Multivariable dynamic ankle mechanical impedance with active muscles

Hyunglae Lee, Hermano Igo Krebs, Neville Hogan

Research output: Contribution to journalArticlepeer-review

64 Scopus citations


Multivariable dynamic ankle mechanical impedance in two coupled degrees-of-freedom (DOFs) was quantified when muscles were active. Measurements were performed at five different target activation levels of tibialis anterior and soleus, from 10% to 30% of maximum voluntary contraction (MVC) with increments of 5% MVC. Interestingly, several ankle behaviors characterized in our previous study of the relaxed ankle were observed with muscles active: ankle mechanical impedance in joint coordinates showed responses largely consistent with a second-order system consisting of inertia, viscosity, and stiffness; stiffness was greater in the sagittal plane than in the frontal plane at all activation conditions for all subjects; and the coupling between dorsiflexion-plantarflexion and inversion-eversion was small-the two DOF measurements were well explained by a strictly diagonal impedance matrix. In general, ankle stiffness increased linearly with muscle activation in all directions in the 2-D space formed by the sagittal and frontal planes, but more in the sagittal than in the frontal plane, resulting in an accentuated "peanut shape." This characterization of young healthy subjects' ankle mechanical impedance with active muscles will serve as a baseline to investigate pathophysiological ankle behaviors of biomechanically and/or neurologically impaired patients.

Original languageEnglish (US)
Article number6825865
Pages (from-to)971-981
Number of pages11
JournalIEEE Transactions on Neural Systems and Rehabilitation Engineering
Issue number5
StatePublished - Sep 1 2014
Externally publishedYes


  • Ankle joint
  • Ankle joint stiffness
  • Ankle stiffness
  • Human ankle
  • Impedance structure
  • Multivariable impedance
  • Multivariable stiffness
  • Stiffness anisotropy

ASJC Scopus subject areas

  • Internal Medicine
  • General Neuroscience
  • Biomedical Engineering
  • Rehabilitation


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