Electromyography (EMG) measures the response of muscles and nerves to electrical activity. It's used to help determine muscle conditions that might be causing muscle weakness, including muscular dystrophy and nerve disorders.

How Is an EMG Done?

Muscles are stimulated by signals from nerve cells called motor neurons. This stimulation causes electrical activity in the muscle, which in turn causes the muscle to contract or tighten. The muscle contraction itself produces electrical signals.

For the purpose of EMG, a needle electrode is inserted into the muscle (the insertion of the needle might feel similar to an injection). The signal from the muscle is then transmitted from the needle electrode through a wire (or more recently, wirelessly) to a receiver/amplifier, which is connected to a device that displays a readout. The results are either printed on a paper strip or, more commonly, on a computer screen.

What Can an EMG Diagnose?

EMGs help diagnose three kinds of diseases that interfere with normal muscle contraction:

1. diseases of the muscle itself (most commonly, muscular dystrophy in children)
2. diseases of the neuromuscular junction, which is the connection between a nerve fiber and the muscle it supplies
3. diseases "upstream" in nerves and nerve roots (which can be due to either nerve damage or ongoing nerve injury)

When Are Results Ready?

Results are available immediately but a trained medical specialist, usually neurologist, is needed to analyze and interpret them.

Reviewed by: Steven Dowshen, MD
Date reviewed: September 2010