Essentials of electrodiagnosis. Assessment of neuromuscular excitability. Role in diagnosis, prognosis, and rehabilitation planning
Electrodiagnosis tests how easily nerve and muscle can be made to fire, to tell whether a muscle still has its nerve. This page covers the classical tests (the strength-duration curve, rheobase, chronaxie, accommodation and the reaction of degeneration), where nerve conduction studies and EMG have taken over, and how the results guide diagnosis, prognosis and rehabilitation.
- 11 min read
- 5 questions
- Checked 05 Nov 2026
Contents4 sections
At a glance
- Electrodiagnosis asks one main question: does this muscle still have its nerve? A nerve fires with short, weak pulses; a muscle without its nerve needs long, strong ones.
- Rheobase is the smallest current that makes a muscle twitch with a long pulse. Chronaxie is the pulse length needed at twice the rheobase: below 1 ms if the nerve is intact, often 10 ms or more if it is lost.
- The strength-duration curve plots threshold current against pulse length. Denervation shifts it to the right and up; a kink means partial denervation; a shift back to the left means reinnervation.
- Healthy nerve accommodates: it ignores a slowly rising current. Denervated muscle does not. This is both a test and the trick for stimulating denervated muscle selectively.
- The reaction of degeneration: no response to short faradic pulses, a slow worm-like contraction to galvanic pulses, often with reversed polarity.
- Tests are normal for the first days after a nerve is cut, until Wallerian degeneration sets in. Nerve conduction studies and needle EMG, best done about three weeks after injury, have replaced most classical tests.
What electrodiagnosis is
Electrodiagnosis uses electricity to examine nerves and muscles. The classical form, taught in physical medicine, stimulates a muscle through the skin and watches what it does. The modern form, nerve conduction studies and EMG, records the electrical signals that nerves and muscles make.
The key idea is excitability: how easily a tissue can be made to fire. Think of a heavy door and a light one. The light door (a nerve fibre) swings with a short push. The heavy one (a muscle fibre on its own, without its nerve) only moves if you lean on it for a while. If a muscle only answers to long, strong pushes, its nerve has been lost.
Why a nerve and a bare muscle differ
When a muscle has its nerve, a stimulus over the muscle fires the nerve's branches inside it, and they fire the muscle. Nerve fibres are thick, myelinated and quick, so they need only a short pulse. When the nerve has degenerated, the current must fire the muscle membrane itself, which is slower to charge and needs a pulse many times longer. The place where a small current gives the best twitch, where the nerve enters the muscle, is the motor point.
The classical tests
Faradic and galvanic testing
The oldest test, from the 19th century, uses two currents.
| Current | What it is | Normal muscle | Denervated muscle |
|---|---|---|---|
| Faradic | short pulses, about 1 ms, repeated about 50 times a second | brisk contraction, a sustained (tetanic) one at 50 Hz | no contraction |
| Galvanic (interrupted) | long pulses of direct current, 100 ms or more | brisk twitch when the current is switched on | slow, sluggish, worm-like twitch |
The normal pattern of twitches follows the polar rule, which goes back to Pflüger's law of contraction (worked out on frog nerve) and was set out for people by Erb: the twitch is strongest when the current is switched on under the negative electrode, the cathodal closing contraction. The reason is that the cathode pulls the membrane towards firing (catelectrotonus), while the anode pushes it away (anelectrotonus). From strongest to weakest, the order is cathode on, then anode on, then anode off, then cathode off; the two anode twitches are close and can swap places.
The reaction of degeneration
When a muscle loses its nerve, testing shows a typical set of changes, called the reaction of degeneration (described by Erb):
- No response to faradic current, because short pulses can no longer fire it.
- A slow, worm-like contraction to galvanic current, because the muscle membrane is firing on its own.
- A change in polarity: the anode may work as well as or better than the cathode.
- A longitudinal reaction: the contraction may be easier to get from the tendon end of the muscle than from the motor point, since the motor point has lost its meaning.
| Degree | Faradic | Galvanic | Meaning |
|---|---|---|---|
| Partial | weak | sluggish, some fibres still brisk | some nerve fibres survive; better outlook |
| Complete | none | slow, worm-like | the whole muscle has lost its nerve |
| Absolute | none | none | muscle fibres replaced by scar after long denervation; recovery not expected |
The strength-duration curve
The strength-duration curve (SD curve) is the classical test made into a graph.
