How Electrical Stimulation Works

What You Will Learn

• Why an electrically stimulated contraction is the same as a natural one
• What the key settings mean (pulse length, current strength, frequency)
• Why standard devices cannot work on a denervated muscle
• How specialist devices overcome this problem

1. The Fundamental Principle

The most important thing to understand about electrical stimulation is this: the muscle contraction it produces is identical to a natural one. Your muscle does not "know" whether the signal came from your brain or from an external device. The contraction is the same, the training effect is real, and the benefits are genuine.

This is the foundation upon which all therapeutic electrical stimulation rests. When a device produces a contraction in your muscle, that contraction imposes the same demands as a voluntary contraction would. The muscle responds by maintaining or building its structure, just as it would with exercise.

However, a minimum signal is required to trigger a contraction. A signal that is too weak or too brief will fail. Understanding this is the key to understanding why a denervated muscle needs different treatment.

2. The Key Settings: What They Mean

Electrical stimulation is controlled by a small number of settings that together determine the treatment's effect. Think of these as the "prescription" for your treatment, similar to a drug's dose and frequency. Getting them wrong does not just reduce effectiveness; it can make the treatment completely ineffective.

Pulse shape (waveform)

The most common pulse type used in rehabilitation is called a biphasic rectangular pulse. Each pulse has two phases, positive and negative, that balance each other. This balance is important for safety: it prevents chemical build-up at the electrode-skin surface that could cause irritation or burns.

A second type, the biphasic triangular pulse, rises gradually rather than switching on instantly. This slowly rising signal has a specific use that we will explain shortly.

Pulse length (pulse duration or pulse width)

This is how long each individual pulse lasts. It is one of the most important settings for determining which tissue responds.

Nerve fibres respond to very short pulses, typically less than half a millisecond. Denervated muscle fibres, which must be stimulated directly, require enormously longer pulses, typically 40 to 200 milliseconds. The difference is like the difference between a quick tap and a sustained push. The nerve responds to the tap; the denervated muscle needs the sustained push.

Frequency

This is how often the pulses repeat, measured in pulses per second (Hertz, or Hz). It determines the type of contraction:

• At very low frequencies (1 to 2 Hz), you might see individual twitches: the muscle contracts and relaxes with each pulse.
• At higher frequencies (above about 15 to 20 Hz), the twitches merge into a smooth, sustained contraction called a tetanic contraction.

For a denervated muscle, treatment usually starts at very low frequencies (single twitches) and progresses to higher frequencies as the muscle recovers its ability to sustain stronger contractions.

Current strength (amplitude)

This is how strong the electrical signal is, measured in milliamperes (mA). Higher current recruits more muscle fibres and produces stronger contractions. Standard devices typically deliver 10 to 130 mA. Specialist devices for denervated muscle can deliver up to 250 mA, because the energy must reach and activate the muscle fibres directly rather than being amplified by the nerve.

Duty cycle

Treatment is usually delivered in bursts of pulses: a period of stimulation (on time) followed by a period of rest (off time). This allows the muscle to recover between contractions and reduces fatigue. The specific pattern of on and off times is tailored to your muscle's needs and changes as treatment progresses.

3. Why Standard Devices Cannot Work on Denervated Muscle

When the nerve is intact, it acts as a sophisticated amplifier. A tiny electrical pulse activates the nerve, and the nerve does the rest: firing the relay station, propagating the signal along the muscle fibre, and engaging the whole contractile system. This is efficient and requires only brief, modest pulses.

When the nerve is absent, this amplifier is gone. Every muscle fibre must be stimulated directly. The electrical properties of the muscle fibre membrane are much slower than those of the nerve, so the pulse must last much longer for the fibre to respond. This is not a matter of turning up the volume. Even at maximum intensity, a short-duration pulse cannot stimulate denervated muscle because the fibre simply needs sustained energy delivery over a much longer timeframe.

