Corticospinal excitability of the biceps brachii is shoulder position dependent

The purpose of this study was to examine the effect of shoulder position on corticospinal excitability (CSE) of the biceps brachii during rest and a 10% maximal voluntary contraction (MVC). Participants (n = 9) completed two experimental sessions with four conditions: 1) rest, 0 degrees shoulder flexion; 2) 10% MVC, 0 degrees shoulder flexion; 3) rest, 90 degrees shoulder flexion; and 4) 10% MVC, 90 degrees shoulder flexion. Transcranial magnetic, transmastoid electrical, and Erb's point stimulation were used to induce motorevoked potentials (MEPs), cervicomedullary MEPs (CMEPs), and maximal muscle compound potentials (M-max), respectively, in the biceps brachii in each condition. At rest, MEP, CMEP, and M-max amplitudes increased (P < 0.01) by 509.7 +/- 118.3%, 113.3 +/- 28.3%, and 155.1 +/- 47.9%, respectively, at 90 degrees compared with 0 degrees. At 10% MVC, MEP amplitudes did not differ (P = 0.08), but CMEP and M-max amplitudes increased (P < 0.05) by 32.3 +/- 10.5% and 127.9 +/- 26.1%, respectively, at 90 degrees compared with 0 degrees. MEP/M-max increased (P < 0.01) by 224.0 +/- 99.1% at rest and decreased (P < 0.05) by 51.3 +/- 6.7% at 10% MVC at 90 degrees compared with 0 degrees. CMEP/M-max was not different (P < 0.22) at rest but decreased (P < 0.01) at 10% MVC by 33.6 +/- 6.1% at 90 degrees compared with 0 degrees. EMG increased (P < 0.001) by 8.3 +/- 2.0% at rest and decreased (P < 0.001) by 21.4 +/- 4.4% at 10% MVC at 90 degrees compared with 0 degrees. In conclusion, CSE of the biceps brachii was dependent on shoulder position, and the pattern of change was altered within the state in which it was measured. The position-dependent changes in M-max amplitude, EMG, and CSE itself all contribute to the overall change in CSE of the biceps brachii. NEW & NOTEWORTHY We demonstrate that when the shoulder is placed into two common positions for determining elbow flexor force and activation, corticospinal excitability (CSE) of the biceps brachii is both shoulder position and state dependent. At rest, when the shoulder is flexed from 0 degrees to 90 degrees, supraspinal factors predominantly alter CSE, whereas during a slight contraction, spinal factors predominantly alter CSE. Finally, the normalization techniques frequently used by researchers to investigate CSE may under- and overestimate CSE when shoulder position is changed.