electromyogram

An electromyogram (EMG) is the record of electrical activity produced by skeletal muscles. It is obtained with electrodes and reflects the summed electrical activity of muscle fibers during rest and contraction. The term EMG is often used to refer both to the recording itself and to the diagnostic procedure, electromyography.

There are two main recording modalities: needle (intramuscular) EMG and surface EMG. Needle EMG uses fine electrodes inserted into a muscle to pick up action potentials from individual motor units and surrounding tissue; surface EMG uses sensors placed on the skin over the muscle to record aggregate electrical activity. Signals are amplified, filtered, and displayed for analysis. Analysis focuses on motor unit action potentials, recruitment patterns, and the presence of spontaneous activity at rest, such as fibrillations or positive sharp waves, which can indicate neuropathic or myopathic conditions. Surface EMG provides information about the timing and relative level of muscle activation during movements or tasks.

EMG is commonly used in the evaluation of neuromuscular disorders, such as peripheral neuropathies, radiculopathies, myopathies, and motor neuron disease. It is typically performed alongside nerve conduction studies to assess peripheral nerves and neuromuscular transmission.

Limitations include discomfort from needle insertion, sampling limitations for deep or small muscles, and the need for skilled interpretation. The recorded electrical activity is an indirect index of muscle force and can be influenced by electrode placement, tissue conductivity, and subject effort.

Beyond clinical use, EMG methods are applied in research, rehabilitation, ergonomics, and sports science to study motor control, muscle coordination, and fatigue.