The effect of chronic resistance training on spinal excitability of the biceps brachii

One major benefit of resistance training (RT) is increasing strength. However, the neurophysiological changes that chronic RT produces in the nervous system remain unclear. To address this, seventeen participants (9 males and 8 females) were recruited and divided into non-resistance and chronic resistance-trained groups to explore different levels of corticospinal tract excitability using a transcranial magnetic stimulation (TMS) that produces motor evoked potentials (MEP), and transmastoid electrical stimulation (TMES) that produces cervicomedullary evoked potentials (CMEP), and Erb’s point stimulation that produces maximal compound muscle action potentials (Mmax). All participants performed a maximum voluntary contraction (MVC) of the biceps brachii, followed by determining the stimulation intensities during a 20% MVC for TMS to achieve a 150-200ms silent period and TMES that produced a CMEP amplitude that was 75% of Mwave. The main task included five isometric elbow contractions at 25%, 50%, and 75% of MVC for eight seconds, with three stimulation techniques applied at 3, 4.5, and 6 seconds in a randomized order. Participants received a Mwave, five TMS, five TMES, and five conditioned TMES (100ms after MEP) at each contraction intensity. Data analysis using t-tests and two-way ANOVAs with contraction intensity and group and contraction intensity and sex as factors revealed that the chronic RT group required less TMS output to achieve the desired silent period (p=0.003) and had a lower active motor threshold (p<0.001). The chronic RT group exhibited 50% and 90.1% higher amplitude in conditioned and unconditioned CMEP, respectively compared to the non-RT group. Furthermore, males had 50.5% and 112.7% higher amplitudes in conditioned and unconditioned CMEP, respectively compared to females. In conclusion, chronic RT induces neurophysiological adaptations that alter the spinal excitability, showing that it is sex dependent. The alteration in corticospinal tract excitability may occur at the spinal motoneuron.