Two electromyography (EMG) experiments related to double bass playing are described. By means of surface electrodes and synchronous recordings of finger movements on the string, it is shown in the first experiment that the production of vibrato is associated with pulsating contractions of two back muscles, the teres minor and the teres major. A failure in the coordination of the activity of these muscles was observed in a student having problems in producing a proper vibrato. The second experiment demonstrates clear effects of a small change in the holding of the instrument on muscular stress in the left and right trapezius muscles. These changes were revealed by the occurrence of different degrees of muscular force developed during playing. The pedagogical implications of these findings are discussed.