Essentia / Devices / EMG

Device Lab 42

EMG

A hand-held bipolar probe delivers a constant-current pulse over the nerve at the wrist, and a pair of surface discs over the muscle it supplies records what arrives. The same stimulus is repeated at the elbow, the two latencies are subtracted, and the surface distance between the two cathode positions, measured with a tape, on the skin, over a nerve that does not run straight, is divided by the difference. That quotient is the only number in the study that is a velocity, and it exists because there were two sites rather than one.

Also called: Electromyograph, Nerve Conduction. Arabic: جهاز تخطيط العضلات الكهربائي. Device Lab 42 of 69 in BMEssentia Medical City, 2 configurations.

Overview: the configurations

How it works: the main parts

What each part does, from the EMG lessons in the 3D Device Lab.

Patient cable and input connectors

Individually screened leads from each electrode gathering into a clip on the limb and continuing as one jacketed multicore to the input box, with a separate heavier lead for the stimulator that is a different colour and runs on the other side of the patient. Nothing here amplifies anything.

Differential preamplifier and gain stage

A small box on a short boom at the patient’s side with the electrode sockets on its face, a few centimetres from where the leads come off the limb. Inside it are an instrumentation amplifier, one resistor that sets the gain, and a guard driver.

High-pass and low-pass filters

Two rotary selectors on the fascia, one for each end of the window, set differently for each of the three things this machine records. They are the settings most often left where the last operator put them, and the ones that most change what a reader sees.

Digitiser, sweep and timebase

A converter, a crystal, and a knob marked in milliseconds per division. Everything after this point in the machine is numbers, and the crystal is the reason a latency is a measurement rather than an estimate.

Display, sweep speed and sensitivity

A screen with a graticule, a knob marked millivolts per division and another marked milliseconds per division. Neither of them changes anything about what was recorded, and both of them change what a reader concludes.

Measurement cursors and the derived-values table

Two vertical markers, a baseline marker, a wheel to drive them, and a table beside the trace showing latency, amplitude, duration, distance and velocity. This is the component where the subtraction that the whole device exists for finally appears as a number.

Loudspeaker and audio monitor

A small cone behind a grille in the front panel with a volume knob beside it. On a nerve conduction study it is turned down and barely used.

Patient ground electrode

A single larger disc or strap on the limb between the recording electrodes and wherever the interference is coming from, with a green lead of its own. It records nothing, it measures nothing, and without it neither configuration works at all.

Explore it in the Device Lab

• What the second site actually removes: Record a distal latency and divide the distal distance by it. Then record a proximal latency, subtract, divide the measured distance by the difference, and compare the two answers.
• The instrument nobody calls an instrument: Take a correct velocity and then change one thing: the distance you claim to have measured. Move it by a centimetre and watch the reported velocity move. Then express that change in degrees of limb cooling, and then in sampling intervals of cursor error.
• The one thing the subtraction cannot remove: Record a velocity at thirty-two degrees. Cool the limb by four degrees and record it again without changing anything else. Then try to remove the difference by subtracting something.
• Recovering from your own stimulus: Compute how large the stimulus is at the skin, compute the largest signal the amplifier can accept, and compare the two. Then watch what the amplifier does when the artefact exceeds that, and how long it takes to be of any use again.
• A subtraction made of metal: Work out the recording area of a concentric needle core from two numbers you can read off the needle, compare it with a surface disc, and then put the same muscle fibre at two distances and compare what arrives.

Common faults and what to check

Most reported device problems (FDA MAUDE)

Raw counts of adverse event reports for this device type (FDA product code IKN), all years. Not adjusted for the number of units in use, so they show what gets reported, not failure rates. Report text is never copied.

Planned maintenance checks

From the BMEssentia procedure library: every 12 months, about 60 minutes, electrical class I, applied part type BF.

Test instruments: Signal simulator; Electrical safety analyser; Load resistor for stimulator output.

Standards engineers test against

Recent FDA recalls

From the FDA recall database (openFDA), newest first, imported 2026-10-22. 1 recalls on record for this device type. Recall text is the FDA's public text; each recall links to the official record.

Learn it, see it in 3D, ask about it

• Ask Essentia about the EMG: answers from named sources, with the device open in its 3D Device Lab beside the answer.
• The EMG on the BMEssentia Platform: the Device Library course, 13 sections from physics to maintenance.
• BMEssentia Medical City: Device Lab 42, the EMG fully assembled in 3D, with lab tasks and the simulations.

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