Essentials of electricity and electrotherapy. Biophysical principles of the interaction between electrical current and biological tissues. Basic electrical quantities and parameters. Physiological and therapeutic effects
What electricity is, how a current behaves when it passes through skin, fat, muscle and nerve, the quantities and pulse settings every current is described by, and the effects a therapist can get from them. The groundwork for every electrotherapy topic that follows.
- 14 min read
- 5 questions
- Checked 05 Nov 2026
Contents5 sections
At a glance
- A current is a flow of charge. In metal wires electrons carry it; in the body ions carry it, through the salty water of tissues.
- Voltage is the push, resistance the narrowness of the pipe, and current what flows: I = U / R (Ohm's law). The dry outer skin is the main resistance; blood, muscle and nerve conduct well; fat and bone poorly.
- Currents are direct (one way, steady), alternating (back and forth) or pulsed (short bursts). By frequency they are low (up to 1,000 Hz), medium (1,000 to 100,000 Hz) and high (above 100,000 Hz) frequency.
- Under the cathode (negative) the nerve membrane depolarises and excitability rises; under the anode (positive) it hyperpolarises and excitability falls.
- A nerve fires when a pulse is strong enough for long enough: rheobase and chronaxie describe that trade-off, and a slowly rising current lets healthy nerve escape by accommodation.
- The effects are chemical (ions, pH, burns), excitatory (sensation, contraction, pain relief) and thermal (heat), and from them come the uses: analgesia, muscle stimulation, better circulation, drug delivery and deep heating.
- Never send a current across the chest or through a pacemaker, and never dose by feel where the skin cannot feel. Most contraindications follow from where the current goes.
Electricity and electrotherapy, the essentials
Electrotherapy is treatment with electric currents and fields. It uses the fact that the body conducts electricity and that nerves and muscles work by electrical signals, so a current from outside can make a nerve fire, a muscle contract, ions move or tissue warm.
Think of a current as water in a pipe. The voltage is the pressure that pushes it, the resistance is how narrow the pipe is, and the current is how much water flows each second. More pressure or a wider pipe means more flow.
The body is not a metal wire. Its "wires" are the tissue fluids, full of sodium, potassium and chloride ions. When a current passes, these ions move: cations towards the cathode (negative electrode), anions towards the anode (positive electrode). That is why a current in the body always has a chemical side as well as an electrical one.
Three kinds of current
| Kind | What the current does | Examples in therapy |
|---|---|---|
| Direct current | Flows one way, at a steady strength, for seconds to minutes | Galvanization, iontophoresis |
| Alternating current | Flows one way then the other, without a break | Medium-frequency carrier currents, high-frequency diathermy |
| Pulsed current | Short pulses with gaps between them, in one direction (monophasic) or both (biphasic) | TENS, NMES, diadynamic currents |
The professor's classification
The book sorts therapeutic currents by two numbers, and the exam expects these bands.
| By frequency | Range | By voltage | Range |
|---|---|---|---|
| Low frequency | up to 1,000 Hz | Low voltage | up to about 150 V |
| Medium frequency | 1,000 to 100,000 Hz | High voltage | traditionally above 2,000 V (high-frequency apparatus) |
| High frequency | above 100,000 Hz |
Combined, they give the three working families: low-voltage low-frequency currents (galvanic, pulsed, diadynamic, TENS), low-voltage medium-frequency currents (interferential, sinusoidal modulated) and high-frequency currents (shortwave, decimetric and microwave), which heat. High-voltage pulsed current (HVPC) is a modern low-frequency current of short pulses at 100 to 500 V; its peak voltage is high but its average current is tiny.
Point at a name or a part to light it.
