Introduction: Chronic unimanual motor practice increases the motor output not only in the trained but also in the nonexercised homologous muscle in the opposite limb. We examined the hypothesis that adaptations in motor cortical excitability of the nontrained primary motor cortex (iM1) and in interhemispheric inhibition from the trained to the nontrained M1 mediate this interlimb cross education.
Methods: Healthy, young volunteers (n = 12) performed 1000 submaximal voluntary contractions (MVC) of the right first dorsal interosseus (FDI) at 80% MVC during 20 sessions.
Results: Trained FDI's MVC increased 49.9%, and the untrained FDI's MVC increased 28.1%. Although corticospinal excitability in iM1, measured with transcranial magnetic stimulation (TMS) before and after every fifth session, increased 6% at rest, these changes, as those in intracortical inhibition and facilitation, did not correlate with cross education. When weak and strong TMS of iM1 were delivered on a background of a weak and strong muscle contraction, respectively, of the right FDI, excitability of iM1 increased dramatically after 20 sessions. Interhemispheric inhibition decreased 8.9% acutely within sessions and 30.9% chronically during 20 sessions and these chronic reductions progressively became more strongly associated with cross education. There were no changes in force or TMS measures in the trained group's left abductor minimi digiti and there were no changes in the nonexercising control group (n = 8).
Conclusions: The findings provide the first evidence for plasticity of interhemispheric connections to mediate cross education produced by a simple motor task.
1East Carolina University, Greenville, NC; 2University of New Mexico, Albuquerque, NM; 3The Bekhterev Neuropsychological Institute, St. Petersburg, RUSSIA; 4The National Institutes of Health, National Institute of Neurological Disorders and Stroke, Bethesda, MD; 5INSERM U731/UPMC, Service de Réadaptation Fonctionnelle, Paris, FRANCE; and 6Tagesklinik, Zentrum für Psychiatrische Rehabilitation, Zürich, SWITZERLAND
Address for correspondence: Tibor Hortobágyi, Ph.D., FACSM, 332a Ward Sports Medicine Bldg., Department of Exercise and Sport Science, East Carolina University, Greenville, NC 27858; E-mail: firstname.lastname@example.org.
Submitted for publication September 2010.
Accepted for publication December 2010.