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

Abstract

It has been suggested that task-specific changes in neurophysiological function (neuroplasticity) should be assessed using testing modalities that replicate the characteristics of the intervention. The squat is a commonly prescribed resistance exercise that has been shown to elicit changes in CNS function. However, previous studies have assessed squat-induced neuroplasticity using isometric knee extension, potentially confounding the results. The aim of the present study was to assess the agreement between corticospinal and intracortical activity relating to the knee extensors during isometric knee extension compared with an isometric squat task. Eleven males completed a neurophysiological assessment in an isometric squat (IS) and knee-extension (KE) task matched for joint angles (hip, knee and ankle). Single- and paired-pulse transcranial magnetic stimulation was delivered during isometric contractions at a range of intensities to assess short-interval cortical inhibition (SICI) and corticospinal excitability. Group mean values for SICI (70 ± 14 versus 63 ± 12% of unconditioned motor evoked potential during IS and KE, respectively) and corticospinal excitability (mean differences 2-5% of the maximal compound muscle action potential at 25, 50, 75 and 100% maximal voluntary contraction between the IS and KE) were not different between the two tasks (P > 0.05) in the vastus lateralis. However, limits of agreement were wide, with poor-to-moderate average intraclass correlation coefficients (ICCs) (SICI, ICC = 0.15; corticospinal excitability, average ICC range = 0.0-0.63), indicating disparate corticospinal and intracortical activity between the IS and KE. These data highlight the importance of task specificity when assessing the modulation of corticospinal excitability and SICI in response to interventions resulting in neuroplastic changes.

Electrical stimulation of human corticospinal axons at the level of the lumbar spinal segments

“…As is well established, MEPs are inherently variable due to the fluctuating nature of corticospinal and motoneuronal excitability (Ellaway et al., ; Kiers, Cros, Chiappa, & Fang, ), randomness in the firing of pyramidal tract neurons and spinal motoneurons (Pitcher, Ogston, & Miles, ) as well as desynchronisation of action potentials (Magistris, Rösler, Truffert, & Myers, ). The variability of responses observed in the present study is comparable to that reported previously when similar numbers of pulses were employed (Biabani, Farrell, Zoghi, Egan, & Jaberzadeh, ; Brownstein, Ansdell, Škarabot, Howatson, et al., ). A greater variability in MEPs compared to LEPs can perhaps be explained by differences in the complexity of the responses to TMS as opposed to LS as discussed above.…”

“…The variability of evoked responses can be reduced by eliciting responses during a contraction (Darling, Wolf, & Butler, ), which is shown in the present data for both MEPs and LEPs. Due to inherent variability of evoked responses, a large quantity of evoked responses are recommended to ascertain a stable index of corticospinal excitability (Brownstein, Ansdell, Škarabot, Howatson, et al., ). The present data suggest that when using LS to evoke LEPs, fewer responses might be required compared to TMS evoked MEPs.…”

“…Thus, when targeting the lower limb muscles, stimuli delivered lower on the spinal tract might provide an alternative methodological paradigm to activate the descending corticospinal axons. One further consideration for assessing lower limb corticospinal excitability is that responses are commonly evoked during muscle contraction (Brownstein, Ansdell, Škarabot, Frazer, et al, 2018;Sidhu, Hoffman, Cresswell, & Carroll, 2012;Škarabot, Tallent, Goodall, Durbaba, & Howatson, 2018). This is recommended when responses during or following locomotion are of interest (Gruet et al, 2013;Kalmar, 2018;Weavil & Amann, 2018).…”

Effects of caffeine on neuromuscular function in a non‐fatigued state and during fatiguing exercise

“…This has also been observed earlier (e.g. Brownstein et al., 2018). Previous studies did not show a significant effect of caffeine on intracortical excitatory mechanisms (de Carvalho et al., 2010; Orth et al., 2005), but we cannot be sure that we were, in fact, testing this association owing to our inability to induce an average ratio of conditioned/unconditioned MEP amplitude >100%.…”

An optimal protocol for measurement of corticospinal excitability, short intracortical inhibition and intracortical facilitation in the rectus femoris

“…Subthreshold CS intensities of 60, 70, 80 and 90% AMT were applied. Inter-stimulus intervals of 2 (Brownstein et al 2018;Brownstein et al 2017;Goodall et al 2018) and 3 ms (O'Leary et al 2016;Thomas et al 2017b) for SICI and 10 (Di Lazzaro et al 2006;Volz et al 2012) and 15 ms (Chen et al 1998;Orth et al 2003) for ICF were examined at each CS intensity since these ISIs successfully elicited inhibition and facilitation in a number of previous studies. The order of conditions was pseudo-randomised and counterbalanced.…”