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Increased spinal reflex excitability is not associated with neural plasticity underlying the cross-education effect

¹Human Neurophysiology Laboratory, Centre for Neuroscience, University of Alberta, Edmonton, Alberta; ²Rehabilitation Neuroscience Laboratory, University of Victoria, Victoria, British Columbia; ³International Collaboration on Repair Discoveries, Vancouver, British Columbia; and ⁴School of Physical Education, University of Victoria, Victoria, British Columbia, Canada

Submitted 5 May 2005 ; accepted in final form 31 August 2005

The purpose of this study was to examine the effects of a 5-wk unilateral, isometric strength-training program on plasticity in the spinal Hoffmann (H-) reflex in both the trained and untrained legs. Sixteen participants, 22–42 yr old, were assigned to either a control (n = 6) or an exercise group (n = 10). Both groups were tested for plantar flexion maximal voluntary isometric contractions (MVIC) and soleus H-reflex amplitude in both limbs, at the beginning and at the end of a 5-wk interval. Participants in the exercise group showed significantly increased MVIC in both legs after training (P < 0.05), whereas strength was unchanged in the control group for either leg. Subjects in the exercise group displayed increased (P < 0.05) H-reflex amplitudes on the ascending limb of the recruitment curve (at an equivalent M wave of 5% of the maximal M wave, H_A) only in the trained leg. Maximal H-reflex and M-wave remained unchanged with training. Increased amplitude of H_A in the trained limb concurrent with increased strength suggests that spinal mechanisms may underlie the changes in strength, possibly because of increased -motoneuronal excitability or reduced presynaptic inhibition. Despite a similar increase in strength in the contralateral limb of the exercise group, H_A amplitude was unchanged. We conclude that the cross-education effect of strength training may be due to supraspinal to a greater extent than spinal mechanisms.

H-reflex; isometric; strength; adaptation; Hebbian synapse

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