Experimental Brain Research

, Volume 217, Issue 1, pp 53–66 | Cite as

Contributions of feed-forward and feedback strategies at the human ankle during control of unstable loads

  • James M. FinleyEmail author
  • Yasin Y. Dhaher
  • Eric J. Perreault
Research Article


The nervous system can regulate the mechanical properties of the human ankle through feed-forward mechanisms such as co-contraction and rapid feedback mechanisms such as stretch reflexes. Though each of these strategies may contribute to joint stability, it is unclear how their relative contribution varies when ankle stability is threatened. We addressed this question by characterizing co-contraction and stretch reflexes during balance of an inverted pendulum simulated by a rotary motor configured as an admittance servo. The stability of this haptic environment was manipulated by varying the stiffness of a virtual spring supporting the pendulum. We hypothesized that co-contraction and stretch reflex amplitude would increase as the stability of the haptic load attached to the ankle was reduced. Electromyographic activity in soleus, medial and lateral gastrocnemius, and tibialis anterior was used to characterize co-contraction patterns and stretch reflex amplitude as subjects stabilized the haptic load. Our results revealed that co-contraction was heightened as stability was reduced, but that the resulting joint stiffness was not sufficient to fully counteract the imposed instability. Reflex amplitude, in comparison, was attenuated as load stability was reduced, contrary to results from upper limb studies using similar paradigms. Together these findings suggest that the nervous system utilizes feed-forward co-contraction rather than rapid involuntary feedback to increase ankle stability during simple balance tasks. Furthermore, since the stiffness generated through co-contraction was not sufficient to fully balance the haptic load, our results suggest an important role for slower, volitional feedback in the control of ankle stability during balancing tasks.


Stretch reflex Joint stability Stiffness Co-contraction Feedback Ankle 



This work was supported by NIH grants T32 HD007418-17, R01 NS053813, and an NSF Graduate Research Fellowship. The authors would like to thank the subjects for their participation and Tim Haswell for technical assistance.


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Copyright information

© Springer-Verlag 2011

Authors and Affiliations

  • James M. Finley
    • 1
    • 2
    • 4
    Email author
  • Yasin Y. Dhaher
    • 1
    • 2
    • 3
  • Eric J. Perreault
    • 1
    • 2
    • 3
  1. 1.Department of Biomedical EngineeringNorthwestern UniversityEvanstonUSA
  2. 2.Sensory Motor Performance ProgramRehabilitation Institute of ChicagoChicagoUSA
  3. 3.Department of Physical Medicine and RehabilitationNorthwestern UniversityEvanstonUSA
  4. 4.Motion Analysis LaboratoryKennedy Krieger InstituteBaltimoreUSA

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