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.
Highlights
For healthy individuals performing sustained contractions, fatigue is an expected and normal physiologic reaction that inevitably leads to task failure [1,2]
Submaximal contractions performed with the elbow flexors at an intensity up to 30% of maximum force result in the same amount of muscle fatigue at task failure; the time to task failure (TTF) is nearly twice as long for the force-matching task compared to the position-matching task [12,15,16]
Women exhibited a greater magnitude of difference in TTF (TTFDiffP-F) between the two fatigue-tasks (TASK 6GENDER: p,0.01); when the absolute 15% MVC target-force was statistically covaried, the effect of GENDER on TTFDiffP-F was no longer significant (TASK 6GENDER: p = 0.81; it should be noted that TASK MAIN EFFECT persisted in the covariate analysis, p,0.01)
Summary
For healthy individuals performing sustained contractions, fatigue is an expected and normal physiologic reaction that inevitably leads to task failure [1,2]. It is well accepted that there is not one single cause of fatigue; instead, the physiologic mechanisms behind the decline in force output that prohibits indefinite task performance are specific to the task demands (i.e., contraction intensity, duration, mode, muscle group, joint angle, limb posture, and stabilization) that collectively stress different regions of the neuromuscular pathway responsible for the contraction in order to sustain the required force output [1,3,4,7,8,9,10,11,12]. Under otherwise identical mechanical demands during sustained submaximal contractions, neurophysiologic measures have been found to differ with load compliance (e.g. exert a constant force against a rigid restraint or hold an equivalent inertial weight) which requires individuals to attend to distinct performance feedback variables, to either force output (force-matching task) or joint angle position (position-matching task) [5,7,12,15,16]. Submaximal contractions performed with the elbow flexors at an intensity up to 30% of maximum force result in the same amount of muscle fatigue at task failure; the time to task failure (TTF) is nearly twice as long for the force-matching task compared to the position-matching task [12,15,16]
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