Treating a denervated muscle is not EMS – at least, not the standard biphasic square-wave EMS used for muscle training or rehabilitation after a stroke. A denervated muscle needs selective stimulation current: a slower-rising, longer-duration impulse that can make the muscle fibre contract directly, without involving the nerve.
Why doesn’t it respond to a standard impulse? – The secret of accommodation
In a healthy muscle, the nerve is the link between the brain and the muscle fibre. A healthy nerve fires extremely quickly: a very short impulse measured in microseconds (millionths of a second) is enough to produce movement. That is why general muscle stimulators use short impulses – a few hundred microseconds.
When the nerve is damaged or lost completely, the muscle fibre has to respond directly to the electrical stimulus. This has two important consequences:
Chronaxie is the impulse duration just sufficient to make a muscle contract (measured using an impulse with twice the rheobase strength). In a healthy muscle-nerve system, chronaxie is very short: 0.1–0.7 milliseconds. If the nerve is damaged, chronaxie increases: to 3–20 ms in partial injury and to over 20 ms in complete denervation. This means a denervated muscle responds only to a longer impulse – a hundred or a thousand times longer than in a healthy muscle. By measuring chronaxie, a specialist can determine the extent of the injury and adjust the treatment parameters accordingly.1
Accommodation is a physiological protective mechanism: healthy nerve fibres “get used to” a slowly rising current and do not respond to it. This is possible because compensatory ion flows begin in the nerve fibres, preventing the stimulation threshold from being reached. The accommodation time of nerves is 20–30 ms, while that of muscle fibres is 100–300 ms.2 So, if we use an impulse that reaches its peak over 100–300 ms (a triangular waveform), healthy nerve fibres accommodate, but denervated muscle fibres – which have lost this ability – respond to it. This is how selective stimulation is achieved.
This may be the most important difference to understand. For a healthy muscle, the impulse duration is 100–500 microseconds (that is, 0.1–0.5 ms). For a denervated muscle, it is 100–900 milliseconds – a hundred to two thousand times longer! The physiotherapist sets the treatment duration for each person according to the severity of the condition: the more severe the injury, the longer the impulse needed. The need for long impulses also explains why denervated muscle treatment is generally slower and uses a lower frequency – usually 0.5–2 Hz – as each impulse needs enough time to discharge and for the pause to follow.3
The four impulse waveforms – when should each be used?
The most important and most frequently misunderstood question in selective stimulation current therapy is: which waveform should be used? The answer depends on how severely the nerve is damaged. Think of it like a dimmer switch: a completely extinguished light calls for a different approach from a flickering one, and a different one again from a light that is almost working normally.
| Waveform | Impulse duration | Rise profile | When is it used? | What does it tell us about the nerve? |
|---|---|---|---|---|
| Triangular | 300–900 ms | Slow rise, then slow fall (90%/10%) | Severe, complete or near-complete denervation, at the start of treatment | The nerve is completely or almost completely non-functional |
| Trapezoidal | 150–400 ms | Steeper rise, sustained peak, then fall | Moderate, partial nerve damage | The nerve is partly damaged and there is some response |
| Square (long) | 100–300 ms | Immediate rise and fall (instantaneous) | Milder denervation, when the condition is improving | The nerve is regenerating and chronaxie has decreased |
| Biphasic square | 100–500 µs (micro!) | Immediate, symmetrical biphasic | Muscles with healthy nerves (EMS, FES, stroke rehabilitation) | The nerve is intact – selective stimulation current is not needed |
Important difference: stroke and peripheral paralysis
In paralysis after a stroke, the motor nerve itself is intact – the problem lies in the brain, in the central nervous system connection. In this case, selective stimulation current is not needed. Standard biphasic square-wave EMS at a low frequency is used instead. Selective stimulation current is needed when the lower motor neuron (the nerve running from the spinal cord to the muscle) is damaged. A physiotherapist or doctor decides this based on a diagnosis.
The “curve” of recovery – how does treatment change over time?
Peripheral nerve injury follows a characteristic course of recovery, which the physiotherapist monitors during regular check-ups. In brief, it looks like this:
Triangular impulse (in severe denervation) → as the nerve regenerates → Trapezoidal impulse (in moderately severe cases) → as recovery continues → Long square impulse (in mild denervation) → once the nerve is intact → Biphasic square wave (standard EMS/FES)
This gradual transition is not automatic – the device does not “decide” on its own. The physiotherapist assesses the muscle response every six months and adjusts the programme based on the findings. The reason for the assessment is that the muscle’s response to an impulse shows how far recovery has progressed. If a particular waveform produces a strong contraction, nerve regeneration is progressing well; if the muscle barely responds, it may be worth continuing with that form of therapy or fine-tuning the parameters.4
What is the “ideal” triangular impulse for severe denervation?
For severe peripheral nerve damage, it is worth starting with a triangular impulse lasting 300–900 ms. If the device allows it, set the impulse rise/fall ratio to about 90%/10% – this means the current rises very slowly and falls more quickly. This configuration maximises the accommodation effect: healthy nerves “ignore” the signal, while denervated muscle fibres contract. According to a study at the Medical University of Vienna, for the tibialis anterior muscle, a 200 ms triangular impulse with the cathode placed proximally produced the best result.1
When may electrical stimulation help?
Selective stimulation current is primarily used as an adjunctive therapy for conditions involving peripheral nerve damage. Below, I review the most common situations.
The peroneal (fibular) nerve is the peripheral nerve most commonly injured in the leg. Damage can make it impossible to lift the foot and toes upwards (dorsiflexion): the person walks with so-called “foot drop” and may trip. Selective stimulation of the foot and the muscles at the front of the lower leg may help prevent atrophy and – if the nerve regenerates – support the return of function. The foot switch supplied with the PeroBravo can also be used to support gait retraining (FES gait assistance).4
In Bell’s palsy, the facial muscles on one side become denervated, which can cause facial asymmetry, difficulty closing the eye and problems with articulation. Selective stimulation of the small facial muscles requires very precise parameter settings – electrode size, polarity and impulse duration are critical. The aim of treatment is to slow atrophy of the facial muscles and help maintain their condition during recovery.
With a spinal disc herniation, extruded disc material presses on a nerve root. This can weaken or partially paralyse the muscle group supplied by that nerve segment. If the pressure noticeably reduces nerve function, selective stimulation may help maintain the condition of the affected muscle group before and after conservative treatment or surgery. Treatment for a herniated disc should always begin on the basis of a medical diagnosis and a physiotherapist’s recommendation.
Nerves can be mechanically injured during surgery, an accident or a fracture – temporarily or permanently, depending on the extent of the damage. In traumatic nerve injury (axonotmesis, neurotmesis), selective stimulation of the affected muscle group is one of the key rehabilitation tools for preventing atrophy. The sooner treatment begins, the more likely the muscle is to retain its functional mass until the nerve grows back.5
In injuries affecting the lower spinal cord and damaging the lower motor neuron (e.g. cauda equina), the muscles of the lower limbs become denervated. Standard FES (functional electrical stimulation) does not work in these cases because it relies on an intact nerve pathway. However, long-pulse-width stimulation (LPWS) – using impulses that may be 100–1,000 times longer – can produce muscle contraction. This may help prevent atrophy and pressure sores caused by prolonged sitting, and in some cases may also partly restore standing and walking.6
Treatment guide for peroneal nerve palsy →
Before you start treatment
Scientific background
Frequently asked questions
Summary – At a glance
