title: Short-interval cortical inhibition and intracortical facilitation during submaximal voluntary contractions changes with fatigue.
abstract: This study determined whether short-interval intracortical inhibition (SICI) and intracortical facilitation (ICF) change during a sustained submaximal isometric contraction. On 2 days, 12 participants (6 men, 6 women) performed brief (7-s) elbow flexor contractions before and after a 10-min fatiguing contraction; all contractions were performed at the level of integrated electromyographic activity (EMG) which produced 25 % maximal unfatigued torque. During the brief 7-s and 10-min submaximal contractions, single (test) and paired (conditioning-test) transcranial magnetic stimuli were applied over the motor cortex (5 s apart) to elicit motor-evoked potentials (MEPs) in biceps brachii. SICI and ICF were elicited on separate days, with a conditioning-test interstimulus interval of 2.5 and 15 ms, respectively. On both days, integrated EMG remained constant while torque fell during the sustained contraction by ~51.5 % from control contractions, perceived effort increased threefold, and MVC declined by 21-22 %. For SICI, the conditioned MEP during control contractions (74.1 ± 2.5 % of unconditioned MEP) increased (less inhibition) during the sustained contraction (last 2.5 min: 86.0 ± 5.1 %; P < 0.05). It remained elevated in recovery contractions at 2 min (82.0 ± 3.8 %; P < 0.05) and returned toward control at 7-min recovery (76.3 ± 3.2 %). ICF during control contractions (conditioned MEP 129.7 ± 4.8 % of unconditioned MEP) decreased (less facilitation) during the sustained contraction (last 2.5 min: 107.6 ± 6.8 %; P < 0.05) and recovered to 122.8 ± 4.3 % during contractions after 2 min of recovery. Both intracortical inhibitory and facilitatory circuits become less excitable with fatigue when assessed during voluntary activity, but their different time courses of recovery suggest different mechanisms for the fatigue-related changes of SICI and ICF.
authors.affiliation: Exercise Science Program, Department of Physical Therapy, Marquette University, P.O. Box 1881, Milwaukee, WI, 53201, USA. Sandra.Hunter@marquette.edu.
authors.forename: Sandra K
authors.initials: SK
authors.lastname: Hunter
authors.affiliation: School of Health and Exercise Sciences, University of British Columbia, Kelowna, BC, Canada.
authors.forename: Chris J
authors.initials: CJ
authors.lastname: McNeil
authors.affiliation: Neuroscience Research Australia, Sydney, Australia.|School of Medical Sciences, The University of New South Wales, Sydney, Australia.
authors.forename: Jane E
authors.initials: JE
authors.lastname: Butler
authors.affiliation: Neuroscience Research Australia, Sydney, Australia.|Prince of Wales Clinical School, The University of New South Wales, Sydney, Australia.
authors.forename: Simon C
authors.initials: SC
authors.lastname: Gandevia
authors.affiliation: Neuroscience Research Australia, Sydney, Australia.|School of Medical Sciences, The University of New South Wales, Sydney, Australia.
authors.forename: Janet L
authors.initials: JL
authors.lastname: Taylor
mesh_terms: D000328:Adult; D004558:Electric Stimulation / Q000379:methods; D019054:Evoked Potentials, Motor / Q000502:physiology*; D005260:Female; D006801:Humans; D008297:Male; D008875:Middle Aged; D009044:Motor Cortex / Q000502:physiology*; D009119:Muscle Contraction / Q000502:physiology*; D018763:Muscle Fatigue / Q000502:physiology*; D018482:Muscle, Skeletal / Q000502:physiology*; D009433:Neural Inhibition / Q000502:physiology; D055815:Young Adult
keywords: Cortical excitation; Cortical inhibition; Elbow flexor muscles; Motor-evoked potentials; Muscle fatigue; Transcranial magnetic stimulation