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±4.8Nm) compared to SOMEP (13.0±3.7Nm; P=0.031), the SIT amplitude at 100% MVC torque was not different (SOMEP: 0.69±0.49Nm vs. SOSIT: 0.74±0.52Nm; 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 prior to measuring VA.