This is the fundamental reason why consumer devices, designed for pulse lengths under one millisecond, cannot produce contractions in denervated muscle regardless of marketing claims. The pulse is simply too brief for the fibre to respond.

Comparison: Innervated vs Denervated Muscle Stimulation

4. Selective Stimulation: Reaching Denervated Muscles Without Disturbing Their Neighbours

In many situations, particularly in peripheral nerve injuries, denervated muscles sit alongside muscles that still have their nerve supply. Stimulating both together with standard pulses would activate the healthy muscles first (because their nerve has a lower threshold), potentially causing unwanted contractions while failing to reach the denervated fibres.

The solution uses the gradually rising triangular pulse. When current rises slowly, intact nerve fibres can adapt and avoid being activated, a property called accommodation. Denervated muscle fibres cannot adapt in this way and will respond when the current reaches their threshold. This allows the specialist device to selectively activate denervated muscle while the neighbouring healthy muscles remain quiet.

5. The Vienna Protocol: A Phased Approach

The evidence-based approach to treating denervated muscle, developed through the RISE programme, follows a phased progression that matches the treatment to the muscle's current state.

Phase 1: Twitch stimulation. Very long pulses (120 to 200 milliseconds) at very low frequencies (below 2 Hz). The aim is to produce visible single twitches and rebuild the muscle's basic ability to respond. This phase may last weeks or months.

Phase 2: Progression. As the muscle responds, pulse lengths are gradually shortened and frequencies increased. The muscle begins to tolerate stronger, more frequent contractions.

Phase 3: Tetanic stimulation. Shorter pulses (40 to 50 milliseconds) at higher frequencies produce sustained contractions suitable for genuine muscle strengthening. This phase represents the transition from tissue preservation to active rehabilitation.

The transition between phases is guided by your clinician's assessment of how your muscle is responding, not by a fixed calendar.

6. The Minimum Effective Intensity

A common assumption is that stronger is better, that maximum current will produce the best results. The evidence suggests otherwise.

The optimal approach is to achieve a visible, functional contraction with the lowest possible current. Your clinician will generally advise you to increase the current until you start to see a contraction, then increase by about 10 per cent above that level. This is the minimum effective intensity.

There are good reasons for this approach. High currents can cause skin irritation, which may lead you to reduce how often you use the device. The treatment requires hundreds of contractions per session, five to six days per week, sustained over months. If the intensity causes significant discomfort, the programme becomes harder to maintain. Research has shown that consistent, moderate-intensity stimulation over months produces better outcomes than sporadic high-intensity sessions.

The principle is simple: more current is not better. More consistency is.

7. Electrodes: Why Size Matters

The electrodes are the interface between the device and your body, and getting them right matters enormously.

For standard stimulation of innervated muscle, small adhesive gel pads are placed over the "motor point," the spot where the nerve is most accessible. For a denervated muscle, this approach is wrong. There is no motor point because there is no nerve to target. Instead, the electrode must cover as much of the target muscle as possible.

This is why denervated muscle stimulation uses large electrodes (100 to 180 cm²), typically made from a wet sponge and carbon rubber combination, rather than small adhesive pads. The large electrodes distribute the current safely over a wide area, keeping the current density (current per unit area) within safe limits. Using small electrodes with the high currents needed for denervated muscle could cause burns.

Chapter Summary

Electrical stimulation produces genuine physiological change: the contraction is identical to a natural one, and the training effect is real. The key settings (pulse length, current strength, frequency, and pulse shape) together define the treatment. Denervated muscle requires dramatically longer pulses (40 to 200 milliseconds), higher currents (up to 250 mA), and larger electrodes than standard stimulation. Standard consumer devices cannot stimulate denervated muscle at any intensity because their pulses are simply too brief. The Vienna protocol provides a phased approach, progressing from single twitches to sustained contractions as the muscle recovers. Consistent treatment at moderate intensity produces better outcomes than sporadic high-intensity sessions.