Frequency decides how a current talks to the body. A direct current (0 Hz) flows one way and moves ions. Low-frequency currents, up to 1,000 Hz, fire a nerve with each pulse. Medium-frequency currents, 1,000 to 100,000 Hz, have cycles too short to fire a nerve one by one, so the nerve fires when many add up; and they pass the skin easily, because the skin's opposition falls as frequency rises. Above 100,000 Hz each cycle is far too brief to fire anything, the energy crosses the skin without contact, and what is left is heat: diathermy, at 27.12 MHz to 2.45 GHz.
Simplified: the lanes are drawn the same width, though each band spans far more frequencies than the one before: frequency rises to the right, not to scale; each wave is a sketch: low-frequency currents come in many shapes, medium-frequency currents are usually modulated, and a few dozen cycles stand for the millions of a high-frequency wave; the nerve's potential is a simple line: the size of each nudge, the leak between them and the shape of the action potential are not to scale; ions and water molecules are drawn far larger than life, and only a few; a water molecule really turns back and forth with the field rather than shifting; the direct-current panel leaves out electrolysis at the electrodes.
6 sources
- Aleksiev A. Physical Medicine and Rehabilitation for English-speaking MD students. Medical University of Sofia, 2002. Electrotherapy, types of electric current; Galvanization; Medium frequency currents; High frequency currents.
- Aleksiev AR. Physical Medicine and Rehabilitation, lecture 1 handnotes. Medical University of Sofia, 2026. Galvanic current, movement of charged and polar molecules.
- Watson T, Nussbaum EL (eds). Electrophysical Agents, evidence-based practice, 13th edition. Elsevier, 2021. Electrical stimulation; interferential and medium-frequency currents; shortwave therapies.
- Robinson AJ, Snyder-Mackler L. Clinical Electrophysiology, electrotherapy and electrophysiologic testing, 3rd edition. Lippincott Williams and Wilkins, 2008.
- Cameron MH. Physical Agents in Rehabilitation, an evidence-based approach to practice, 5th edition. Elsevier, 2018. Chapters on electrical currents and on diathermy.
- Hall JE, Hall ME. Guyton and Hall Textbook of Medical Physiology, 14th edition. Elsevier, 2021. Membrane potentials and action potentials.
How current meets the body
Tissues are uneven conductors
Current takes the easiest path, like water through the widest pipes. Tissues full of water and ions conduct best; those with little water conduct worst.
| Conducts well | Conducts poorly |
|---|---|
| Blood, cerebrospinal fluid, muscle (best along its fibres), nerve, internal organs | Dry skin (the stratum corneum), fat, bone, tendon, nail, hair |
The stratum corneum, the dead outer layer of skin, is the biggest barrier. Dry skin can oppose a direct current with tens of thousands of ohms; wet, warm, washed skin with far less. So skin is cleaned and electrodes sit on moist pads or gel: the aim is to get the current through the doorway without forcing it.
The skin and the cell membrane behave like capacitors
A capacitor is two conducting plates with an insulator between them, like a bucket that fills with charge. Skin is one (sweat-filled tissue, then the insulating stratum corneum, then the pad) and so is every cell membrane (fluid, lipid layer, fluid). A capacitor blocks a steady current once it is full, but lets a changing current through, and the faster the change, the more easily it passes.
So the opposition of skin to a current, its impedance, falls as frequency rises. A direct current meets the skin's full resistance and stings; a 4,000 Hz current passes with less voltage and less sensation. That is the main reason medium-frequency currents were invented.
What a current does to a nerve membrane
At rest, the inside of a nerve or muscle cell is about 70 to 90 mV negative to the outside: its resting membrane potential. The cell fires an action potential when this difference shrinks to threshold, about 15 to 20 mV less negative.
- Under the cathode, the outside of the membrane is made more negative, so the difference across it shrinks: the membrane depolarises and comes nearer threshold. This is catelectrotonus, and excitability rises.
- Under the anode, the outside is made more positive, the difference grows, the membrane hyperpolarises and moves away from threshold. This is anelectrotonus, and excitability falls.
