A measure of the electrical activity a muscle produces when it works.
Electromyography (EMG) records the electrical activity generated by muscle fibres when they are recruited, picked up via surface electrodes over the muscle belly or fine-wire electrodes inserted into deeper or smaller muscles. It tells you which muscles are switching on, roughly how much activity is present, and the timing of that activity relative to a movement, rather than measuring force or tension directly.
On the clinic floor, EMG helps answer questions about activation strategy: is a muscle firing at all, is it firing too early or too late relative to its synergists, or is one muscle compensating for another that appears underactive. This is useful when reasoning about motor control issues, guiding retraining or biofeedback work, or informing discussions with a multidisciplinary team about a patient's movement pattern.
Surface EMG is the common clinical option, with electrodes placed over the target muscle belly after skin preparation to reduce impedance, then recorded during a rest, submaximal or functional contraction and often normalised to a reference contraction for comparison. Fine-wire EMG is reserved for deep or small muscles that surface electrodes cannot isolate, and is typically performed by trained specialists. Data is usually reviewed as raw waveform amplitude and onset/offset timing rather than as an absolute force value.
Surface EMG amplitude is heavily influenced by electrode placement, skin preparation, adipose tissue, crosstalk from nearby muscles, and normalisation method, so comparing raw values across sessions, muscles or patients without standardised protocol is unreliable. It reflects electrical activation, not mechanical force or muscle quality, so a muscle can show high EMG activity without producing proportionate force, and clinicians should avoid over-interpreting single recordings as definitive evidence of dysfunction.
This guide was auto-drafted and is pending editorial review.