title: Fatigue diminishes motoneuronal excitability during cycling exercise.
abstract: Exercise-induced fatigue influences the excitability of the motor pathway during single-joint isometric contractions. This study sought to investigate the influence of fatigue on corticospinal excitability during cycling exercise. Eight men performed fatiguing constant-load (80% Wpeak; 241 ± 13 W) cycling to exhaustion during which the percent increase in quadriceps electromyography (ΔEMG; vastus lateralis and rectus femoris) was quantified. During a separate trial, subjects performed two brief (∼45 s) nonfatiguing cycling bouts (244 ± 15 and 331 ± 23W) individually chosen to match the ΔEMG across bouts to that observed during fatiguing cycling. Corticospinal excitability during exercise was quantified by transcranial magnetic, electric transmastoid, and femoral nerve stimulation to elicit motor-evoked potentials (MEP), cervicomedullary evoked potentials (CMEP), and M waves in the quadriceps. Peripheral and central fatigue were expressed as pre- to postexercise reductions in quadriceps twitch force (ΔQtw) and voluntary quadriceps activation (ΔVA). Whereas nonfatiguing cycling caused no measureable fatigue, fatiguing cycling resulted in significant peripheral (ΔQtw: 42 ± 6%) and central (ΔVA: 4 ± 1%) fatigue. During nonfatiguing cycling, the area of MEPs and CMEPs, normalized to M waves, similarly increased in the quadriceps (∼40%; P < 0.05). In contrast, there was no change in normalized MEPs or CMEPs during fatiguing cycling. As a consequence, the ratio of MEP to CMEP was unchanged during both trials (P > 0.5). Therefore, although increases in muscle activation promote corticospinal excitability via motoneuronal facilitation during nonfatiguing cycling, this effect is abolished during fatigue. We conclude that the unaltered excitability of the corticospinal pathway from start of intense cycling exercise to exhaustion is, in part, determined by inhibitory influences on spinal motoneurons obscuring the facilitating effects of muscle activation.
authors.affiliation: Department of Exercise & Sport Science, University of Utah, Salt Lake City, Utah.
authors.forename: Joshua C
authors.initials: JC
authors.lastname: Weavil
authors.affiliation: Department of Internal Medicine, University of Utah, Salt Lake City, Utah; Discipline of Physiology, University of Adelaide, Australia; and.
authors.forename: Simranjit K
authors.initials: SK
authors.lastname: Sidhu
authors.affiliation: Department of Exercise & Sport Science, University of Utah, Salt Lake City, Utah.
authors.forename: Tyler S
authors.initials: TS
authors.lastname: Mangum
authors.affiliation: Department of Exercise & 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, George E. Whalen Veterans Affairs Medical Center, Salt Lake City, Utah.
authors.forename: Russell S
authors.initials: RS
authors.lastname: Richardson
authors.affiliation: Department of Exercise & 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, George E. Whalen Veterans Affairs Medical Center, Salt Lake City, Utah markus.amann@hsc.utah.edu.
authors.forename: Markus
authors.initials: M
authors.lastname: Amann
mesh_terms: D000328:Adult; D001642:Bicycling / Q000502:physiology*; D004558:Electric Stimulation; D004576:Electromyography; D019054:Evoked Potentials, Motor; D015444:Exercise / Q000502:physiology*; D005267:Femoral Nerve / Q000502:physiology; D006801:Humans; D007866:Leg / Q000502:physiology*; D008297:Male; D009046:Motor Neurons / Q000502:physiology*; D018763:Muscle Fatigue / Q000502:physiology*; D018482:Muscle, Skeletal / Q000502:physiology*; D011712:Pyramidal Tracts / Q000502:physiology; D050781:Transcranial Magnetic Stimulation
keywords: corticospinal pathway; electromyography; motor cortex; motor neuron