How strong, for how long: rheobase and chronaxie
A nerve needs enough charge to reach threshold. A strong pulse can be short; a weak pulse must last longer. Plotting the weakest current that fires the nerve against pulse duration gives the strength–duration curve.
- Rheobase is the smallest current that fires the tissue with a very long pulse: below it, nothing happens however long you wait.
- Chronaxie is the pulse duration needed at twice the rheobase. It measures how quickly a tissue responds.
- chronaxie of healthy motor nerve, roughly
- 0.05–0.5ms
- chronaxie of denervated muscle, which must be fired directly
- 10 or morems
The gap is the whole basis of electrodiagnosis and of stimulating denervated muscle: healthy nerve answers short pulses, denervated muscle needs long ones.
Accommodation: why slow currents slip by
If a current rises slowly, a healthy nerve adapts: its sodium channels inactivate and potassium channels open as the membrane creeps towards threshold, so threshold runs ahead and no impulse fires. This is accommodation. It is why switching a direct current on suddenly makes a muscle twitch but a steady current does not, and why currents are ramped up and down for comfort. Denervated muscle accommodates poorly, so a slowly rising (triangular or exponential) pulse fires it while healthy nerves and skin receptors ignore it.
Which fibres fire first
Large, myelinated fibres have the lowest threshold to an outside current. With surface electrodes, as intensity rises, the order is roughly:
- 1
Sensory threshold
Large touch fibres (Aβ) fire: tingling. This is where analgesic currents work. - 2
Motor threshold
Motor axons (Aα) fire: a twitch or a contraction. This is where muscle stimulation works. - 3
Pain threshold
Small Aδ and C pain fibres fire: sharp, then burning pain. Treatment stays below it.
The space between the motor and pain thresholds is the working room for muscle stimulation; short pulses (about 0.1 to 0.4 ms) make it widest.
Polarisation of tissue: the professor's zones
The book describes where a direct current acts. The polar zones lie right under the electrodes, the peripolar zones just beneath them in the subcutis, and the interpolar zone deep between the two. Because tissues are a patchwork of membranes, charge piles up at many boundaries along the path, so small "virtual electrodes" of reversed polarity appear inside the peripolar zones. This is a model, not something measured in each patient, but it explains why effects are strongest under the electrodes and weaker and less specific in between.
Heat
Any current warms what it passes through (Joule's law: heat = I² × R × t). With low- and medium-frequency currents at treatment intensities the heat is negligible, because the current is small. With high-frequency currents the current or field is large, and heat is the main effect.
Basic electrical quantities and parameters
The quantities
| Quantity | Symbol and unit | What it means | Everyday picture |
|---|---|---|---|
| Charge | Q, coulomb (C) | An amount of electricity | The amount of water |
| Current | I, ampere (A); in therapy mA or µA | Charge flowing per second (I = Q / t) | Litres per second |
| Voltage | U or V, volt (V) | The push between two points | Water pressure |
| Resistance | R, ohm (Ω) | Opposition to a steady current | A narrow pipe |
| Impedance | Z, ohm (Ω) | Opposition to a changing current, resistance plus the capacitor effect; falls as frequency rises | A pipe that widens when the flow pulses |
| Power | P, watt (W) | Energy per second (P = U × I) | How hard the pump works |
| Current density | mA/cm² | Current divided by electrode area | How crowded the flow is at one spot |
Ohm's law ties the first three: I = U / R. Double the push, double the flow; double the resistance, half the flow. Most modern stimulators are constant-current devices: they adjust the voltage themselves so that the set current flows even if the skin's resistance changes.
