Electromyography is a diagnostic tool used by neurologists and other specialists. For this procedure, small needles are placed into a muscle to record the electrical activity at that site. The activity is pictured on a computer screen and heard through a speaker.

Normal resting muscle produces no electrical activity. However, even the slightest flex of a muscle produces some electrical activity, which becomes more apparent as the flex grows stronger. In muscular, peripheral nerve and spinal motor neuron diseases, this electrical activity is abnormal.

The speed that the motor nerves send and receive electrical impulses can be measured with nerve conduction studies. A small charge of electricity to a motor nerve triggers an impulse. The impulse moves along the nerve until it reaches the muscle and makes it contract. By measuring the time the impulse takes to get to the muscle, a doctor can calculate the impulse speed.

The same measurements can be taken of sensory nerves. If muscle weakness is caused by a muscular disease, the nerve conduction speed remains normal. If muscle weakness is caused by a neurologic disease, the nerve conduction speed is usually slowed.

Measuring the electrical activity in muscles and nerves can help detect the presence, location and extent of diseases that can damage muscle tissue (such as muscular dystrophy) or nerves (such as amytrophic lateral sclerosis). In the case of nerve injury, the actual site of nerve damage can often be located. EMG and nerve conduction studies are often done together to provide more complete information.