Cortical and Spinal Mechanisms of Task Failure of Sustained Submaximal Fatiguing Contractions

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.

Short-interval cortical inhibition and intracortical facilitation during submaximal voluntary contractions changes with fatigue

“…The decrease in SICI we observed during sustained fatiguing voluntary activity of the elbow flexor muscles is consistent with the reduction in intracortical inhibition (SICI) when tested after fatiguing exercise while the muscle was at rest (Benwell et al 2006; Maruyama et al 2006; Takahashi et al 2009; Vucic et al 2011), and the recent report of decreasing inhibition during sustained submaximal contractions with a constant target force and hence, increasing EMG (Williams et al 2014). Voluntary activity that is not fatiguing lessens SICI (Ridding et al 1995; Fisher et al 2002; Ortu et al 2008) but here SICI decreased from control contractions to the end of the fatiguing contraction with a constant EMG (Figure 2 shows an increase in values toward the unconditioned MEP, indicating less inhibition).…”

“…Others however, showed no significant changes in ICF of the exercised limb after fatigue (Maruyama et al 2006), although fatigue suppressed ICF in the homologous non-exercising muscle for up to 6 min after a fatiguing contraction of the first dorsal interosseous (FDI) (Baumer et al 2002). In addition, Williams et al (2014) found no change in ICF during a fatiguing submaximal contraction. However, facilitation prior to fatigue was minimal (conditioned MEP 104–107% of unconditioned).…”

“…In the non-fatigued muscle, voluntary drive reduces the excitability of the cortical inhibitory circuits, as measured by SICI, in cortical areas that represent the active muscle (Ridding et al 1995; Reynolds and Ashby 1999; Ortu et al 2008). Fatigue may further decrease the excitability of these circuits during voluntary drive (Williams et al 2014), although it is not known if this decrease in SICI is independent of the increased voluntary drive that occurs during a constant-force contraction. The progressive increase in voluntary drive during a submaximal constant-force contraction presumably occurs because the active muscle fibers become fatigued and more motor units are recruited to maintain the required force.…”

“…Similar to SICI, ICF has been tested with the muscle at rest after fatiguing exercise and yielded conflicting results; i.e., it either did not change (Maruyama et al 2006) or decreased (Tergau et al 2000). During a fatiguing voluntary contraction, ICF was not altered (Williams et al 2014); although, like SICI, ICF is reduced during voluntary contraction compared with rest in the absence of fatigue (Ridding et al 1995; Hanajima et al 2002). It is not known how ICF responds to fatigue if one eliminates the increase in voluntary drive that occurs when force is maintained by keeping the EMG constant.…”

Sex comparisons of agonist and antagonist muscle electromyographic parameters during two different submaximal isometric fatiguing tasks

“…; Williams et al. ), our more extended (135‐degree) elbow joint provided a slightly different position. In addition to the obvious biomechanical changes, potential changes in the muscle activation patterns of the synergistic muscles (e.g., deltoid muscles, rotator cuffs) might also have occurred (Rudroff et al.…”