Electromyography

Electromyography

Electromyography (EMG) is a method used to assess and record the electrical signals generated by skeletal muscles. An electromyograph, an instrument used for EMG, produces a record called an electromyogram. It detects the electrical potential produced by muscle cells when they are activated either electrically or neurologically. These signals can be analyzed to identify medical abnormalities, assess activation levels, determine recruitment order, or analyze the biomechanics of human or animal movement.

The electrical source of EMG signals is the muscle membrane potential, typically around -90 mV. The measured EMG potentials range from less than 50 μV to up to 20 to 30 mV, depending on the muscle being observed.

There are two common types of EMG: surface EMG and intramuscular (needle and fine-wire) EMG. Intramuscular EMG involves inserting a needle electrode or a needle containing two fine-wire electrodes through the skin into the muscle tissue. A trained professional observes the electrical activity while inserting the electrode, providing valuable information about the muscle's condition and its innervating nerve.

The muscle's electrical activity at rest is analyzed, and abnormal spontaneous activity may indicate nerve or muscle damage. The patient is then asked to contract the muscle smoothly, and the resulting motor unit potentials are assessed. This process is repeated until sufficient data is collected. Intramuscular EMG provides a localized view of muscle activity and requires electrodes to be placed at various locations to obtain accurate results.

In some cases, intramuscular EMG may be considered too invasive or unnecessary. Surface electrodes can be used instead to monitor overall muscle activation, providing a broader picture of muscle activity. This technique is often used in physiotherapy clinics, where patients receive auditory or visual feedback to help them understand when they are activating their muscles (biofeedback).

During EMG, motor units are evaluated. A motor unit comprises one motor neuron and all the muscle fibers it innervates. When a motor unit fires, an action potential is transmitted down the motor neuron to the muscle fibers it innervates, resulting in a motor unit action potential (MUAP). The shape of MUAPs in the myogram is influenced by factors such as motor unit composition, muscle fiber count, and metabolic type.

Nerve conduction testing is frequently conducted alongside EMG to diagnose neurological conditions.

Some patients may find the procedure mildly painful, while others experience minimal discomfort during needle insertion. The tested muscles may feel slightly sore for a day or two afterward.

Normally, muscle tissue is electrically inactive at rest. After the initial electrical activity caused by needle insertion diminishes, the electromyograph should detect no abnormal spontaneous activity. When the muscle contracts voluntarily, action potentials begin to appear, increasing in frequency and amplitude as the muscle contraction intensifies. A full muscle contraction results in a varied group of action potentials, indicating complete recruitment and interference pattern.