Voluntary activation does not differ when using two different methods to determine transcranial magnetic stimulator output.

ABSTRACT

According to current guidelines, when measuring voluntary activation (VA) using transcranial magnetic stimulation (TMS), stimulator output (SO) should not exceed the intensity that, during a maximal voluntary contraction (MVC), elicits a motor evoked potential (MEP) from the antagonist muscle >15%-20% of its maximal M-wave amplitude. However, VA is based on agonist evoked-torque responses [i.e., superimposed twitch (SIT) and estimated resting twitch (ERT)], which means limiting SO based on electromyographic (EMG) responses will often lead to a submaximal SIT and ERT, possibly underestimating VA. Therefore, the purpose of this study was to compare elbow flexor VA calculated using the original method (i.e., intensity based on MEP size; SOMEP) and a method based solely on eliciting the largest SIT at 50% MVC torque (SOSIT), regardless of triceps brachii MEP size. Fifteen healthy, young participants performed 10 sets of brief contractions at 100%, 75%, and 50% MVC torque, with TMS delivered at SOMEP (73.0 ± 13.5%) or SOSIT (92.0 ± 10.8%) for five sets each. Although the mean ERT torque was greater using SOSIT (15.2 ± 4.8 Nm) compared with SOMEP (13.0 ± 3.7 Nm; P = 0.031), the SIT amplitude at 100% MVC torque was not different (SOMEP 0.69 ± 0.49 Nm vs. SOSIT 0.74 ± 0.52 Nm; P = 0.604). Despite the ERT disparity, VA scores were not different between SOMEP (94.6 ± 3.5%) and SOSIT (95.0 ± 3.3%; P = 0.572). Even though SOSIT did not lead to a higher VA score than the SOMEP method, it has the benefit of yielding the same result without the need to record antagonist EMG or perform MVCs when determining SO, which can induce fatigue before measuring VA.NEW & NOTEWORTHY When using transcranial magnetic stimulation (TMS) to determine voluntary activation (VA) of the elbow flexors, we hypothesized that a stimulator output designed to limit antagonist muscle activity would evoke submaximal agonist superimposed twitch amplitudes, thus underestimating VA. Contrary to our hypothesis, VA was not greater with an output based on maximal superimposed twitch amplitude. Nevertheless, our findings advance methodological practices by simplifying the equipment and minimizing the time required to determine VA using TMS.