🔵 Note: This article is intended for healthcare professionals and contains advanced medical information.
1. Introduction
Electromyography (EMG) is a diagnostic test used in clinical neurophysiology to assess the electrical activity produced by skeletal muscles. It helps evaluate the health of muscles and the nerve cells (motor neurons) that control them.
Why it matters: EMG is essential in diagnosing neuromuscular disorders, localizing lesions, guiding treatment plans, and differentiating between nerve vs. muscle vs. neuromuscular junction disorders.
2. Basic Anatomy & Physiology Overview
Motor Units: Each motor neuron + all the muscle fibers it innervates = motor unit. Disorders can affect different parts (anterior horn cell, peripheral nerve, neuromuscular junction, muscle fiber). PubMed+1
Resting vs active muscle state: In a healthy resting muscle, EMG should show electrical silence (with some small “end-plate potentials” near neuromuscular junction). With slight activation, individual motor unit potentials can be detected.
3. Components of EMG + Instrumentation
Needle EMG: Insertion of needle electrode into muscle to measure electrical activity. Used to examine motor unit potential (MUP) shapes, recruitment pattern, spontaneous activity (fibrillations, positive sharp waves etc.).
- Nerve conduction studies (NCS): Stimulating peripheral nerves and measuring the speed and amplitude of responses (motor & sensory). Helps locate whether lesion is demyelinating vs axonal.
- Electrodes: Different types: monopolar, concentric, etc. Amplifiers, filters, display (visual and auditory).
4. Technique / How EMG is Done
Patient prep: Explain to patient, relax muscle, avoid caffeine, etc.
Electrode placement: Choice of muscles depends on suspected pathology. Use standardized landmarks.
Recording at rest: Look for spontaneous activity: fibrillation potentials, fasciculations, positive sharp waves.
With minimal contraction: Examine motor unit potentials: shape, duration, amplitude, number of phases.
Strong contraction testing: Evaluate recruitment (how many motor units fire as contraction increases).
5. Normal vs. Abnormal Findings
Â
| Parameter | Normal | Abnormal / What it suggests |
|---|---|---|
| Resting state | Electrical silence (no spontaneous activity) | Spontaneous activity → indicates denervation, muscle membrane irritability |
| Motor unit potential shape & duration | Triphasic, symmetrical, regular amplitude | Increased duration / polyphasic → reinnervation or chronic denervation |
| Recruitment pattern | Progressive, orderly | Reduced recruitment → fewer motor units available / neuropathy; Early recruitment → myopathy |
| Nerve conduction velocity | Within expected norms for age, limb | Slowed conduction → demyelination |
6. Clinical Applications
Peripheral neuropathies (e.g. diabetic neuropathy, compressive neuropathy)
Myopathies (muscular dystrophy, inflammatory myositis)
Motor neuron diseases (e.g. ALS)
Neuromuscular junction disorders (e.g. myasthenia gravis)
Radiculopathies / plexus injuries
7. Limitations & Pitfalls
Patient discomfort with needles
Need good technical setup (filtering, electrode types) to avoid artifacts
Interpretation requires experience
Sometimes can’t detect very early pathology
8. Recent Advances
Quantitative EMG techniques
Single-fiber EMG (for neuromuscular junction disorders)
Better signal processing, integration with imaging
9. Conclusion
EMG is a powerful tool in clinical neurophysiology. When done correctly, it gives highly specific info about where the problem lies (nerve, muscle, junction). Combining EMG with nerve conduction studies, clinical exam and imaging gives best diagnostic yield.
10. ReferencesÂ
- Clinical electromyography. Principles and practice. PubMed article.
- Neurophysiology in clinical practice.
- Basics in clinical neurophysiology: Nerve conduction and needle electromyography in nerve entrapment syndromes.
