What does an EMG record?

The sum of the action potentials of motor units near the electrode in skeletal muscle

The electrical activity of the heart

The electrical activity of the brain

The mechanical force of the muscle

What is seen in a normal muscle?

Electrical silence at complete rest, single motor unit potentials in weak contraction, interference pattern in strong contraction

Continuous large spikes at rest

Silence even during strong contraction

How is nerve conduction velocity measured?

By stimulating at two points and dividing the distance between them by the difference in latency

By measuring the amplitude of the EMG

By multiplying the latency by the distance

By stimulating at one point only

2075 Chaitra · 4 marks

Short note: 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.

Write a short note on Electromyography.

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).

 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).

✓ Answer checked by Claude Opus 5.5 (high effort)

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