Submitted on August 9, 2007
Accepted on March 20, 2008
Sway-dependent modulation of the triceps surae H-reflex during standing
Craig D Tokuno1, S. Jayne Garland2, Mark G Carpenter3, Alf Thorstensson4, and Andrew G Cresswell5*
1 Neuroscience, Karolinska Institute, Stockholm, Sweden; Human Movement Studies, The University of Queensland, Brisbane, Queensland, Australia
2 School of Physical Therapy, University of Western Ontario, London, Canada; Department of Physiology and Pharmacology, University of Western Ontario, London, Canada
3 School of Human Kinetics, University of British Columbia, Vancouver, Canada
4 Neuroscience, Karolinska Institute, Stockholm, Sweden
5 Human Movement Studies, The University of Queensland, Brisbane, Queensland, Australia
* To whom correspondence should be addressed. E-mail: a.cresswell{at}uq.edu.au.
Previous research has shown that changes in spinal excitability occur during the postural sway of quiet standing. In the present study, it was of interest to examine the independent effects of sway position and sway direction on the efficacy of the triceps surae Ia pathway, as reflected by the H-reflex amplitude, during standing. Eighteen participants, tested under two different experimental protocols, stood quietly on a force platform. Percutaneous electrical stimulation was applied to the posterior tibial nerve when the position and direction of antero-posterior center of pressure signal satisfied the criteria for the various experimental conditions. It was found that regardless of sway position, a larger amplitude of the triceps surae H-reflex (difference of 9-14%; P=0.005) occurred when subjects were swaying in the forward as compared to the backward direction. The effects of sway position, independent of the sway direction, on spinal excitability exhibited a trend (P=0.075), with a 8.9±3.7% increase in the H-reflex amplitude occurring when subjects were in a more forward position. The observed changes to the efficacy of the Ia pathway cannot be attributed to changes in stimulus intensity, as indicated by a constant M-wave amplitude, or to the small changes in the level of background electromyographic activity. One explanation for the changes in reflex excitability with respect to the postural sway of standing, is that the neural modulation may be related to the small lengthening and shortening contractions occurring in the muscles of the triceps surae.
