Introduction. Non-human primate research has highlighted the reticulospinal tract (RST) as a likely site of adaptation to resistance training (Glover & Baker, 2020) however, longitudinal observations in untrained humans have yet to be performed. This study examined the possible time-course adaptations of the RST over a 6-week resistance training intervention.

Methods. Eight untrained (age: 28 ± 5 years) and six untrained (age: 31 ± 4 years) participants were pseudo-randomly assigned into training and control groups, respectively. The isometric resistance training intervention consisted of 4 sets of 8 repetitions at 80% maximal voluntary contraction (MVC) of the dominant elbow flexor, twice weekly for 6-weeks. Neuromuscular assessments were performed at baseline and end of weeks 1, 2, 3 and 6 in both training and control groups. The neuromuscular assessments consisted of measures of cortical function; short-interval cortical inhibition (SICI) intra-cortical facilitation (ICF) and voluntary activation (V-wave), assessment of corticospinal tract (CST) excitability through motor evoked potentials (MEP) and motoneuronal function through silent period cervico-medullary stimulations (spCMEP). RST function was indirectly inferred from responses to cervico-medullary stimulations (CMEP_CON) and transcranial magnetic stimulation (TMS_CON), paired with a conditioning startling auditory stimuli of ≥ 110 dB, either 80 ms (CMEP_CON) or 50 ms (TMS_CON) apart (Furubayashi et al., 2000). Conditioned responses were expressed as a percentage of the unconditioned response. Evoked responses were recorded at 10% MVC with surface electromyography. The StartReact protocol, which assess reaction times in response to visual (VRT), auditory (VART), and startling auditory stimuli (VASRT) was also employed (Baker & Perez, 2017). RST gain, an index of RST function, was quantified as the difference in VRT and VASRT with respect to the difference in VRT and VART.       

Results. Training group strength increased compared to controls following the 6-week isometric intervention (11 ± 4% vs. 4 ± 4%, p = 0.006). Of the measures of RST function, compared to control there was a training group interaction effect with StartReact VASRT decreasing at week 2 (−12 ± 8%; p = 0.029), week 3 (−22 ± 10%; p = 0.003) and week 6 (−25 ± 12%; p = 0.004) compared to baseline. RST gain was also greater at week 1 compared to baseline (44 ± 32%; p = 0.043). There were no interaction effects found in the control group (p = ≥0.055). CMEP_CON facilitation, and TMS_CON inhibition were not different following the intervention (p = ≥0.43). No differences in SICI, ICF, V-Wave, MEPs or spCMEP (p = ≥0.48) were found across both groups following the intervention.     

Conclusion. Elbow flexor strength, StartReact rection times and RST gain all improved in the training group following the isometric resistance training intervention but not in the control group. This, along with the lack of cortical or CST changes, strongly indicates that RST adaptations drive initial increases in strength when beginning a resistance training programme.