Electromyograhic or EMG signal is the muscle electrical activity. It is the superposition of the evoked action potentials of all the active motor units in a muscle.
It can be recorded either in Monopolar or Bipolar configuration.
In Monopolar configuration one electrode is place over the belly of the muscle and one electrode some where farther as the reference and the signal between the two electrodes is amplified and recorded.
Click here for an example of an electrode for monopolar EMG.
In Bipolar configuration two electrodes are placed over the belly of the muscle within 1-2 cm from each other and one electrode some where farther as the reference and the signal between the two electrodes over the belly of the muscle is amplified differentially respect to the reference electrode. The advantage in this configuration is the common noise between the two electrodes is eliminated and hence we’ll have a cleaner EMG signal. The the signal-to-noise ratio will be much better.
Click here to see an example of an electrode for bipolar EMG.