Abstract: An elongate hollow cylindrical member houses electronic circuitry and has disposed on its surface, on either end, a live electrode and a spaced common electrode. The electronic circuitry includes a difference amplifier, and electromyogram signals detected by the electrodes are applied to opposite polarity inputs of the difference amplifier. As the electromyogram signals are of substantially equal amplitude and phase, the electromyogram signals will be subtracted from each other in the difference amplifier to provide a substantially zero output due to electromyogram signals at the output of the difference amplifier. However, electrocardiograph signals, being of opposite phase, will be added in the difference amplifier so that the output of the difference amplifier is substantially due to only the electrocardiograph signal.