An electromyogram tests the functions of the nerve roots and the way that the nerves affect muscle function. It does this by looking at how well the electrical currents in the nerves are being transmitted to the muscles. Pressure on the nerves or damage to the nerves changes the way they transmit electrical current. This shows up in the muscles as they react to the information being sent to them from the brain, by the nerves.
The test is done by inserting fine needle electrodes (like acupuncture needles) into the muscles of the arms and or legs being tested. You may feel some slight discomfort as the needle is placed in the muscle. The electric signals from the muscles show up on a screen and can be heard over a speaker. The electrical activity of the muscles is examined during both contraction and rest. Muscle twitching and contraction is felt by the patient.
By looking for abnormal electrical signals in the muscles, the EMG can show if a nerve is being irritated, or pinched as it leaves the spine.