Electromyography (EMG) is the study of motor unit and muscle fibre electrical activity. They can be studied individually or collectively. EMG evaluates the electrical activity at rest, and during voluntary muscle contraction.
The EMG is usually done alongside nerve conduction studies (NCS). A needle electrode is inserted into the muscle of interest, stimulated, and then the action potentials generated by groups of muscle fibres are recorded. The muscle fibres being stimulated are the motor unit potentials (MUPs). The muscle is tested at rest, with slight contraction, and with stronger contraction. In normal situations the muscle is silent at rest. Spontaneous action potentials from single-muscle fibres (fibrillation potentials) may occur in severe or inflammatory myopathies, and in an active neuropathic process. Spontaneous contractions of a single-motor unit (fasciculation potentials) may be observable in certain neuropathic processes such as motor neuron disease and diseases of the proximal root.
There may be changes in MUP parameters in neurogenic or myopathic processes. In neurogenic processes, the MUPs are often increased in amplitude, duration, and degree of polyphasia with reduced fibre recruitment with increase in effort. In myopathic processes, the MUPs may be decreased in amplitude, decreased in duration, and increased in polyphasia and early recruitment.
Single-fibre EMG is possible which studies single-muscle fibre action potentially. This is used to evaluate neuromuscular junction function, but it can also be abnormal in neuropathic and myopathic disease.
- Biller, José. Practical neurology. Philadelphia: Wolters Kluwer, 2017.