The effect of shoulder position on motor evoked and maximal muscle compound action potentials of the biceps brachii

The purpose of the study was to assess the effect of shoulder position, 0° versus 90° shoulder flexion, on stimulation intensity and maximal muscle compound action potentials (Mmax) and motor evoked potentials (MEP) of the biceps brachii during both rest and 10% maximum voluntary contraction (MVC). Nine participants completed two experimental sessions with four conditions. During each condition, transcranial magnetic (TMS) and Erb's point stimulation were used to elicit MEPs and Mmax, respectively. During rest, the TMS intensity to elicit a MEP response (p < 0.001), was significantly lower by 28.6 ± 6.8%, in the 90° compared to the 0° position, but the stimulation intensity to elicit a Mmax was not different. MEP (p < 0.001) and Mmax (p < 0.001) amplitudes were significantly higher by 212.4 ± 43.3% and 86.5 ± 38.0%, respectively in the 90° compared to the 0° position. During 10% MVC the Mmax stimulation intensity (p = 0.022), but not TMS intensity, was significantly lower by 7.4 ± 3.8% in the 90° compared to the 0° position. Mmax (p < 0.001) amplitudes were significantly higher by 92.2 ± 20.2% in the 90° compared to the 0° position whereas MEP (p < 0.001) amplitudes were significantly lower by 24.5 ± 6.0% in the 90° compared to the 0° position. In conclusion, TMS intensity and Mmax intensity were both shoulder-position and state-dependent, whereas MEP and Mmax amplitudes were only shoulder position-dependent.