The parameters of a pulse
| Parameter | What it is | Why it matters |
|---|---|---|
| Amplitude (intensity) | How strong each pulse is, in mA or V | Decides how many and which fibres fire |
| Phase and pulse duration | How long one phase or pulse lasts, in µs or ms | Short pulses fire nerve; long ones are needed for denervated muscle |
| Frequency (pulse rate) | Pulses per second, in Hz | Below about 10 Hz, separate twitches; from about 20 to 30 Hz, contractions start to fuse; 50 Hz gives a smooth tetanic contraction |
| Interpulse interval | The pause between pulses | Gives the membrane time to recover |
| Waveform | Rectangular, triangular, trapezoidal, sinusoidal, semi-sinusoidal, exponential | A steep rise fires healthy nerve; a slow rise spares it |
| Polarity and balance | Monophasic (one direction, has a direct-current component) or biphasic (both, can be charge-balanced) | A net direct current causes chemical effects and burns; balanced biphasic pulses avoid them |
| Modulation | Changes in amplitude, frequency or grouping over time (bursts, surges) | Fights the nerve's habit of getting used to a steady stimulus |
| Ramp, on time, off time | How fast a train builds up, how long it lasts, how long it rests | Comfort, and avoiding muscle fatigue |
| Treatment time and number of sessions | Minutes per session, sessions per course | The dose |
Point at a name or a part to light it.
Every pulsed current is described by the same handful of settings. Amplitude decides how many and which fibres fire; pulse width, with amplitude, how much charge each pulse carries, short pulses for nerve and long ones for denervated muscle; the frequency, how many pulses come each second, decides whether a muscle twitches or contracts smoothly. The ramp lets the current build up gently. Monophasic pulses leave a net direct current behind them, with its chemical effects; balanced biphasic pulses leave none.
Simplified: neither time nor current is to scale: pulses are usually far narrower than the gaps between them; the pulses are drawn square; real pulses can be triangular, trapezoidal, sinusoidal or exponential, and rise and fall with a slope of their own; a real ramp lasts a second or more and takes many pulses, not three.
5 sources
- Aleksiev AR. Physical Medicine and Rehabilitation, lecture 1 handnotes. Medical University of Sofia, 2026. Pulsed currents: waveform, polarity, pulse duration, frequency, amplitude, modulation.
- American Physical Therapy Association, Section on Clinical Electrophysiology. Electrotherapeutic terminology in physical therapy. APTA, 2001.
- Robinson AJ, Snyder-Mackler L. Clinical Electrophysiology, electrotherapy and electrophysiologic testing, 3rd edition. Lippincott Williams and Wilkins, 2008.
- Watson T, Nussbaum EL (eds). Electrophysical Agents, evidence-based practice, 13th edition. Elsevier, 2021. Electrical stimulation, introduction and general concepts.
- Cameron MH. Physical Agents in Rehabilitation, an evidence-based approach to practice, 5th edition. Elsevier, 2018. Chapters on electrical currents and their effects.
- the upper limit of low-frequency currents
- 1,000Hz
- mains frequency in Europe, and the frequency of a smooth tetanic contraction
- 50Hz
- where high-frequency currents begin
- 100,000Hz
Physiological and therapeutic effects
Every effect comes from one of three things a current can do: move ions, fire excitable cells, or make heat.
| What the current does | Physiological effect | Therapeutic use | Main currents |
|---|---|---|---|
| Moves ions | Electrolysis, pH change under the electrodes, ion and water shifts (electrophoresis and electro-osmosis), membrane polarisation | Drug delivery, changing excitability, local hyperaemia | Galvanic current, iontophoresis |
| Fires sensory fibres | Tingling, closing the spinal "gate", release of endorphins | Pain relief | TENS, diadynamic, interferential, pulsed galvanic |
| Fires motor fibres or muscle | Twitches and contractions | Strengthening, preventing atrophy, re-education, functional movement, muscle pump for oedema | NMES, FES, tetanic, exponential, Russian current |
| Makes heat | Vasodilation, faster metabolism, softer collagen | Deep heating of stiff, painful tissue | Shortwave, decimetric, microwave |
On top come indirect effects: more blood flow (from axon reflexes, released histamine-like substances and muscle pumping), better tissue nutrition (the "trophic" effect) and, the book says, reflex effects in the same spinal segment.
