Transcranial magnetic stimulation intensity affects exercise-induced changes in corticomotoneuronal excitability and...

Transcranial magnetic stimulation (TMS) of the motor cortex during voluntary contractions elicits electrophysiological and mechanical responses in the target muscle. The effect of different TMS intensities on exercise-induced changes in TMS-elicited variables is unknown, impairing data interpretation. This study aimed to investigate TMS intensity effects on maximal voluntary activation (VATMS), motor-evoked potentials (MEPs), and silent periods (SPs) in the quadriceps muscles before, during, and after exhaustive isometric exercise. Eleven subjects performed sets of ten 5-s submaximal isometric quadriceps contractions at 40% of maximal voluntary contraction (MVC) strength until task failure. Three different TMS intensities (I100, I75, I50) eliciting MEPs of 53 ± 6%, 38 ± 5% and 25 ± 3% of maximal compound action potential (Mmax) at 20% MVC were used. MEPs and SPs were assessed at both absolute (40% baseline MVC) and relative (50%, 75%, and 100% MVC) force levels. VATMS was assessed with I100 and I75. When measured at absolute force level, MEP/Mmax increased during exercise at I50, decreased at I100 and remained unchanged at I75. No TMS intensity effect was observed at relative force levels. At both absolute and relative force levels, SPs increased at I100 and remained stable at I75 and I50. VATMS assessed at I75 tended to be lower than at I100. TMS intensity affects exercise-induced changes in MEP/Mmax (only when measured at absolute force level), SPs, and VATMS. These results indicate a single TMS intensity assessing maximal voluntary activation and exercise-induced changes in corticomotoneuronal excitability/inhibition may be inappropriate.