The treatment of a denervated muscle is not EMS — at least not the usual biphasic square-wave EMS used for muscle training or post-stroke rehabilitation. A denervated muscle requires selective stimulation current: a more slowly rising, longer-duration pulse that can directly trigger contraction of the muscle fiber without the nerve.
Why doesn’t it respond to the usual pulse? – The secret of accommodation
In a healthy muscle the nerve is the connecting link between the brain and the muscle fiber. A healthy nerve fires extremely quickly: a very short, microsecond (millionth-of-a-second) pulse is sufficient to produce movement. That is why general-purpose stimulators use short pulses — a few hundred microseconds.
When the nerve is injured or lost altogether, the muscle fiber must respond directly to the electrical stimulus. This has two important consequences:
Chronaxie is the pulse duration that is just sufficient to produce muscle contraction (measured at twice the rheobase intensity). In a healthy neuromuscular system chronaxie is very short: 0.1–0.7 milliseconds. If the nerve is damaged, chronaxie increases: with partial injury it is 3–20 ms, and in complete denervation it rises to above 20 ms. This means a denervated muscle responds only to much longer pulses — hundreds to thousands of times longer than a healthy muscle. A clinician can determine the severity of injury by measuring chronaxie and tailor the treatment parameters accordingly.1
Accommodation is a physiological protective mechanism: healthy nerve fibers "get used to" slowly rising current and do not respond. This is possible because compensatory ion flows in the nerves prevent the threshold from being crossed. Nerve accommodation time is about 20–30 ms, whereas muscle fibers have accommodation times of 100–300 ms.2 Therefore, if we use a pulse that reaches its peak over 100–300 ms (triangle-shaped), healthy nerve fibers accommodate, but denervated muscle fibers — which have lost that ability — will respond. This achieves selective stimulation.
This is perhaps the most important difference you need to know. For treating a healthy muscle the pulse duration is 100–500 microseconds (i.e. 0.1–0.5 ms). For a denervated muscle the duration is 100–900 milliseconds — hundreds to thousands of times longer. The physiotherapist sets the treatment duration individually according to severity: the more severe the injury, the longer the pulse required. The need for long pulses also explains why denervated treatment is generally slower and at lower frequencies — usually 0.5–2 Hz — because each pulse requires sufficient time for discharge and rest.3
The four pulse shapes – when to use which?
The most important and most often misunderstood question in selective stimulation therapy is: which waveform should be used? The answer depends on how severely the nerve is damaged. Imagine it like a light switch scale: the approach differs for a completely out lamp, a flickering one, and one that almost lights normally.
| Waveform | Pulse duration | Rise characteristic | When to use? | What it indicates about nerve state? |
|---|---|---|---|---|
| Triangle | 300–900 ms | Slowly rising, then slowly falling (90%/10%) | Severe, complete or near-complete denervation, at the start of treatment | The nerve is completely or almost completely lost |
| Trapezoid | 150–400 ms | Steeper rise, maintained plateau, then fall | Moderate, partial nerve damage | The nerve is partially damaged, some response is present |
| Square (long) | 100–300 ms | Immediate rise and fall (instantaneous) | Milder denervation, improving condition | The nerve is regenerating, chronaxie decreasing |
| Biphasic square | 100–500 µs (micro!) | Immediate, symmetric two-phase | Muscles with intact nerves (EMS, FES, stroke rehab) | The nerve is present – selective stimulation not required |
Important distinction: stroke vs. peripheral paralysis
In post-stroke paralysis the motor nerve itself is intact — the issue lies in the brain, at the level of central nervous system connections. In that case selective stimulation current is not needed; instead the usual biphasic square-wave EMS at low frequency is appropriate. Selective stimulation current is required where the lower motor neuron (the nerve running from the spinal cord to the muscle) is damaged. This should be decided by a physiotherapist or physician based on diagnosis.
The regeneration "nice curve" – how treatment changes over time
Peripheral nerve injury treatment follows a characteristic progression that the physiotherapist monitors during regular checks. The path can be summarized as follows:
Triangle pulse (in severe denervation) → as the nerve regenerates → Trapezoid pulse (moderate damage) → improving further → Long square pulse (mild denervation) → when the nerve is intact → Biphasic square wave (normal EMS/FES)
This gradual transition is not automatic — the device does not "decide" on its own. The physiotherapist assesses the muscle response every few months and adjusts the program accordingly. The reason for assessment: the nature of the muscle’s response to a pulse reveals the stage of regeneration. If a given waveform produces a strong contraction, nerve regeneration is progressing; if the muscle barely responds, it is advisable to continue the current therapy or fine-tune parameters.4
What is the "ideal" triangle pulse for severe denervation?
For severe peripheral nerve damage it is reasonable to start with a 300–900 ms triangle pulse. If the device allows it, set the rise/fall ratio to approximately 90%/10% — meaning the current rises very slowly and falls faster. This configuration maximizes the accommodation effect: healthy nerves "ignore" the signal while denervated muscle fibers contract. According to a Vienna medical university study, for the tibialis anterior muscle a 200 ms triangle pulse with the cathode placed proximally produced the best result.1
In which conditions can electrical stimulation help?
Selective stimulation current therapy is primarily used as an adjunctive treatment for conditions involving peripheral nerve damage. Below are the most common situations.
The peroneal (fibular) nerve is the most commonly injured peripheral nerve in the leg. When damaged, dorsiflexion of the foot and toes becomes impossible: the patient walks with a so-called "dropping foot" and trips. Selective stimulation of the foot and anterior lower-leg muscles can help prevent wasting and — if the nerve regenerates — facilitate functional recovery. With the foot switch supplied with the PeroBravo device, treatment can also be used to relearn gait (FES gait assistance).4
In Bell’s palsy the mimetic muscles on one side of the face become denervated, causing facial droop, difficulty closing the eye, and articulation problems. Selective stimulation of the small facial muscles requires very fine parameter adjustments — electrode size, polarity and pulse duration are critical. The treatment aims to slow muscle wasting and preserve muscle condition during nerve recovery.
With a herniated disc the protruding material can press on a nerve root, causing weakness or paresis in the muscle group supplied by that segment. If the compression noticeably impairs nerve function, selective stimulation can help preserve the affected muscle group's condition before and after conservative or surgical treatment. In herniated disc cases treatment should always be initiated based on medical diagnosis and physiotherapist recommendation.
Nerves can be mechanically damaged during surgery, accidents or fractures — temporarily or permanently depending on severity. In traumatic nerve injuries (axonotmesis, neurotmesis), selective stimulation of the affected muscle group is one of the most important rehabilitation tools to prevent wasting. The earlier treatment begins, the greater the chance the muscle will retain functional mass until nerve regrowth occurs.5
In injuries affecting the lower spinal cord that damage lower motor neurons (e.g. cauda equina), the muscles of the lower limbs become denervated. In these cases standard FES (functional electrical stimulation) does not work because it assumes intact nerve pathways. With long pulse width stimulation (LPWS) — using pulse durations up to 100–1000 times longer — it is possible to elicit muscle contractions, thereby preventing wasting, pressure ulcers from sitting, and in some cases partially restoring standing and walking.6
Peroneal palsy treatment guide →
Before you start treatment
Scientific background
Frequently asked questions
Summary – Quick overview