The professor's two ways of sorting the effects
The book's school, from Bulgarian and Russian physiotherapy, sorts effects in two ways, and both come up in the exam.
- By where they arise. Specific effects arise in the polar and peripolar zones, where excitable tissue meets the current: muscle contraction, acetylcholine and cholinesterase release, sensations from the special senses (a flash of light, a metallic taste near the head). Nonspecific effects arise in the interpolar zone: histamine-like substances, hyperaemia, metabolic change.
- By direction. Irritative (stimulating) effects include electrostimulation of muscle, the revulsive effect (a reddening counter-irritation), raising tone and a bone-building (osteoblastic) effect. Inhibitory effects include electroanalgesia, lowering tone and an osteoclastic effect. The cathode is the irritative electrode, the anode the inhibitory one.
How strong is the evidence
The firmest evidence is for effects that can be seen: muscle contraction from NMES and FES, drug delivery by iontophoresis for a few uses, and short-term pain relief from TENS in some conditions. For many traditional indications of galvanic, diadynamic and interferential currents, good trials are few or show little benefit over sham. Electrotherapy is best used as one part of an active rehabilitation programme, not on its own.
Safety: the rules for every current
The contraindications of electrotherapy look long, but most follow from four reasons.
| Reason | Contraindication or caution |
|---|---|
| The current could reach the heart or an implant | Electrodes across the chest, a pacemaker or implanted defibrillator in or near the current path, other implanted stimulators; serious arrhythmias |
| The current could harm the skin, and nobody would notice | Broken, infected or diseased skin; an area without normal sensation; a patient who cannot report what they feel (confused, very young, unconscious) |
| More blood flow or tissue activity would spread harm | Malignant tumour in the area, active bleeding or a bleeding disorder, acute deep vein thrombosis, acute infection, fever, active tuberculosis |
| A sensitive area or condition | The pregnant abdomen, pelvis and low back, the anterior neck and carotid sinus, the eyes, the head in a person with epilepsy; metal in the current path with direct or monophasic currents, which concentrates current at its surface |
Electric shock
Treatment currents are tiny compared with the currents that kill, but the numbers explain the rules. At mains frequency (50 Hz), roughly:
- felt as a tingle, hand to hand
- about 1mA
- muscles clamp and the person cannot let go
- about 10mA
- across the chest for a second, can cause ventricular fibrillation
- about 100mA
Mains current at 50 Hz is especially dangerous because it falls right in the range that excites nerve and muscle best. A current applied straight to the heart (through a catheter or pacing wire) can cause fibrillation at microamperes, which is why electrodes are never placed so that the current crosses the chest.
- 1
Before
Check the referral, the contraindications, implants and skin sensation (test sharp and blunt, hot and cold). Explain what the patient should feel. - 2
Set up
Check the device and cables, clean the skin, use well-wetted pads or fresh gel, choose electrode size and placement, start from zero. - 3
During
Raise the intensity slowly, ask about sensations, look at the skin. Pain or burning means reduce or stop. - 4
After
Turn the current down to zero before removing electrodes, inspect the skin, record the parameters and the response.
Check yourselfAnswer, then say how sure you were: a lucky guess still counts as a weak point.
1A student says that the anode is the electrode that excites nerves, because it is positive and positive ions trigger the action potential. What is the correct teaching?
2Why do medium-frequency currents pass the skin with less discomfort than a direct current of the same strength?
3The same 10 mA current runs between a 10 cm² electrode and a 100 cm² electrode. Where is the effect, and the risk of a burn, greatest?
4A current is made to rise very slowly from zero to a strength that, as a sudden square pulse, makes a healthy muscle contract. The muscle does not contract. Why?
5A patient with a cardiac pacemaker is referred for electrotherapy of the opposite knee. What is the safest reasoning?