Electromyography (EMG) is a diagnostic test to evaluate the health of muscles and the nerve cells that control them. Motor neurons transmit electrical signals that cause muscles to contract. An EMG uses tiny devices called electrodes to transmit or detect electrical signals. These are then interpreted by a specialist. EMGs can include placing a needle electrode into a specific muscle to measure its electrical activity, and a nerve conduction study, which is using electrodes taped to the skin to measure the speed and strength of signals traveling between two or more points.