Electromyography is an electrophysiological technique used to evaluate the electrical activity of skeletal muscles and, in clinical practice, is commonly performed together with nerve conduction studies as part of an electrodiagnostic assessment. Needle electromyography uses a fine electrode inserted into selected muscles to record electrical potentials generated by muscle fibres. The examiner assesses activity at rest and during voluntary contraction. The characteristics of motor-unit potentials and spontaneous activity can provide information about the integrity of motor neurons, peripheral nerves, neuromuscular junctions and muscle tissue. Surface electrodes can also record muscle activity non-invasively, although surface recordings provide different information from needle EMG.
During needle EMG, the examiner may evaluate insertional activity, spontaneous activity at rest and motor-unit recruitment during voluntary contraction. Normal resting skeletal muscle is electrically quiet apart from brief activity associated with needle insertion. Abnormal spontaneous activity can include fibrillation potentials and positive sharp waves, which may indicate denervation or muscle membrane instability in appropriate clinical contexts. Motor-unit action-potential morphology includes features such as amplitude, duration and configuration. Recruitment describes how additional motor units are activated as force increases. The pattern of abnormalities helps distinguish different types of neuromuscular disease.
EMG is particularly useful in evaluating suspected peripheral nerve disorders, radiculopathies, plexopathies, motor neuron disorders and selected myopathies. In a radiculopathy, abnormalities may occur in muscles supplied by the affected nerve root, sometimes with associated abnormalities in paraspinal muscles. Peripheral neuropathies can produce patterns reflecting axonal loss or demyelination when interpreted alongside nerve conduction studies. Motor neuron disorders can produce widespread denervation and reinnervation changes. Myopathies may produce relatively short-duration, low-amplitude motor-unit potentials with characteristic recruitment patterns, although findings vary considerably among diseases.
Nerve conduction studies complement Electromyography by measuring the electrical response of peripheral nerves to controlled stimulation. Motor studies assess compound muscle action potentials, while sensory studies assess sensory nerve responses. Measurements include latency, amplitude and conduction velocity. Patterns can help distinguish demyelinating processes from axonal loss in appropriate settings. Needle EMG then provides information about muscle and motor-unit physiology that nerve conduction studies alone cannot provide. The two techniques are frequently combined because they investigate related but different components of the peripheral neuromuscular system.
Electromyography is technically demanding and operator-dependent. Electrode placement, muscle selection, patient effort, temperature and the timing of the examination after an injury can influence findings. A muscle may appear normal if it is examined too early after certain nerve injuries, while abnormalities can evolve over time. The examiner must interpret results according to anatomy because muscles share overlapping innervation patterns. A comprehensive study therefore involves selecting muscles strategically rather than inserting the needle randomly. Clinical history and neurological examination are essential for choosing appropriate muscles and interpreting abnormalities.
Needle EMG can cause brief discomfort, bruising or minor bleeding, and infection is uncommon when appropriate technique is used. Particular precautions may be relevant in people with bleeding disorders or those taking anticoagulant medicines, depending on the procedure and muscle selected. The electrical recording itself is diagnostic rather than therapeutic. Surface EMG is less invasive but is more susceptible to cross-talk from neighbouring muscles and generally provides less detailed information about individual motor-unit potentials. Both approaches have research, rehabilitation and clinical applications, but they answer somewhat different questions.
EMG findings can help distinguish neurogenic from myopathic processes, but they are rarely interpreted in isolation. Imaging such as MRI can identify structural lesions, while laboratory tests can reveal metabolic or inflammatory muscle disease. EMG can show physiological consequences of nerve or muscle dysfunction even when structural imaging is unrevealing. Conversely, structural abnormalities on imaging do not necessarily prove that they are causing the patient's symptoms. Electrodiagnostic testing is therefore most useful when its findings are integrated with symptoms, neurological examination and other investigations.
Electromyography developed from early investigations of muscle electrical activity into sophisticated clinical electrophysiology. Improvements in amplifiers, recording systems, electrode technology and signal analysis have enabled detailed examination of motor-unit physiology. Modern EMG remains particularly valuable because it can localise dysfunction within the complex pathway extending from motor neurons through peripheral nerves to neuromuscular transmission and muscle. Its principal strengths are physiological specificity and the ability to distinguish patterns of neuromuscular dysfunction, while its limitations include invasiveness for needle studies, operator dependence, timing effects and the need for anatomical and clinical expertise.
