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J Physiol. 2018 Oct;596(19):4789-4801. doi: 10.1113/JP276460. Epub 2018 Sep 3.

Fatigue-related group III/IV muscle afferent feedback facilitates intracortical inhibition during locomotor exercise.

Author information

1
Department of Internal Medicine, University of Utah, Salt Lake City, UT, USA.
2
Adelaide Medical School, Discipline of Physiology, The University of Adelaide, Australia.
3
Geriatric Research, Education, and Clinical Center, VAMC, Salt Lake City, UT, USA.
4
Department of Nutrition and Integrative Physiology, University of Utah, Salt Lake City, UT, USA.
5
Department of Anesthesiology, University of Utah, Salt Lake City, UT, USA.
6
Department of Circulation and Medical Imaging, Faculty of Medicine, Norwegian University of Science and Technology, Trondheim, Norway.
7
Centre for Heart, Lung and Vascular Health, School of Health and Exercise Sciences, University of British Columbia, Kelowna, British Columbia, Canada.

Abstract

KEY POINTS:

This study investigated the influence of group III/IV muscle afferents on corticospinal excitability during cycling exercise and focused on GABAB neuron-mediated inhibition as a potential underlying mechanism. The study provides novel evidence to demonstrate that group III/IV muscle afferent feedback facilitates inhibitory intracortical neurons during whole body exercise. Firing of these interneurons probably contributes to the development of central fatigue during physical activity.

ABSTRACT:

We investigated the influence of group III/IV muscle afferents in determining corticospinal excitability during cycling exercise and focused on GABAB neuron-mediated inhibition as a potential underlying mechanism. Both under control conditions (CTRL) and with lumbar intrathecal fentanyl (FENT) impairing feedback from group III/IV leg muscle afferents, subjects (n = 11) cycled at a comparable vastus-lateralis EMG signal (∼0.26 mV) before (PRE; 100 W) and immediately after (POST; 90 ± 2 W) fatiguing constant-load cycling exercise (80% Wpeak; 221 ± 10 W; ∼8 min). During, PRE and POST cycling, single and paired-pulse (100 ms interstimulus interval) transcranial magnetic stimulations (TMS) were applied to elicit unconditioned and conditioned motor-evoked potentials (MEPs), respectively. To distinguish between cortical and spinal contributions to the MEPs, cervicomedullary stimulations (CMS) were used to elicit unconditioned (CMS only) and conditioned (TMS+CMS, 100 ms interval) cervicomedullary motor-evoked potentials (CMEPs). While unconditioned MEPs were unchanged from PRE to POST in CTRL, unconditioned CMEPs increased significantly, resulting in a decrease in unconditioned MEP/CMEP (P < 0.05). This paralleled a reduction in conditioned MEP (P < 0.05) and no change in conditioned CMEP. During FENT, unconditioned and conditioned MEPs and CMEPs were similar and comparable during PRE and POST (P > 0.2). These findings reveal that feedback from group III/IV muscle afferents innervating locomotor muscle decreases the excitability of the motor cortex during fatiguing cycling exercise. This impairment is, at least in part, determined by the facilitating effect of these sensory neurons on inhibitory GABAB intracortical interneurons.

KEYWORDS:

central fatigue; intracortical inhibition; sensory muscle afferents; whole body exercise

PMID:
30095164
PMCID:
PMC6166070
[Available on 2019-10-01]
DOI:
10.1113/JP276460

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