Corticospinal responses following strength training: a systematic review and meta‐analysis

Abstract

Strength training results in changes in skeletal muscle; however, changes in the central nervous system also occur. Over the last 15 years, non-invasive brain stimulation techniques, such as transcranial magnetic stimulation, have been used to study the neural adaptations to strength training. This review explored the hypothesis that the neural adaptations to strength training may be due to changes in corticospinal excitability and inhibition and, such changes, contribute to the gain in strength following strength training. A systematic review, according to PRISMA guidelines, identified studies by database searching, hand-searching and citation tracking between January 1990 and the first week of February 2017. Methodological quality of included studies was determined using the Downs and Black quality index. Data were synthesised and interpreted from meta-analysis. Nineteen studies investigating the corticospinal responses following strength training were included. Meta-analysis found that strength training increased strength [standardised mean difference (SMD) 0.84, 95% CI 0.55 to 1.13], decreased short-interval intracortical inhibition (SMD -1.00, 95% CI -1.84 to -0.17) and decreased the cortical silent period (SMD -0.66, 95% CI -1.00 to -0.32). Strength training had no effect on motor threshold (SMD -0.12, 95% CI -0.49 to 0.25), but a borderline effect for increased corticospinal excitability (SMD 0.27, 95% CI 0.00 to 0.54). In untrained healthy participants, the corticospinal response to strength training is characterised by reduced intracortical inhibition and cortical silent period duration, rather than changes in corticospinal excitability. These data demonstrate that strength training targets intracortical inhibitory networks within the primary motor cortex (M1) and corticospinal pathway, characterising an important neural adaptation to strength training.

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

“…Here, we found no change after unilateral acute ST at a low or high intensity. These results combined with the SICI data suggest that, although chronic ST could lead to reductions in SICI and SP, just a single session of isometric ST does not reduce the efficacy of the GABAa and GABAb receptor–mediated inhibitory intracortical circuits projecting to the cortical excitatory neurons. Acute reductions in intracortical inhibition could be a compensatory mechanism to diminish the effect of peripheral fatigue on force output .…”

“…Therefore, although there is still no evidence of a relationship between chronic changes in intracortical inhibition and the force of a muscle contraction, decreases in the efficacy of those inhibitory intracortical circuits can release cM1 from inhibition, increasing cM1 excitability, the efficacy of the motor command, and the drive to muscles to contract more forcefully. In fact, chronic ST tends to decrease short‐interval intracortical inhibition (SICI) and silent period (SP) duration, suggesting that a release of intracortical inhibition could be one mechanism underlying the chronic increases in cM1 excitability and in the effectiveness of the motor command to increase MVC force. However, the time course of such adaptations is unclear because results from acute studies are inconsistent .…”

Training‐, muscle‐ and task‐specific up‐ and downregulation of cortical inhibitory processes

“…At first sight, it may be surprising that the present study found a reduced level of SICI after the explosive training as a recent review by Berghuis, Semmler, Opie, Post, and Hortobagyi (2017) failed to identify changes in SICI as a consequence of ballistic motor learning in young as well as old subjects. From a functional perspective, however, it makes sense that cortical inhibition is reduced after explosive training in order to ensure high levels of cortical excitatory drive (Kidgell, Bonanno, Frazer, Howatson, & Pearce, 2017). Furthermore, the difference to the results of Berghuis et al (2017) might simply be explained by the observation that in the majority of studies included in this review, SICI had been tested at rest rather than during activity even though inhibition is well known to be modulated in a task-dependent manner (Opie & Semmler, 2016;Papegaaij et al, 2016;Sidhu et al, 2013;Soto et al, 2006).…”

“…At first sight, it may be surprising that the present study found a reduced level of SICI after the explosive training as a recent review by Berghuis, Semmler, Opie, Post, and Hortobagyi () failed to identify changes in SICI as a consequence of ballistic motor learning in young as well as old subjects. From a functional perspective, however, it makes sense that cortical inhibition is reduced after explosive training in order to ensure high levels of cortical excitatory drive (Kidgell, Bonanno, Frazer, Howatson, & Pearce, ). Furthermore, the difference to the results of Berghuis et al.…”

Myths and Methodologies: How loud is the story told by the transcranial magnetic stimulation‐evoked silent period?

“…Elucidation of these mechanisms is highly meaningful, because inhibitory neural systems are essential for the modulation of excitatory input and maintenance of synaptic stability. Although the SP is often not the primary outcome variable, it is used as one of the main measures of central nervous system (CNS) inhibition, with the changes in SP being implicated in phenomena such as neurological disease (Nantes et al, 2016), exercise-induced neuroplasticity (Kidgell, Bonanno, Frazer, Howatson, & Pearce, 2017) and fatigue The present review describes current knowledge of mechanisms underpinning the SP, discusses approaches to measuring the SP and the potential confounding influences on its measurement and provides recommendations for best practice of SP measurement.…”