Training intensity‐dependent increases in corticospinal but not intracortical excitability after acute strength training

Abstract

The purpose of this study was to determine whether the increases in corticospinal excitability (CSE) observed after one session of unilateral isometric strength training (ST) are related to changes in intracortical excitability measured by magnetic brain stimulation (TMS) in the trained and the contralateral untrained biceps brachii (BB) and whether such changes scale with training intensity. On three separate days, 15 healthy young men performed one ST session of 12 sets of eight isometric contractions of the right elbow flexors at 0% (control session), 25%, or 75% of the maximal voluntary contraction (MVC) in a random order. Before and after each session separated at least by 1 week, motor evoked potential (MEP) amplitude, short‐interval intracortical inhibition (SICI), contralateral silent period (SP), and intracortical facilitation (ICF) generated by TMS were measured in the trained and the untrained BBs. Compared with baseline, MEPs recorded from the trained BB increased by ~47% after training at 75% of MVC (P < .05) but not after training at 0% (~4%) or 25% MVC (~5%, both P > .05). MEPs in the untrained BB and SICI, SP, and ICF in either BB did not change. Therefore, acute high‐intensity but not low‐intensity unilateral isometric ST increases CSE in the trained BB without modifications in intracortical inhibition or facilitation. Thus, increases in corticospinal neurons or α‐α‐motoneuron excitability could underlie the increases in CSE. Regardless of contraction intensity, acute isometric ST did not modify the excitability of the ipsilateral primary motor cortex measured by TMS.

High-Volume Light-Load Strength Training, but Not Low-Volume Heavy-Load Strength Training Increases Corticospinal Excitability

“…To date, it remains unclear how rapidly these neural responses to strength training occur following the commencement of a strength training program. Transcranial magnetic stimulation (TMS) studies have emerged in recent years to study the effects of strength training on the synaptic activity of the cortico-cortical circuitry of the primary motor cortex and of the corticospinal tract (CST) following a bout of strength training [2][3][4][5][6][7][8][9][10]. Leung et al [6] reported that a single session of heavy-load strength training (75%1RM) of the biceps brachii increased the amplitude of motor-evoked potentials (MEPs) induced by single-pulse TMS, a finding consistent with Latella et al [5] who also showed increased MEP amplitude following both heavy-load and hypertrophy-based strength training (<75% 1RM).…”

“…Leung et al [6] reported that a single session of heavy-load strength training (75%1RM) of the biceps brachii increased the amplitude of motor-evoked potentials (MEPs) induced by single-pulse TMS, a finding consistent with Latella et al [5] who also showed increased MEP amplitude following both heavy-load and hypertrophy-based strength training (<75% 1RM). Recently, Mason et al [7] and Colomer-Poveda et al [2] reported that an acute bout of heavy-load strength training of the biceps brachii increased CSE, but interestingly, both of these studies included a light-load ST group. However, the findings of Mason et al [7] were in contrast to Colomer-Poveda et al [2], whereby light-light strength training increased CSE in the former study, but remained unchanged in the latter study.…”

“…Recently, Mason et al [7] and Colomer-Poveda et al [2] reported that an acute bout of heavy-load strength training of the biceps brachii increased CSE, but interestingly, both of these studies included a light-load ST group. However, the findings of Mason et al [7] were in contrast to Colomer-Poveda et al [2], whereby light-light strength training increased CSE in the former study, but remained unchanged in the latter study. To add further confusion to the acute corticospinal responses to strength training, Latella et al [4] and Selvanayagam et al [10] both reported reduced MEP amplitude after a single session of strength training.…”

“…A major caveat to the previous studies is that the authors have not determined the time-course responses or examined whether high-volume light-load strength training or low-volume heavy-load strength training differently modulate the corticospinal responses. The previous work of Mason et al [7] and Colomer-Poveda et al [2] only measured the responses immediately post-training. Furthermore, the findings of the systematic review by Mason et al [8] are confounded by the heterogeneity of the strength training interventions, whereby training-load and training-volume all varied.…”

“…Furthermore, the findings of the systematic review by Mason et al [8] are confounded by the heterogeneity of the strength training interventions, whereby training-load and training-volume all varied. For example, the previous work of Nuzzo et al [9] completed high-volume (12 sets of 8, quick bursts of maximal force production per repetition ), whilst Mason et al [7] included a high (4 sets of 20) and lowvolume (4 sets of 6-8) strength training group, whilst Colomer-Poveda et al [2] had equal training volume (12 sets of 8), but different training loads (25% or 75% of maximal voluntary contraction [MVC] force). There is a need to determine if the acute motor cortical responses to strength training are due to training-volume or training-load and how these responses differ over time.…”