The ability to perform tasks of daily living is dependant upon the capacity of the central nervous system to voluntarily activate muscles over prolonged durations. This neural drive is termed voluntary activation (VA) and can be measured using twitch interpolation¹. It has been reported that VA is reduced following both maximal and prolonged submaximal contractions, reflecting central fatigue. The trapezius muscle is activated submaximally for prolonged periods during tasks of daily living such as bag carrying, but it remains to be established if this task causes central fatigue. It has been reported that VA of trapezius can be measured using electrical stimulation (ES) of the motor nerve²; however this technique provides no information regarding the changes occurring at a cortical level following a fatiguing task. Transcranial magnetic stimulation (TMS) of the motor cortex might provide this information, and the aim of this study was to identify if TMS can be used to asses cortical VA of trapezius. With ethical approval and written informed consent eleven healthy subjects were recruited. Electromyographic recordings were made from the right trapezius, ES was delivered to the right accessory nerve and TMS applied to the left motor cortex. Subjects sat on a chair with their feet suspended, and a force transducer was positioned over their right shoulder. Maximum voluntary contractions (MVC) of the right trapezius were carried out to identify target forces of 10, 30, 50, 70, 90 and 100% MVC. Subjects performed contractions to these target forces in a random order during which ES or TMS was applied. For ES, superimposed twitch (SIT) size decreased linearly as contraction strength increased (mean r²=0.75, p<0.05 (n=10)). For TMS this relationship was polynomial across all contraction strengths (mean r²=0.67, p<0.05 (n=9)) or linear >70% MVC (mean r²=0.62, p<0.05 (n=8)). VA during MVCs of the trapezius was 81.0 (12.9)% when measured with ES (n=8) and 78.1 (13.3)% when measured with TMS (n=8). SITs evoked by TMS were significantly larger than by ES at 10-90% MVC (p<0.05). Maximally evoked direct motor responses to ES (M-waves) were significantly larger than motor evoked potentials (MEPs) at contraction strengths ≤90% MVC (p<0.05). There was a polynomial relationship between contraction strength and both M-wave (r²=0.89) and MEP (r²=0.99) amplitudes with ES and TMS, respectively. The non-linearity of the SITs to TMS at contraction strengths of <70% MVC may reflect lowered levels of corticospinal excitability, as supported by smaller MEP amplitudes, at these strengths. A similar result has been found using TMS in other muscle groups^3,4. TMS can be used to assess cortical VA of the trapezius and this technique could be useful to assess the changes that occur at a cortical level as a result of central fatigue⁵.