Transcranial magnetic stimulation intensity affects exercise-induced changes in corticomotoneuronal excitability and inhibition and voluntary activation

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 (VA_TMS), 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 (M_max) 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. VA_TMS was assessed with I₁₀₀ and I₇₅. When measured at absolute force level, MEP/M_max 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. VA_TMS assessed at I₇₅ tended to be lower than at I₁₀₀. TMS intensity affects exercise-induced changes in MEP/M_max (only when measured at absolute force level), SPs, and VA_TMS. These results indicate a single TMS intensity assessing maximal voluntary activation and exercise-induced changes in corticomotoneuronal excitability/inhibition may be inappropriate.