D5 Electromyography (EMG)

Topic

Electromyography records muscle electrical activity using surface or needle electrodes: surface electrodes capture this activity from the skin without penetrating the tissue, whereas needle electrodes capture it directly from within the muscle; this difference in depth determines the precision and invasiveness of each method.

By recording this electrical activity, electromyography directly operationalizes muscle activation: the electrical signal captured by the electrode is, in itself, a direct reflection of the level of muscle activation the nervous system is sending to the muscle at any given moment.

In addition to muscle activation, electromyography operationalizes motor controller parameters: it estimates the motor controller's proportional term based on how muscle electrical activity changes with movement speed; it estimates the derivative term based on how that activity evolves over time; and by simultaneously recording the electrical activity of two opposing muscles, it detects whether co-activation is occurring between them.

With muscle activation operationalized, electromyography allows for the quantification of weakness caused by inhibition: if a muscle receives less activation than it should—even though the muscle itself and the neuromuscular junction are functioning normally—electromyography records lower-than-expected electrical activity, revealing that the weakness originates in neural activation rather than the muscle.

With motor controller parameters operationalized, electromyography enables the detection of abnormal activation patterns associated with motor neurological disorders: an increase in the motor controller's proportional term—dependent on movement speed—indicates spasticity; reduced muscle activation indicates paresis or plegia; and abnormal co-activation between opposing muscles indicates dystonia. Electromyography also assesses fatigue in the respiratory muscles: by recording the electrical activity of the diaphragm and intercostal muscles during breathing, it detects characteristic changes in this activity as the muscles fatigue, revealing whether the diaphragm or intercostal muscles are losing the ability to sustain ventilatory mechanics.

By directly recording muscle electrical activity, electromyography transforms processes that would otherwise remain invisible—muscle activation, motor control function, and specific muscle fatigue—into a measurable signal that makes it possible to distinguish between weakness due to inhibition, motor neurological disorders, and respiratory muscle fatigue.

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