Abstract: Apparatus for monitoring a brain signal, especially indicative of muscle fatigue, comprises at least three sensors sensing neuronal activity, wherein one sensor (11) comprises a reference electrode and the other sensors (15, 16) comprise sensing electrodes, wherein the reference sensor (11) serves as a common reference for the other sensors (15, 16). The reference sensor (11) is placed within the quadrangle defined by the four positions [F5-F1-C 1-C5] or on a corridor of 10 millimetres width along the line between Fpz to Oz, the first sensing sensor is placed within the quadrangle [C1-C2-P2-P1], and the second sensing sensor is placed within the quadrangle [FT7-FC3-CP3-TP7], with the proviso that the reference electrode is not positioned on the same position as is one of the two sensing sensors and wherein the positions of the sensors delimiting the quadrangles are defined according to the 10-10 Electrode Placement System of the American Clinical Neurophysiology Society.