Cortical and spinal mechanisms of task failure of sustained submaximal fatiguing contractions

Williams, Petra S. and Hoffman, Richard L. and Clark, Brian C. (2014) Cortical and spinal mechanisms of task failure of sustained submaximal fatiguing contractions. PLoS ONE, 9 (3). e93284. ISSN 1932-6203

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Abstract

In this and the subsequent companion paper, results are presented that collectively seek to delineate the contribution that supraspinal circuits have in determining the time to task failure (TTF) of sustained submaximal contractions. The purpose of this study was to compare adjustments in supraspinal and spinal excitability taken concurrently throughout the performance of two different fatigue tasks with identical mechanical demands but different TTF (i.e., force-matching and position-matching tasks). On separate visits, ten healthy volunteers performed the force-matching or position-matching task at 15% of maximum strength with the elbow flexors to task failure. Single-pulse transcranial magnetic stimulation (TMS), paired-pulse TMS, paired cortico-cervicomedullary stimulation, and brachial plexus electrical stimulation were delivered in a 6-stimuli sequence at baseline and every 2–3 minutes throughout fatigue-task performance. Contrary to expectations, the force-matching task TTF was 42% shorter (17.5±7.9 min) than the position-matching task (26.9±15.11 min; p<0.01); however, both tasks caused the same amount of muscle fatigue (p = 0.59). There were no task-specific differences for the total amount or rate of change in the neurophysiologic outcome variables over time (p>0.05). Therefore, failure occurred after a similar mean decline in motorneuron excitability developed (p<0.02, ES = 0.35–0.52) coupled with a similar mean increase in measures of corticospinal excitability (p<0.03, ES = 0.30–0.41). Additionally, the amount of intracortical inhibition decreased (p<0.03, ES = 0.32) and the amount of intracortical facilitation (p>0.10) and an index of upstream excitation of the motor cortex remained constant (p>0.40). Together, these results suggest that as fatigue develops prior to task failure, the increase in corticospinal excitability observed in relationship to the decrease in spinal excitability results from a combination of decreasing intracortical inhibition with constant levels of intracortical facilitation and upstream excitability that together eventually fail to provide the input to the motor cortex necessary for descending drive to overcome the spinal cord resistance, thereby contributing to task failure.

Item Type:	Article
Publisher’s Statement:	Specific CC license required, no statement seen
ID number or DOI:	10.1371/journal.pone.0093284
Keywords:	Adult; Anatomy; Biology and life sciences; brain; conscious humans; corticocortical inhibition; corticomotor excitability; elbow flexor muscles; Electroporation; Evoked Potentials, Motor; exercise intensity; fatigue; Female; human motor cortex; Humans; Integrative physiology; interval intracortical inhibition; isometric contractions; load type; Male; Mechanical treatment of specimens; Medicine and health sciences; motor neurons; muscle contraction; muscle fatigue; Muscle functions; Muscle physiology; Muscles; Musculoskeletal system; Nervous system; Nervous system physiology; Neuroscience; Physiology; Research and analysis methods; Research Article; Specimen disruption; Specimen preparation and treatment; spinal cord; spine -- Physiology; Systems biology; Time Factors; transcranial magnetic stimulation; voluntary contractions
Subjects:	R Medicine > RC Internal medicine
NAU Depositing Author Academic Status:	Faculty/Staff
Department/Unit:	College of Health and Human Services > Physical Therapy and Athletic Training
Date Deposited:	16 Oct 2015 17:23
URI:	http://openknowledge.nau.edu/id/eprint/1689

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