Electromyography (EMG) is a physiological measurement that evaluates the electrical activity of the skeletal muscle. Muscle tissue is an electrically quiet tissue during rest. It is controlled by motor nerves only. An Electromyograph measures the activity of the motor nerve, the conduction between the nerve and the muscle, and the muscle contraction itself. In sleep research, EMG is used to measure muscle tone during sleep. Muscle weakness, known as atonia, is expected in healthy REM sleep.