Logo for the Journal of Rehab R and D

Volume 46 Number 3, 2009
   Pages 361 — 374

Abstract – Agonist-antagonist active knee prosthesis: A preliminary study in level-ground walking

Ernesto C. Martinez-Villalpando, SM;1 Hugh Herr, PhD1-2*

1Massachusetts Institute of Technology (MIT) Media Laboratory, Cambridge, MA; 2Harvard-MIT Division of Health Sciences and Technology, Cambridge, MA

Abstract — We present a powered knee prosthesis with two series-elastic actuators positioned in parallel in an agonist-antagonist arrangement. To motivate the knee's design, we developed a prosthetic knee model that comprises a variable damper and two series-elastic clutch units that span the knee joint. Using human gait data to constrain the model's joint to move biologically, we varied model parameters using an optimization scheme that minimized the sum over time of the squared difference between the model's joint torque and biological knee values. We then used these optimized values to specify the mechanical and control design of the prosthesis for level-ground walking. We hypothesized that a variable-impedance control design could produce humanlike knee mechanics during steady-state level-ground walking. As a preliminary evaluation of this hypothesis, we compared the prosthetic knee mechanics of an amputee walking at a self-selected gait speed with those of a weight- and height-matched nonamputee. We found qualitative agreement between prosthetic and human knee mechanics. Because the knee's motors never perform positive work on the knee joint throughout the level-ground gait cycle, the knee's electrical power requirement is modest in walking (8 W), decreasing the size of the onboard battery required to power the prosthesis.

Clinical Trial Registration: ClinicalTrials.gov; Active Knee Prosthesis Study for Improvement of Locomotion for Above Knee Amputees, NCT00771589; http://clinicaltrials.gov/ct/show/NCT00771589.

Key words: agonist-antagonist actuation, biomimetic design, finite state machine, gait evaluation, human walking, impedance control, knee mechanics, knee prosthesis, rehabilitation, series-elastic actuation.

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