A Quasi-Passive Robotic Ankle Foot Orthosis With Speed-Adaptive Stiffness

Samuel Hopkins, Collin Bowersock, Elliott J. Rouse, Zachary F. Lerner

Research output: Contribution to journalArticlepeer-review


Ankle foot orthoses (AFOs) are one of the most prescribed mobility devices for individuals with walking disability from conditions like cerebral palsy (CP) and stroke. Current AFO designs offer a fixed stiffness during walking, functioning optimally at only a single speed. The primary goal of this study was to develop and validate a quasi-passive robotic AFO that automatically adjusted stiffness to a user's walking speed. We designed a leaf spring AFO with an adjustable pivot point actuated by a compact linear servo motor. We developed a walking speed estimator using onboard sensors to automatically adjust the pivot point location, and therefore device stiffness. First, we characterized stiffness range, adjustment response time, and battery life during walking. Next, we performed clinical device validation testing in five individuals with CP during stand-to-run acceleration bouts and at constant walking speeds. The AFO exhibited a stiffness range of 60 to 250 Nm/rad during normal walking and up to 300-400 Nm/rad under certain conditions (e.g., running) for the CP participants. The device was able to adjust stiffness by ∼200 Nm/rad during swing phase in ∼0.25 seconds. Battery life approached 6000 steps. The on-board controller accurately predicted relative changes in walking speed for the five participants with CP (R2 = 0.92 ± 0.04), demonstrating the ability to automatically increase device stiffness with ambulation speed. This study advances the state-of-the-art for quasi-passive AFOs that can function optimally at different ambulation speeds.

Original languageEnglish (US)
Pages (from-to)1740-1747
Number of pages8
JournalIEEE Robotics and Automation Letters
Issue number2
StatePublished - Feb 1 2024
Externally publishedYes


  • Biologically-inspired robots
  • compliance and impedance control
  • compliant joints and mechanism
  • human-robot interaction
  • wearable robotics

ASJC Scopus subject areas

  • Control and Systems Engineering
  • Biomedical Engineering
  • Human-Computer Interaction
  • Mechanical Engineering
  • Computer Vision and Pattern Recognition
  • Computer Science Applications
  • Control and Optimization
  • Artificial Intelligence


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