title: Intensity-dependent alterations in the excitability of cortical and spinal projections to the knee extensors during isometric and locomotor exercise.
abstract: We investigated the role of exercise intensity and associated central motor drive in determining corticomotoneuronal excitability. Ten participants performed a series of nonfatiguing (3 s) isometric single-leg knee extensions (ISO; 10-100% of maximal voluntary contractions, MVC) and cycling bouts (30-160% peak aerobic capacity, W peak). At various exercise intensities, electrical potentials were evoked in the vastus lateralis (VL) and rectus femoris (RF) via transcranial magnetic stimulation (motor-evoked potentials, MEP), and electrical stimulation of both the cervicomedullary junction (cervicomedullary evoked potentials, CMEP) and the femoral nerve (maximal M-waves, M max). Whereas M max remained unchanged in both muscles (P > 0.40), voluntary electromyographic activity (EMG) increased in an exercise intensity-dependent manner for ISO and cycling exercise in VL and RF (both P < 0.001). During ISO exercise, MEPs and CMEPs progressively increased in VL and RF until a plateau was reached at ∼ 75% MVC; further increases in contraction intensity did not cause additional changes (P > 0.35). During cycling exercise, VL-MEPs and CMEPs progressively increased by ∼ 65% until a plateau was reached at W peak. In contrast, RF MEPs and CMEPs progressively increased by ∼ 110% throughout the tested cycling intensities without the occurrence of a plateau. Furthermore, alterations in EMG below the plateau influenced corticomotoneuronal excitability similarly between exercise modalities. In both exercise modalities, the MEP-to-CMEP ratio did not change with exercise intensity (P > 0.22). In conclusion, increases in exercise intensity and EMG facilitates the corticomotoneuronal pathway similarly in isometric knee extension and locomotor exercise until a plateau occurs at a submaximal exercise intensity. This facilitation appears to be primarily mediated by increases in excitability of the motoneuron pool.
authors: Department of Exercise and Sport Science, University of Utah, Salt Lake City, Utah;
authors: J C
authors: JC
authors: Weavil
authors: Department of Internal Medicine, University of Utah, Salt Lake City, Utah;
authors: S K
authors: SK
authors: Sidhu
authors: Department of Exercise and Sport Science, University of Utah, Salt Lake City, Utah;
authors: T S
authors: TS
authors: Mangum
authors: Department of Exercise and Sport Science, University of Utah, Salt Lake City, Utah; Department of Internal Medicine, University of Utah, Salt Lake City, Utah; Geriatric Research, Education, and Clinical Center, Salt Lake City Veterans Affairs Medical Center, Salt Lake City, Utah.
authors: R S
authors: RS
authors: Richardson
authors: Department of Exercise and Sport Science, University of Utah, Salt Lake City, Utah; Department of Internal Medicine, University of Utah, Salt Lake City, Utah; Geriatric Research, Education, and Clinical Center, Salt Lake City Veterans Affairs Medical Center, Salt Lake City, Utah markus.amann@hsc.utah.edu.
authors: M
authors: M
authors: Amann
mesh_terms: D000328:Adult; D001642:Bicycling; D004558:Electric Stimulation / Q000379:methods; D004576:Electromyography; D019054:Evoked Potentials, Motor; D015444:Exercise; D005267:Femoral Nerve / Q000502:physiology; D006801:Humans; D007537:Isometric Contraction; D007717:Knee; D008297:Male; D009043:Motor Activity; D009044:Motor Cortex / Q000502:physiology*; D009046:Motor Neurons / Q000502:physiology*; D011712:Pyramidal Tracts / Q000502:physiology*; D052097:Quadriceps Muscle / Q000294:innervation*; D013997:Time Factors; D050781:Transcranial Magnetic Stimulation
keywords: cervicomedullary stimulation; motor cortex; spinal motoneurons; transcranial stimulation