Electronic Devices in Physiotherapy
(Physics for BPT – Practical Overview)
1. Simple Explanation
Electronic devices used in physiotherapy are small electric circuits that generate controlled electrical signals (current, voltage, or waves) to influence the body. They are built from basic electronic components—resistors, capacitors, inductors, diodes, transistors, oscillators—and use either direct current (DC) or alternating current (AC) to produce the therapeutic effect.
| Device | What it does | Typical Signal | Main Components |
|---|---|---|---|
| TENS (Transcutaneous Electrical Nerve Stimulation) | Pain relief via electrical stimulation | AC pulse train (0–500 Hz) | Pulse generator, capacitors, electrodes |
| EMS (Electrical Muscle Stimulation) | Muscle contraction | 20–50 Hz AC pulses | Pulse generator, current‑controlled driver |
| Interferential Therapy | Deep muscle stimulation | Two AC currents (≈ 4000 Hz) that interfere | Dual‑channel oscillator, mixing circuit |
| Ultrasound | Tissue heating / micro‑vibration | 1–3 MHz mechanical wave | Transducer, RF oscillator |
| Magnetotherapy | Blood flow improvement | Pulsed magnetic field | Coil, current source |
Key point: The electrical signal is delivered to the body through electrodes or transducers, and the therapeutic effect depends on the frequency, amplitude, pulse width, and waveform.
2. Key Physics Concepts
| Concept | Formula | Typical Physiotherapy Values |
|---|---|---|
| Electric Current (I) | ( I = \frac{V}{R} ) (Ohm’s Law) | 0.1 mA – 30 mA |
| Voltage (V) | ( V = I \times R ) | 5 V – 200 V (peak‑to‑peak) |
| Resistance (R) | ( R = \frac{V}{I} ) | 1 kΩ – 10 kΩ (tissue) |
| Power (P) | ( P = V \times I ) | < 1 W (safe for human skin) |
| Impedance (Z) | ( Z = \sqrt{R^2 + (X_L - X_C)^2} ) | < 5 kΩ for most devices |
| Frequency (f) | ( f = \frac{1}{T} ) | 1 Hz – 5 kHz (TENS/EMS) |
| Capacitance (C) | ( C = \frac{Q}{V} ) | 1 µF – 100 µF (pulse shaping) |
Safety Note: Maximum safe skin current is ≈ 10 mA for a brief period. Devices incorporate current‑limiting resistors or constant‑current drivers.
3. Worked Example – Calculating Current for a TENS Device
Problem
A TENS unit delivers a peak‑to‑peak voltage of 30 V to a pair of electrodes. The skin‑tissue impedance between electrodes is 60 kΩ.
Find the resulting current delivered to the patient and the power dissipated.
Solution
-
Ohm’s Law
[ I = \frac{V}{R} = \frac{30,\text{V}}{60,000,\Omega} = 0.0005,\text{A} = 0.5,\text{mA} ] -
Power
[ P = V \times I = 30,\text{V} \times 0.0005,\text{A} = 0.015,\text{W} = 15,\text{mW} ]
Interpretation
- 0.5 mA is well below the 10 mA safety threshold, so the device is safe for routine use.
- 15 mW of power is minimal, ensuring no excessive heating of skin or tissue.
Tip: Most TENS units allow the therapist to adjust the pulse width and frequency to achieve the desired sensation without exceeding safe current limits.
4. Safety & Practical Considerations
| Issue | What to Check | Why It Matters |
|---|---|---|
| Grounding / Insulation | Device housing, electrode straps, patient skin | Prevents accidental shocks |
| Current Limiting | Series resistor or constant‑current driver | Avoids exceeding 10 mA |
| Electrode Quality | Conductive gel, proper size | Reduces skin resistance & burns |
| Frequency Selection | 1–100 Hz for muscle, 100–500 Hz for pain | Optimises therapeutic effect |