- 1
Find the motor point
A small electrode over the motor point, a larger one elsewhere on the limb. - 2
Long pulse first
Start with a long rectangular pulse, 100 to 300 ms. Raise the current until a minimal visible contraction appears. Write it down. - 3
Shorter and shorter
Repeat with shorter pulses, for example 100, 30, 10, 3, 1, 0.3, 0.1 and 0.03 ms. Each time, find the current that gives the minimal contraction. - 4
Plot
Put pulse duration across the bottom (on a log scale) and current up the side. Join the points.
The curve always rises to the left: the shorter the pulse, the more current it takes, until below a certain length no current will do. Its position tells the story.
| Measure | Definition | Normal (nerve intact) | Denervated |
|---|---|---|---|
| Rheobase | the smallest current that fires the tissue with a long pulse | a few mA, depends on skin and electrode | may fall at first, then rise as the muscle wastes and scars |
| Chronaxie | the pulse duration that fires the tissue at twice the rheobase | below 1 ms, often 0.1 to 0.5 ms | above 1 ms, often 10 to 50 ms |
| Utilisation time | the shortest pulse that fires the tissue at rheobase current | short | long |
| Shape of the curve | flat until about 1 ms, then rises steeply only for the shortest pulses | shifted right and up; no response to short pulses |
Point at a name or a part to light it.
For the curve, the current that just makes the muscle twitch is found for pulses from long to very short. A muscle with its nerve answers short pulses, so its curve sits to the left. Without its nerve the muscle needs pulses many times longer: the curve shifts up and to the right, and short pulses do nothing. The chronaxie, read at twice the rheobase, sums it up: below 1 ms with the nerve, often 10 to 50 ms without it.
Simplified: the curves are drawn from a simple law, in which the threshold falls as the pulse lengthens and is twice the rheobase at the chronaxie, not from measurements; each curve is drawn at one chronaxie from its range; the denervated rheobase is drawn higher, as later in denervation; early on it may be lower; partial denervation, whose curve has a kink, and the utilisation time are not drawn.
5 sources
- Robinson AJ, Snyder-Mackler L. Clinical Electrophysiology: electrotherapy and electrophysiologic testing, 3rd edition. Lippincott Williams and Wilkins, 2008. Strength-duration testing.
- Watson T, Nussbaum EL (eds). Electrophysical Agents, evidence-based practice, 13th edition. Elsevier, 2021. Chapters on the physiology of electrical stimulation and on stimulation of denervated muscle.
- Cifu DX (ed). Braddom’s Physical Medicine and Rehabilitation, 6th edition. Elsevier, 2021. Chapters on electrodiagnostic medicine.
- Geddes LA. Accuracy limitations of chronaxie values. IEEE Transactions on Biomedical Engineering, 2004;51(1):176–181.
- Aleksiev A. Physical Medicine and Rehabilitation for English-speaking MD students. Medical University of Sofia, 2002. Electrodiagnostics: rheobase, chronaxie and the strength-duration curve.
- the current at which chronaxie is read
- 2×rheobase
- chronaxie with the nerve intact
- below 1ms
- chronaxie typical of complete denervation
- 10–50ms
Chronaxie alone can mislead in partial denervation, because it reflects whichever fibres dominate; the whole curve is more useful. Rheobase depends heavily on the skin and electrodes, so it is compared only on the same patient and the same set-up.
Accommodation
Accommodation is a nerve's ability to adapt to a current that rises slowly. If the current creeps up, the sodium channels have time to close (inactivate) and potassium channels to open, and the nerve does not fire, even at a strength that would fire it if switched on suddenly. It is like getting into a cold bath slowly: you hardly notice the change.
- Healthy nerve accommodates well. A slowly rising (triangular) pulse must be several times stronger than a rectangular one to fire it, often quoted as about two to six times, depending on the rise time and the source.
- Denervated muscle accommodates poorly. A triangular pulse fires it at about the same current as a rectangular one.
The ratio between the two thresholds, the accommodation quotient, falls towards 1 in denervation. This gives the test its practical use: slowly rising pulses can make a denervated muscle contract while the healthy nerves around it, including the skin's sensory nerves, stay quiet. That is how denervated muscle is stimulated in treatment (see low-frequency pulsed currents and NMES).
Where EMG and nerve conduction studies fit today
The classical tests are cheap and need only a stimulator, but they are crude: they see a contraction, not a signal, and they cannot say where along the nerve the damage lies. Since the second half of the 20th century, nerve conduction studies and needle EMG have become the standard. The classical tests survive in physiotherapy, to follow a denervated muscle and to set stimulation parameters.
