---
title: "Short note: Electromyography"
subject: "Biomedical Instrumentation"
chapter: 4
key: "semester_7/biomedical_instrumentation:short-note-electromyography"
source: https://www.bctengineer.com/q/semester_7/biomedical_instrumentation/short-note-electromyography
site: BCT Engineering
---

# Short note: Electromyography

Biomedical Instrumentation · Chapter 4: Bioelectric Signals Measurement and Recording System

- Asked in: 2075 Chaitra, 4 marks
- Video: [Electromyography | EMG Machines | Biomedical Engineers TV |](https://www.youtube.com/watch?v=eRxU4Pa39ME) — Biomedical Engineers TV
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_7/biomedical_instrumentation/short-note-electromyography
- Whole chapter: https://www.bctengineer.com/md/semester_7/biomedical_instrumentation/chapter-4

## Question

Write a short note on Electromyography.

## Short answer

Electromyography (EMG) records skeletal muscle electrical activity (50 µV-5 mV, 20 Hz-10 kHz) using concentric needle electrodes for single motor units or surface Ag/AgCl electrodes for whole muscle. The signal is amplified differentially, filtered, displayed on oscilloscope, played via loudspeaker for audio feedback, and recorded/integrated. Normal muscle is silent at rest; weak contraction shows single motor unit potentials, strong contraction shows interference pattern. Nerve conduction velocity is measured by stimulation at two points. Used for neuromuscular disease diagnosis and prosthetic control.

## Answer

**Electromyography (EMG)** is the recording of the electrical activity produced by skeletal muscles. The record is called an electromyogram. It gives the sum of the action potentials of the many motor units near the electrode.

#### Signal and recording
- Amplitude: 50 µV to 5 mV; frequency range 20 Hz to 10 kHz (energy mostly 50 to 500 Hz).
- **Electrodes:** concentric needle electrode (inserted in the muscle, for single motor unit study), or surface Ag/AgCl electrodes (for the whole muscle).

```text
 Muscle -> [Electrode] -> [Differential preamp] -> [Filter] -> [Main amp]
                                                       |
                  +------------------------------------+
                  v               v                v
              [CRO display]   [Loudspeaker]   [Recorder/computer]
```
**Block functions:** differential amplifier with high CMRR (gain about 1000), band-pass filter, display on an oscilloscope, audio loudspeaker (the sound helps the doctor to judge the muscle), and a recorder or integrator (RMS) for the quantitative record.

#### Findings
- At complete rest a normal muscle is electrically silent.
- A weak contraction gives single motor unit potentials; a strong contraction gives an interference pattern.
- Nerve conduction velocity is measured by stimulation at two points: v = distance/difference in latency.

**Uses:** diagnosis of neuromuscular diseases (muscular dystrophy, neuropathy, myasthenia), nerve injury, and control of prosthetic limbs (myoelectric).

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. What does an EMG record?
   A. The electrical activity of the heart
   B. The sum of the action potentials of motor units near the electrode in skeletal muscle
   C. The electrical activity of the brain
   D. The mechanical force of the muscle
   Answer: B. EMG records skeletal-muscle electrical activity, the sum of motor unit action potentials. (Key point: EMG definition)

2. What is seen in a normal muscle?
   A. Continuous large spikes at rest
   B. Electrical silence at complete rest, single motor unit potentials in weak contraction, interference pattern in strong contraction
   C. Interference pattern at rest
   D. Silence even during strong contraction
   Answer: B. Normal muscle is silent at rest; recruitment of more units produces an interference pattern. (Key point: EMG findings)

3. How is nerve conduction velocity measured?
   A. By stimulating at two points and dividing the distance between them by the difference in latency
   B. By measuring the amplitude of the EMG
   C. By multiplying the latency by the distance
   D. By stimulating at one point only
   Answer: A. v = distance / difference in latency between two stimulation points. (Key point: Nerve conduction velocity)