Electromyography, or EMG, is a medical test that records the tiny electrical signals produced by muscles. These signals come from motor neurons telling muscle fibers when to contract. EMG matters because it helps doctors find problems in muscles, nerves, or the connection between them.
It is commonly used when a patient has weakness, numbness, cramping, or unexplained muscle pain.
During an EMG test, electrodes detect voltage changes in or near a muscle while the muscle is at rest and while it contracts. Surface electrodes stick to the skin, while needle electrodes can be inserted into a muscle for more detailed measurements. The EMG machine amplifies the small signals and displays them as waveforms that show timing, size, and pattern of muscle activity.
Abnormal waveforms can help identify conditions such as nerve compression, peripheral neuropathy, muscle disease, or motor neuron disorders.
Key Facts
- EMG records muscle electrical activity as voltage over time, usually shown as a waveform.
- Muscle activation begins when a motor neuron sends an action potential to muscle fibers.
- A motor unit is one motor neuron and all the muscle fibers it controls.
- Signal amplitude is often measured in microvolts or millivolts, where 1 mV = 0.001 V.
- Frequency measures how often signal cycles or pulses occur, with f = 1/T.
- A stronger muscle contraction usually recruits more motor units and increases EMG signal amplitude.
Vocabulary
- Electromyography
- Electromyography is a test that measures the electrical activity of muscles to help diagnose nerve and muscle disorders.
- Electrode
- An electrode is a sensor that detects electrical signals from the body and sends them to a recording device.
- Motor unit
- A motor unit is a single motor neuron together with all the muscle fibers it activates.
- Action potential
- An action potential is a rapid electrical signal that travels along a nerve or muscle cell membrane.
- Waveform
- A waveform is a graph of how an electrical signal changes over time.
Common Mistakes to Avoid
- Confusing EMG with ECG is wrong because EMG measures muscle activity, while ECG measures the electrical activity of the heart.
- Thinking EMG measures muscle force directly is wrong because EMG measures electrical activation, not the exact mechanical force produced.
- Assuming a flat EMG signal always means no problem is wrong because electrode placement, loose connections, or poor skin contact can also reduce the recorded signal.
- Ignoring the difference between rest and contraction recordings is wrong because abnormal activity at rest can indicate a different problem than abnormal patterns during contraction.
Practice Questions
- 1 An EMG waveform repeats every 0.020 s during a steady contraction. What is the signal frequency using f = 1/T?
- 2 An EMG machine records a peak signal of 0.75 mV. Convert this voltage to volts.
- 3 A patient has normal EMG signals when the muscle is stimulated directly but weak signals when the nerve is tested. Explain why this pattern may suggest a nerve problem rather than a muscle fiber problem.