Ankle-Foot-Orthosis 'Hermes' Compensates Pathological Ankle Stiffness of Chronic Stroke: A Proof of Concept

Karen E. Rodriguez Hernandez*, Jurriaan H. De Groot, Frank Baas, Marjon Stijntjes, Eveline R.M. Grootendorst-Heemskerk, Sven K. Schiemanck, Frans C.T. Van Der Helm, Herman Van Der Kooij, Winfred Mugge

*Corresponding author for this work

Research output: Contribution to journalArticleAcademicpeer-review

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Individuals with an upper motor neuron syndrome, e.g., stroke survivors, may have a pathological increase of passive ankle stiffness due to spasticity, that impairs ankle function and activities such as walking. To improve mobility, walking aids such as ankle-foot orthoses and orthopaedic shoes are prescribed. However, these walking aids generally limit the range of motion (ROM) of the foot and may therewith negatively influence activities that require a larger ROM. Here we present a new ankle-foot orthosis 'Hermes', and its first experimental results from four hemiparetic chronic stroke patients. Hermes was designed to facilitate active ankle dorsiflexion by mechanically compensating the passive ankle stiffness using a negative-stiffness mechanism. Four levels of the Hermes' stiffness compensation (0%, 35%, 70% and 100%) were applied to evaluate active ROM in a robotic ankle manipulator and to test walking feasibility on an instrumented treadmill, in a single session. The robotic tests showed that Hermes successfully compensated the ankle joint stiffness in all four patients and improved the active dorsiflexion ROM in three patients. Three patients were able to walk with Hermes at one or more Hermes' stiffness compensation levels and without reducing their preferred walking speeds compared to those with their own walking aids. Despite a small sample size, the results show that Hermes holds great promise to support voluntary ankle function and to benefit walking and daily activities.

Original languageEnglish
Pages (from-to)3535-3544
Number of pages10
JournalIEEE transactions on neural systems and rehabilitation engineering
Publication statusPublished - 30 Aug 2023


  • Clinical and Translational Impact Statement-In this early clinical research study
  • Equinus deformity
  • joint range of motion
  • muscle spasticity
  • orthotic devices
  • stroke
  • the Hermes ankle-foot orthosis increased the voluntary ankle dorsiflexion by compensating ankle stiffness of chronic ambulant stroke patients with spastic hemiparesis and equinus foot


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