Intrinsic and reflex contributions to human ankle stiffness : variation with activation level and position

A parallel-cascade system identification method was used to identify intrinsic and reflex contributions to dynamic ankle stiffness over a wide range of tonic voluntary contraction levels and ankle positions in healthy human subjects. Intrinsic stiffness dynamics were described well by a linear pathw...

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Bibliographic Details
Published in:Experimental brain research 2000-12, Vol.135 (4), p.423-436
Main Authors: MIRBAGHERI, M. M, BARBEAU, H, KEARNEY, R. E
Format: Article
Language:English
Subjects:
Adult
Ankle Joint - physiology
Biological and medical sciences
Electromyography
Female
Fundamental and applied biological sciences. Psychology
Humans
Male
Motor control and motor pathways. Reflexes. Control centers of vegetative functions. Vestibular system and equilibration
Movement - physiology
Muscle Contraction - physiology
Neural Pathways - physiology
Posture - physiology
Reflex, Stretch - physiology
Torque
Vertebrates: nervous system and sense organs
Volition - physiology
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Summary:A parallel-cascade system identification method was used to identify intrinsic and reflex contributions to dynamic ankle stiffness over a wide range of tonic voluntary contraction levels and ankle positions in healthy human subjects. Intrinsic stiffness dynamics were described well by a linear pathway having elastic, viscous, and inertial properties. A velocity-sensitive pathway comprising a delay, a static non-linearity, resembling a half-wave rectifier, followed by a low-pass filter, described reflex stiffness dynamics. The absolute magnitude of intrinsic and reflex stiffness parameters varied from subject to subject but the relative changes with contraction level and position were consistent. Intrinsic stiffness increased monotonically with contraction level while reflex stiffness was maximal at low contraction levels and then decreased. Intrinsic and reflex stiffness both increased as the ankle was dorsiflexed. As a result, reflex mechanics made their largest relative contributions near the neutral position at low levels of activity. The size of the maximum reflex contribution varied widely among subjects, in some it was so small (ca 1%) that it would be unlikely to have any functional importance; however, in other subjects, reflex contributions were large enough (as high as 55% in one case) to play a significant role in the control of posture and movement. This variability may have arisen because stretch reflexes were not useful for the torque-matching task in these experiments. It will be of interest to examine other tasks where stretch reflexes would have a direct impact on performance.
ISSN:0014-4819
1432-1106
DOI:10.1007/s002210000534