title: Behaviour of the motoneurone pool in a fatiguing submaximal contraction.
abstract: During fatigue caused by a sustained maximal voluntary contraction (MVC), motoneurones become markedly less responsive when tested during the silent period following transcranial magnetic stimulation (TMS). To determine whether this reduction depends on the repetitive activation of the motoneurones, responses to TMS (motor evoked potentials, MEPs) and to cervicomedullary stimulation (cervicomedullary motor evoked potentials, CMEPs) were tested during a sustained submaximal contraction at a constant level of electromyographic activity (EMG). In such a contraction, some motoneurones are repetitively activated whereas others are not active. On four visits, eight subjects performed a 10 min maintained-EMG elbow flexor contraction of 25% maximum. Test stimuli were delivered with and without conditioning by TMS given 100 ms prior. Test responses were MEPs or CMEPs (two visits each, small responses evoked by weak stimuli on one visit and large responses on the other). During the sustained contraction, unconditioned CMEPs decreased ∼20% whereas conditioned CMEPs decreased ∼75 and 30% with weak and strong stimuli, respectively. Conditioned MEPs were reduced to the same extent as CMEPs of the same size. The data reveal a novel decrease in motoneurone excitability during a submaximal contraction if EMG is maintained. Further, the much greater reduction of conditioned than unconditioned CMEPs shows the critical influence of voluntary drive on motoneurone responsiveness. Strong test stimuli attenuate the reduction of conditioned CMEPs which indicates that low-threshold motoneurones active in the contraction are most affected. The equivalent reduction of conditioned MEPs and CMEPs suggests that, similar to findings with a sustained MVC, impaired motoneurone responsiveness rather than intracortical inhibition is responsible for the fatigue-related impairment of the MEP during a sustained submaximal contraction.
authors: Neuroscience Research Australia, and University of New South Wales, Barker Street, Randwick, New South Wales, Australia, 2031.
authors: Chris J
authors: CJ
authors: McNeil
authors: Sabine
authors: S
authors: Giesebrecht
authors: Simon C
authors: SC
authors: Gandevia
authors: Janet L
authors: JL
authors: Taylor
references: J Appl Physiol (1985). 2009 Jul;107(1):112-20
references: 19443741
references: Exp Brain Res. 2007 May;179(2):255-62
references: 17464523
references: J Neurosci. 2003 Nov 12;23(32):10224-30
references: 14614080
references: J Physiol. 2001 Aug 1;534(Pt 3):903-12
references: 11483719
references: Exp Brain Res. 2011 Mar;209(2):287-97
references: 21267549
references: J Neurophysiol. 2006 Jun;95(6):3512-8
references: 16481454
references: Electroencephalogr Clin Neurophysiol. 1991 Aug;81(4):257-62
references: 1714819
references: Electroencephalogr Clin Neurophysiol. 1992 Dec;85(6):355-64
references: 1282453
references: J Neurophysiol. 2008 Feb;99(2):554-63
references: 18046002
references: J Appl Physiol (1985). 2007 Aug;103(2):560-8
references: 17463302
references: Muscle Nerve. 1996 Aug;19(8):1017-24
references: 8756168
references: J Physiol. 1993 Jan;460:549-72
references: 8387589
references: J Physiol. 1996 Jan 15;490 ( Pt 2):529-36
references: 8821149
references: J Physiol. 1996 Jan 15;490 ( Pt 2):519-28
references: 8821148
references: J Physiol. 2009 Dec 1;587(Pt 23):5517-8
references: 19959549
references: J Physiol. 1989 Jun;413:351-78
references: 2600855
references: J Neurosci. 2006 May 3;26(18):4796-802
references: 16672652
references: Brain Res. 1981 Aug 24;219(1):45-55
references: 7260629
references: J Physiol. 2009 Dec 1;587(Pt 23):5601-12
references: 19805743
references: Scand J Work Environ Health. 1990;16 Suppl 1:55-8
references: 2345867
references: Muscle Nerve. 1997 Jun;20(6):710-7
references: 9149078
references: J Physiol. 1986 Oct;379:451-9
references: 3560001
references: J Physiol. 2002 Jun 15;541(Pt 3):949-58
references: 12068053
references: Electroencephalogr Clin Neurophysiol. 1991 Jun;81(3):240-2
references: 1710974
references: Electroencephalogr Clin Neurophysiol. 1993 Aug;89(4):211-20
references: 7688683
references: J Physiol. 1993 May;464:75-120
references: 8229829
references: Exp Brain Res. 1993;94(3):489-98
references: 8359263
references: Physiol Rev. 2001 Oct;81(4):1725-89
references: 11581501
references: Neurosci Lett. 1993 Jun 25;156(1-2):167-71
references: 8414181
references: J Physiol. 2000 Jun 15;525 Pt 3:793-801
references: 10856130
references: J Neurophysiol. 1997 Nov;78(5):2246-53
references: 9356378
references: Exp Brain Res. 1982;46(2):191-6
references: 6284539
references: J Physiol. 1999 Dec 15;521 Pt 3:749-59
references: 10601504
references: J Physiol. 2006 Jun 1;573(Pt 2):511-23
references: 16556656
references: Rev Physiol Biochem Pharmacol. 2001;143:137-263
references: 11428264
references: Ann Neurol. 1991 Apr;29(4):418-27
references: 1929211
references: Exp Brain Res. 2004 Feb;154(4):479-87
references: 14574429