Abstract
While neural changes are thought to be responsible for early increases in strength following resistance training (RT), the exact changes in motor unit (MU) firing properties remain unclear. This review aims to synthesize the available evidence on the effect of RT on MU firing properties. MEDLINE (OVID interface), EMBASE (OVID interface), Web of Science (all databases), Cochrane Library, EBSCO CINAHL Plus, PubMed, and EBSCO SportDiscus were searched from inception until June 2021. Randomized controlled trials and non-randomized studies of interventions that compared RT to no intervention (control) were included. Two reviewers independently extracted data from each trial, assessed the risk of bias and rated the cumulative quality of evidence. Motor unit discharge rate (MUDR), motor unit recruitment threshold (MURT), motor unit discharge rate variability (MUDRV), MU discharge rate at recruitment vs. recruitment threshold relationship, and MU discharge rate vs. recruitment threshold relationship were assessed. Seven trials including 167 participants met the inclusion criteria. Meta-analysis (four studies) revealed that MUDR did not change significantly (P = 0.43), but with considerable heterogeneity likely to be present (I2 = 91). Low to moderate evidence supports changes in MUDRV, MUDR at recruitment vs. recruitment threshold relationship, and the MUDR vs. recruitment threshold relationship. Overall, this systematic review revealed that there is a lack of high-quality evidence for the effect of RT on MU firing properties. Heterogeneity across studies undermines the quality of the evidence for multiple outcomes and affects the conclusions that can be drawn.
Highlights
Resistance training (RT) involves resisted movements with the overall goal to increase an individual’s strength
The ROB-2 tool was applied to five randomized controlled trials (RCTs), and the ROBINS-I tool was applied to two nonrandomized studies on intervention effects (NRSIs)
Three trials used maximal isometric or isometric ballistic voluntary contractions to form the exercises making up their resistance training (RT) regimens (Rich and Cafarelli, 2000; Pucci et al, 2006; Del Vecchio et al, 2019a), whereas the other four studies used a combination of multiple lower body exercises including exercises that did not isolate the muscles that were being used for measurement of motor unit firing properties; except for Stock and Thompson (2014) who used a RT program consisting of conventional deadlifts only
Summary
Resistance training (RT) involves resisted movements with the overall goal to increase an individual’s strength. Resistance Training and Motor Units system such as, decreased activity of inhibitory networks in the primary motor cortex (Kidgell et al, 2017), increased corticospinal axon excitability at the spinal level (Mason et al, 2019) and changes in motor unit activation (Siddique et al, 2020) are responsible for early changes in strength output (Siddique et al, 2020). Pucci et al (2006) found no significant change in MUDR after 3-weeks of isometric RT These diverse findings make changes in motor unit firing properties following a regimen of RT unclear, and no systematic review has evaluated the overall evidence of changes in motor unit firing properties in response to RT in order to synthesize the available evidence and draw conclusions from the available studies. This systematic review aims to synthesize current evidence on the effect of RT on motor unit firing properties in order to determine the direction and strength of evidence
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