Electromyography, or EMG, is a medical technique for measuring muscle response to nervous stimulation. EMG is performed using an instrument called an electromyograph, to produce a record called an electromyogram. An electromyograph detects the electrical potential generated by muscle cells when these cells contract. EMG can be performed with needle electrodes, in order to study very localized potentials, or with surface electrodes to study larger muscle (group) contractions (known as surface EMG or sEMG).
Measurements are often taken with Ag/AgCl temporary electrodes, or with metal electrodes into which conductive/adhesive gel is injected. The surface of the skin should be shorn and abraised lightly to ensure high conductivity. Dead, dry skin reduces the effectiveness of the induced EMG signal. Three electrodes are suggested for a fully-differential setup: two difference electrodes and a ground electrode. The 'difference' electrodes are placed on either side of the muscle under study. The ground electrode should be placed on the skin above a non-muscled portion of the body, near the difference electrodes. For example, when measuring the surface potential of the calf muscle, the ground electrode would be placed on the knee.
Clinically, EMG is sometimes done with both an excitatory stimulus pulse, which causes the muscles to contract, as well as the EMG sensing. This provides a measure of nerve conduction and propagation time. EMG electrodes were originally coupled to audio amplifiers as the sonified electrical activity is contained in an audible spectrum. It was especially useful to diagnose myotonia, where a frequency modulation would be audible creating sounds similar to a “dive-bomber”.
The electrodes are often wired to an instrumentation amplifier which provides high-gain differential amplification with a high impedance input stage (See Texas Instruments' INA128 or INA129).
Following amplification, a filtering stage is also suggested (though active filtering within the amplification stage is also possible), as low-frequency motion artifacts and high-frequency RF noise may affect signal quality. A bandpass filter with 80Hz and 1000Hz (or down to 300Hz depending on the information of interest) corner frequencies is a reasonable starting point. In order to generate a measure of muscle contraction, the filtered EMG signal can be full-wave rectified and smoothed.