Nerve conduction studies
A nerve conduction study fires a nerve with a brief pulse through the skin and records the response further along.
| Measure | What it is | What a change means |
|---|---|---|
| CMAP amplitude | the size of the muscle's response to nerve stimulation | falls when axons are lost |
| SNAP amplitude | the size of a sensory nerve's response | falls when sensory axons are lost; normal if the damage is at the root, before the sensory ganglion |
| Latency and conduction velocity | how fast the signal travels | slowed in demyelination; normal or only mildly slowed in axonal loss |
| Conduction block | a response much smaller above a point than below it | focal demyelination at that point, as in a neurapraxia or entrapment |
| F wave and H reflex | responses that travel to the spinal cord and back | test the roots and the proximal nerve |
Needle EMG
Electromyography (EMG) records the electrical activity of muscle through a fine needle.
| Finding | When | Meaning |
|---|---|---|
| Fibrillation potentials and positive sharp waves at rest | from about 2 to 3 weeks after injury, later in distant muscles | single muscle fibres firing on their own because they have lost their nerve: active denervation |
| Reduced recruitment | at once | fewer motor units can be called up |
| Small, polyphasic, unstable nascent units | weeks to months | the first regrowing axons reaching the muscle: early reinnervation |
| Large, long, polyphasic units | months | surviving axons have adopted orphaned fibres (collateral sprouting): chronic reinnervation |
Timing matters
After an axon is cut, the part beyond the cut keeps working for some days before Wallerian degeneration breaks it down. Motor responses beyond the cut fade from about day 3 and, after a complete cut, are gone by about day 9; sensory responses follow a few days later (from about day 6, gone by about day 11). Fibrillations take two to three weeks or more to appear. So:
- In the first days, every test can be normal, even with a complete cut. Only stimulation above and below the lesion can show a block.
- The classical SD curve shows denervation only after about two to three weeks.
- EMG gives the fullest picture at about three to four weeks, and is repeated to follow recovery.
Role in diagnosis, prognosis and rehabilitation planning
Diagnosis
Electrodiagnosis tells whether a weak muscle is weak from the nerve, the muscle, or neither; where along the nerve the damage lies (root, plexus, nerve, junction); whether the damage is to the myelin or the axon; and how severe it is. It sorts nerve injuries into Seddon's three types.
| Seddon type | What is damaged | Electrodiagnosis | Outlook |
|---|---|---|---|
| Neurapraxia | myelin only, the axon intact | normal response below the lesion, block across it; normal SD curve; no fibrillations | full recovery in days to weeks, up to about three months |
| Axonotmesis | axons, with the sheath intact | denervation: SD curve to the right, fibrillations, lost CMAP | regrowth along the old sheath, about 1 mm a day; often good |
| Neurotmesis | the whole nerve | as axonotmesis, with no sign of reinnervation over time | no recovery without surgical repair |
Prognosis
- The amount of axon loss, best judged by the CMAP compared with the healthy side, predicts recovery.
- A neurapraxia recovers well; a lesion with no surviving axons recovers only by regrowth, slowly, from the injury down.
- Signs of reinnervation (nascent units, a curve moving left) often come weeks before movement and are good news.
- No sign of reinnervation where it was expected, usually by about three to four months, is a reason to consider surgical exploration.
Rehabilitation planning
The results set the goals and the methods. A neurapraxia needs protection and exercise while it recovers. A denervated muscle needs to be kept in good state until the nerve returns: positioning, splints, passive movement, and in some centres electrical stimulation of the denervated muscle, whose evidence is mixed. Repeat testing shows when to move from passive work to active exercise.
The book sets the parameters for stimulating denervated muscle from the tests, each reflecting the logic above:
| Parameter | Taken from | As denervation deepens |
|---|---|---|
| Intensity | the rheobase | higher |
| Pulse duration | the chronaxie | longer, often tens to hundreds of ms |
| Pulse shape | accommodation | slower rise, triangular or exponential |
| Pause between pulses | the refractory period and fatigue | longer |
| Frequency | how often the muscle can respond | lower, a few contractions a minute when complete |
Check yourselfAnswer, then say how sure you were: a lucky guess still counts as a weak point.
1On a strength-duration curve, what is the chronaxie?
2A man cuts his wrist on glass; the median nerve may be divided. On day 2 a nerve conduction study below the cut is normal. What does this mean?
3A muscle does not contract to short (faradic) pulses but gives a slow, worm-like contraction to long galvanic pulses, stronger when the anode is the stimulating electrode. What is this?
4Why are slowly rising (triangular or exponential) pulses used to stimulate a denervated muscle that lies next to healthy muscles?
5Three months after a fibular (peroneal) nerve injury there is still no visible ankle dorsiflexion. Which finding best shows that reinnervation has begun?