An electromyogram (EMG) and nerve conduction study are tests that are used in combination to diagnose sources of pain. An electromyogram (EMG) measures the activity of the muscles supplied by the nerves that are being irritated by a ruptured or herniated disc. The electromyogram may be used to diagnose nerve or muscle disorders. A nerve conduction study tests the velocity of how the nerve transmits its signal. In other words, if the nerve is pinched, its ability to transmit a signal is reduced. Abnormal muscle activity and slowed nerve conduction may suggest a pinched or irritated nerve from a disc rupture or more serious problems with the spinal cord.