Paired-pulse rTMS at trans-synaptic intervals increases corticomotor excitability and reduces the rate of force loss during a fatiguing exercise of the hand

Previous studies have shown that the motor evoked potential (MEP) amplitude increases as force declines during a fatiguing muscle contraction, indicating that there is an increase in corticomotor excitability. In spite of this there is a progressive reduction in voluntary motor drive, as shown by an increase in the interpolated twitch force as fatigue develops. The aim of this study was to determine whether, by further increasing corticomotor excitability using a paired-pulse rTMS protocol designed to induce I-wave facilitation (iTMS), force loss during a sustained voluntary contraction could be reduced. We designed a cross-over study incorporating a 15-min period of iTMS (ISI 1.5 ms; 0.2 Hz; similar to AMT), following which MEP amplitude (first dorsal interosseous muscle) increased to 194 +/- 38% of baseline (P < 0.05), compared to a control period of stimulation that did not increase MEP amplitude (single-pulse TMS; 0.2 Hz; similar to 1.2 AMT). Eight right-handed healthy subjects received both iTMS and control stimulation, in a randomized order, a week apart. We measured percentage force loss at the end of a 10-s maximum right hand key-pinch task, and compared force loss before and after stimulation. There was an improvement in task performance following iTMS, with a reduction in force loss compared to pre-stimulation baseline (11.3 +/- 2.0 vs. 17.6 +/- 2.4%; post vs. pre; P < 0.05). There was no significant difference in force loss before and after control stimulation. The results indicate that by increasing corticomotor excitability using paired-pulse rTMS at trans-synaptic intervals, maximum voluntary force can be sustained at a higher level during a brief fatiguing maximal voluntary contraction.