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This Article Full Text Full Text (PDF) All Versions of this Article: 107/1/112    most recent 91541.2008v1 Submit a response Alert me when this article is cited Alert me when eLetters are posted Alert me if a correction is posted Citation Map Services Email this article to a friend Similar articles in this journal Similar articles in PubMed Alert me to new issues of the journal Download to citation manager Google Scholar Articles by Hoffman, B. W. Articles by Cresswell, A. G. PubMed PubMed Citation Articles by Hoffman, B. W. Articles by Cresswell, A. G.

Increases in corticospinal responsiveness during a sustained submaximal plantar flexion

¹School of Human Movement Studies and ³School of Health and Rehabilitation Sciences, University of Queensland, Brisbane; and ²School of Medical Sciences, Faculty of Medicine, University of New South Wales, Sydney, Australia

Submitted 25 November 2008 ; accepted in final form 8 May 2009

Studying the responsiveness of specific central nervous system pathways to electrical or magnetic stimulation can provide important information regarding fatigue processes in the central nervous system. We investigated the changes in corticospinal responsiveness during a sustained submaximal contraction of the triceps surae. Comparisons were made between the size of motor-evoked potentials (MEPs) elicited by motor cortical stimulation and cervicomedullary motor-evoked potentials (CMEPs) elicited by magnetic stimulation of the descending tracts to determine the site of any change in corticospinal responsiveness. Participants maintained an isometric contraction of triceps surae at 30% of maximal voluntary contraction (MVC) for as long as possible on two occasions. Stimulation was applied to the motor cortex or the cervicomedullary junction at 1-min intervals during contraction until task failure. Peripheral nerve stimulation was also applied to evoke maximal M waves (M_max) and a superimposed twitch. Additionally, MEPs and CMEPs were evoked during brief contractions at 80%, 90%, and 100% of MVC as a nonfatigue control. During the sustained contractions, MEP amplitude increased significantly in soleus (113%) and medial gastrocnemius (108%) muscles and, at task failure, matched MEP amplitude in the prefatigue MVC (20–25% M_max). In contrast, CMEP amplitude increased significantly in medial gastrocnemius (51%), but not in soleus (63%) muscle and, at task failure, was significantly smaller than during prefatigue MVC (5–6% M_max vs. 11–13% M_max). The data indicate that cortical processes contribute substantially to the increase in corticospinal responsiveness during sustained submaximal contraction of triceps surae.

lower limb; central nervous system; muscle; transcranial magnetic stimulation; electromyogram