Effect of short-term resistance training on corticospinal and reticulospinal excitability in biceps brachii

Abstract

BACKGROUND: Descending pathways, such as the corticospinal (CST) and reticulospinal tracts (RST), are highly influential structures that mediate movement. Although the two tracts have differences in their motor function (Lemon 2008), it has been postulated that both may contribute to neural adaptations to short-term resistance training (Siddique et al. 2020; Glover & Baker 2020). Studies using transcranial magnetic stimulation (TMS), a non-invasive form of brain stimulation, to measure intracortical and CST adaptations to resistance training have yielded equivocal results (Siddique et al. 2020). The effect of resistance training on RST excitability has yet to be investigated in humans. Therefore, the aim of this study was to measure adaptations of the CST and RST to a 6-week resistance training program. Two forms of resistance training (explosive and sustained contractions) were implemented to ascertain whether motor pathway adaptations are modulated by training type.

METHODS: Thirteen healthy young males were recruited and distributed into 3 groups: control (n = 5), explosive-contraction training (n = 4), and sustained-contraction training (n = 4). The 6-week at-home resistance training programs were conducted with a novel device that measured force production from isometric elbow flexion. Maximal strength was quantified by maximal voluntary contractions (MVCs) and explosive strength by rate of force development (RFD). Corticospinal adaptations were assessed by posterior-anterior (PA) current-induced motor evoked potential (MEP) amplitude, MEP latency, cortical silent period, and recruitment curve. Reticulospinal adaptations were measured using the StartReact test, loud acoustic stimulation (LAS) paired with TMS, and MEPs induced by anterior-posterior (AP) current orientation. Paired-sample t-tests were used for within-subject comparison to detect changes after the resistance training program. The alpha level of 0.05 was adjusted to α = 0.016 to account for multiple comparisons.

RESULTS: Strength assessments did not yield significant changes in MVC or RFD in any group. Significant StartReact effects were observed across groups in both experimental sessions (p = 0.001), but there were no changes in the effect after training. Significant changes were not observed in MEP parameters for any group. No changes were observed in MEP suppression from the LAS paired with TMS protocol in any group, but MEP suppression occurred in the majority of participants in both experimental sessions (pre-intervention: 66.7%, post-intervention: 72.7%).

CONCLUSION: The lack of significant improvements in MVC or RFD in either training group indicates that the resistance training programs were not effective. It is also possible that small sample sizes limited statistical power, masking increases in MVC and RFD comparable to other resistance training studies. There were negligible changes in neural measures, and therefore, modulation of CST or RST excitability cannot be concluded. Baseline data from the neurophysiological tests were similar to previous studies, which substantiates the validity of these tests. Therefore, it is likely that the minimal observed neural adaptations are likely due to a lack of changes in neural function, rather than limitations in neural assessments.