was incorporated to perform research and development on the energy-recycling technology reported in this manuscript. None of the data presented here is proprietary, however the design of the artificial foot is partially covered by US Provisional Patent no. 60/705,019. Foot prostheses based on this technology are under development, but none are currently available as marketed products. We will make available all data used to reach the conclusions presented in this study.
Humans normally dissipate significant energy during walking, largely at the transitions between steps. The ankle then acts to restore energy during push-off, which may be the reason that ankle impairment nearly always leads to poorer walking economy. The replacement of lost energy is necessary for steady gait, in which mechanical energy is constant on average, external dissipation is negligible, and no net work is performed over a stride. However, dissipation and replacement by muscles might not be necessary if energy were instead captured and reused by an assistive device.
We developed a microprocessor-controlled artificial foot that captures some of the energy that is normally dissipated by the leg and “recycles” it as positive ankle work. In tests on subjects walking with an artificially-impaired ankle, a conventional prosthesis reduced ankle push-off work and increased net metabolic energy expenditure by 23% compared to normal walking. Energy recycling restored ankle push-off to normal and reduced the net metabolic energy penalty to 14%.
These results suggest that reduced ankle push-off contributes to the increased metabolic energy expenditure accompanying ankle impairments, and demonstrate that energy recycling can be used to reduce such cost.
The ankle normally produces a larger burst of work than any other joint during walking
Much of the dissipation in normal walking occurs when the body center of mass velocity is redirected at the transition between steps. During each step, the stance leg behaves similarly to an inverted pendulum as it transports the center of mass along an arced path (
(
We developed an energy-recycling artificial foot (
(
We tested the artificial foot on able-bodied human subjects (N = 11, male, 19–28 yrs) walking with an artificially-immobilized ankle. Subjects wore the device (1.37 kg) on one leg using a prosthesis simulator
The Conventional Prosthesis reduced ankle push-off and increased metabolic expenditure for all subjects. The Energy Recycling artificial foot captured collision energy and returned it as positive ankle work later in stance, resulting in greater push-off and lower metabolic expenditure than with the Conventional Prosthesis.
Normal walking yielded an average rate of ankle push-off work of 17.7±3.4 W (mean ± s.d., rate of positive work over a stride,
Power produced by normal and artificial ankles (top), and rate of work performed on the center of mass by the entire leg and device (bottom), with (
(
The Conventional Prosthesis also increased metabolic energy expenditure, an energetic penalty that was reduced with the Energy Recycling foot (
This reduction in metabolic energy expenditure compares favorably against a variety of conventional elastic prostheses, which have been found not to significantly reduce the metabolic penalty
The precise relationship between push-off work and metabolic energy expenditure, however, is more complex than these results first imply. With the Conventional Prosthesis, ankle push-off decreased by 45% and net metabolic expenditure increased by 23% compared to Normal. The Energy Recycling foot restored push-off to 7% above the Normal level, but only reduced net metabolic energy expenditure by 9%. Some of the residual penalty may be due imprecise capture of energy, which appears to have caused additional positive work by the human leg during early stance (
This energy-recycling device may nevertheless provide a basis for the design of prosthetic feet that improve walking economy for amputees. The design would benefit from a reduction in weight and size, tuning of shape and stiffness characteristics for amputee gait, and improved cosmesis. Potential complexities due to the interface between residual limb and prosthesis would need to be studied.
Our results also suggest ways to improve other assistive devices. Energy recycling could be applied to other prosthetic limbs and orthotic devices, using configurations in parallel with the leg joints in addition to the series configuration examined here. Parallel devices would have the added advantage of reducing costs associated with force production
Additional descriptions of the artificial foot construction, experimental methods, and analysis methods. Includes supporting figures and captions.
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Experimental setup. (A) Prosthesis simulator boots worn by intact subjects, fitted with the Energy Recycling foot or with the Conventional Prosthesis. Simulator boots were worn unilaterally (on the Affected leg), with a height-matched lift shoe on the opposite foot (Contralateral leg). Prosthesis simulator boots were comprised of AirCast© pneumatic boots augmented with a prosthetic pyramidal adaptor
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Ground reaction forces. Normalized to body weight (BW, 82.6±7.1 N) and presented in components: (A) vertical component of the ground reaction force acting on the subject, with positive defined as opposing gravity, (B) fore-aft component with positive defined as along the direction of travel, and (C) lateral component with positive defined as rightward. Solid lines correspond to the leg on which the prosthesis simulator was worn (Affected leg), dashed lines correspond to the opposite limb (Contralateral leg). The stride begins at heel strike of the Affected limb. The first peak in vertical ground reaction force on the Contralateral limb was reduced with the Energy Recycling artificial foot as compared to the Conventional Prosthesis, apparently due to increased push-off impulse.
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Center of mass work decomposition. Work performed on the center of mass over four phases of the gait cycle by the entire leg and by the human leg (un-shaded bars, estimated by subtracting separately-measured prosthesis work) for (A) the Affected leg (on which the prosthesis simulator was worn) and (B) the Contralateral leg. Collision, rebound, preload, and push-off refer to four characteristic phases of positive or negative center of mass work
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Lower-limb joint mechanics. Joint angles (top row), joint torques (middle row), and joint powers (bottom row) for the biological ankle (left column), knee (middle column), and hip (right column) as calculated using inverse dynamics [34,35]. Clinical phases of joint work [36] for the Affected side are marked as A1, A2, etc., as defined in the analysis methods section of
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Lower limb joint work decomposition. Joint work was decomposed into clinical phases for (A) the Affected leg (on which the prosthesis simulator was worn) and (B) the Contralateral leg. Clinical phases of gait for each leg are defined in
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Energy recycling with the artificial foot. High-speed video of the energy-recycling artificial foot, played back at 6% of actual speed. Camera rate was 500 frames per second. In the video, the foot proceeds through the phases described in
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We thank K. E. Zelik for data collection assistance and technical comments. We also thank P. G. Adamczyk, J. M. Cziernecki, J. M. Donelan, J. Harlaar, G. K. Klute, L. A. Lau, C. L. Lewis, M. S. Orendurff, A. Ruina, M. M. van der Krogt, M. Wisse, and an anonymous reviewer for technical and editorial comments. We thank R. Ching, and E.Yliniemi for assistance with making the high-speed video.