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

Abstract

This study determined whether short-interval intracortical inhibition (SICI) and intracortical facilitation (ICF) change during a sustained submaximal isometric contraction. On two days, 12 participants (6 men, 6 women) performed brief (7-s) elbow flexor contractions before and after a 10-minute fatiguing contraction; all contractions were performed at the level of integrated electromyographic activity (EMG) which produced 25% maximal unfatigued torque. During the brief 7-s and 10-minute submaximal contractions, single (test) and paired (conditioning-test) transcranial magnetic stimuli were applied over the motor cortex (5s apart) to elicit motor evoked potentials (MEPs) in biceps brachii. SICI and ICF were elicited on separate days, with a conditioning-test interstimulus interval of 2.5ms and 15ms respectively. On both days, integrated EMG remained constant while torque fell during the sustained contraction by ~51.5% from control contractions, perceived effort increased 3 fold and MVC declined by 21–22%. For SICI, the conditioned MEP during control contractions (74.1±2.5% of unconditioned MEP) increased (less inhibition) during the sustained contraction (last 2.5 min: 86.0±5.1%; P<0.05). It remained elevated in recovery contractions at 2 minutes (82.0±3.8%; P<0.05) and returned toward control at 7-mins recovery (76.3±3.2%). ICF during control contractions (conditioned MEP 129.7±4.8% of unconditioned MEP) decreased (less facilitation) during the sustained contraction (last 2.5 min: 107.6±6.8% P<0.05) and recovered to 122.8±4.3% during contractions after 2 minutes of recovery. Both intracortical inhibitory and facilitatory circuits become less excitable with fatigue when assessed during voluntary activity, but their different time courses of recovery suggest different mechanisms for the fatigue-related changes of SICI and ICF.

Anodal transcranial direct current stimulation does not influence the neural adjustments associated with fatiguing contractions in a hand muscle

“…In addition, the increased duration in EMG silent period that follows the MEP suggests greater responsiveness of intracortical inhibitory circuits. This may reflect an adaptive modulation in central activation and inhibition processes to optimize the descending drive (Benwell et al 2006a(Benwell et al , b, 2007Lévénez et al 2008;Søgaard et al 2006). Even if there is a safety margin (Cooper et al 1988;Duchateau and Hainaut 1985), the reduction in M max amplitude may reflect possible alterations in neuromuscular propagation, contributing thereby to reduce the force developed by the muscle.…”

Motor cortical and corticospinal function differ during an isometric squat compared with isometric knee extension

“…Single‐pulse TMS permits the quantitative assessment of corticospinal excitability through the size of the compound electromyography (EMG) response, whereas paired‐pulse TMS separated by 2–5 and 10–15 ms can be used to examine intracortical inhibitory (termed short‐interval intracortical inhibition; SICI) and facilitatory circuits (termed intracortical facilitation; ICF), respectively (Kujirai et al., ). Single‐ and paired‐pulse TMS protocols have been used as tools to investigate responses to exercise, such as fatiguing isometric single‐limb contractions (Goodall, Howatson, & Thomas, ; Hunter, McNeil, Butler, Gandevia, & Taylor, ; Kennedy, McNeil, Gandevia, & Taylor, ) and locomotor exercise (Brownstein et al., ; Sidhu, Cresswell, & Carroll, ; Thomas, Dent, Howatson, & Goodall, ), mechanisms of locomotion (Sidhu, Cresswell, & Carroll, ) and neural adaptations to strength training (Weier & Kidgell, ).…”

Investigating the effects of muscle contraction and conditioning stimulus intensity on short‐interval intracortical inhibition

“…This raises an empirical question from a study design perspective, as to whether outcome measures should be obtained when controlling for force output (about a joint) or muscular EMG, when assessing neuromuscular adaptations following both acute‐ and long‐term interventions. It is likely that an experimental design that utilizes rmsEMG to standardize target muscle activation (rather than total force output) would have detected more subtle changes in SICI and excitability that were not observed in this experiment (Hunter et al., ). As such, we suggest that our results should be interpreted as the effects of BB SICI on elbow flexion force output, not BB activation.…”

“…Overall, our data indicate that the SICI CS paradigm should be optimized for individual participants (i.e., evoke a pre‐determined level of inhibition across participants). Such an approach has been used in previous investigations and considered sensitive to detect changes in SICI (Benwell et al., ; Hunter et al., ) and thus may be considered in future investigations.…”

“…Moreover, changes in SICI have also been suggested to underpin deficits in central motor control of physical function (e.g., strength or fine motor control) during and immediately following maximal or submaximal muscle actions. The evidence is, however, mixed with the reports of increased inhibition of the hand and leg extensors (Maruyama, Matsunaga, Tanaka, & Rothwell, ; Takahashi et al., ; Teo, Rodrigues, Mastaglia, & Thickbroom, ), less inhibition of the hand and elbow flexors (Benwell et al., ; Hunter, McNeil, Butler, Gandevia, & Taylor, ) or no change in the leg extensors (Goodall, Howatson, & Thomas, ).…”