The Elements of NMES
The precise manner for applying Neuromuscular Electrical Stimulation (NMES) differs per stimulator. However, every stimulation works with the following principles: frequency, amplitude, and pulse width. If you’re first wondering what electrical stimulation actually is and why it’s great, hit the link. Furtherly, if you’re thinking of applying electrical stimulation, always follow the instructions in the manual that belongs to your stimulator. Also be sure to check out the contraindications for applying electrical stimulation before doing anything.
The frequency is the number of pulses applied per minute. When the frequency is increased, the intensity of the muscle tension and the produced EMG-signal also increase. The applicable frequency range differs per stimulator. In some, only a standard unchangeable frequency is possible, other stimulators offer the possibility to set the frequency within a certain range. For example between 0 and 70 Hz (Hz=Hertz; number of pulses per second). The frequency that is used for smooth muscles tensions is usually about 20 to 50 Hz. A frequency that is too low makes a movement jerky and a frequency that’s too high makes the stimulation painful.
The Pulse Width
Pulse width stands for the time to which every group of stimulation pulses is set. This is usually set in microseconds (µs), but sometimes in milliseconds (ms). In most stimulators, the pulse width is about 200-300 µs. A short pulse width may feel better, but a longer pulse width has the advantage that more motoneurons will be innervated, leading to an increased muscle tension.
In neuromuscular electrical stimulation, the Amplitude represents the electric current of the stimulation. This is measured in Milli Ampere (mA). The amplitude determines how deep the stimulation will go and therefore how many motoneurons are stimulated, and thus how strong the muscle tension will become. The frequency, pulse width, and amplitude form the intensity/powerfulness of the stimulation together.
It is thus possible to make some adjustments in these settings, while still maintaining an intensity that is roughly similar. For example by increasing the pulse width while decreasing the amplitude. You can make tehse adjustments to make sure that the approximate same effect is reached, while the person undergoing it feels better.
What settings should be used for neuromuscular electrical stimulation?
You can most probably also find the applicable settings in the user manual of your stimulator. The to-be-used settings depend on your goal with neuromuscular electrical stimulation. The following applies to generating a muscle contraction that leads to movement.
From personal experience, it is usually best to first set the frequency (between 20 and 50 Hz) and the pulse width (200 to 300 µs) and consequently increase the amplitude. The needed amplitude for good stimulation may strongly differ per person. Start low and slowly increase this: for small muscles, start at e.g. 5mA and keep on increasing this with a couple mA every time. For bigger muscles, e.g. the leg muscles that make the knees extend, you can start higher, for example at 10 mA.
Only increase the intensity as long as it feels good and don’t go too far. If a strong contraction is reached, it is often not needed to increase the intensity much more. Also, When you apply electrical stimulation, always make sure that the joint(s) that take(s) part in the movement cannot overstretch. For example by fixing them or by restricting the movement range. Furthermore, make sure the one who applies ES is alert and able to turn off the stimulation at any moment.
What settings should be used for to fight pain?
Different settings should be used when you want to fight pain. In order to fight pain, electrical stimulation can either be applied on the spinal cord or the muscles. When applied to the spinal cord, it’s usually called epidural stimulation. When electrical stimulation is applied to the muscles or nerves to fight pain, it is called Transcutaneous Electrical Nerve Stimulation (TENS). Clicking the links can offer you more information about this.
Factors in NMES that influence your needed intensity
In some cases, it may be more useful to apply a different frequency, pulse width, or amplitude. A lower frequency of 1 to 10 Hz, for example, might work better when a muscle gets exhausted too fast. This is mainly useful when it is not the goal to make fluent movements, but to just train the muscle. After training with low frequencies, a muscle may become more fatigue-resistant, allowing you to apply higher frequencies and perform more functional movements.
With a lower pulse width, between 50 µs and 200 µs, the stimulation will not go very deep and the stimulation might feel more comfortable. This could be useful when small muscles nearby the skin need to be stimulated. On the other hand, a higher pulse width (350-500 µs) could be useful in bigger muscles. Changing the pulse width causes the intensity of the movement to change. This difference can be compensated to some extent by altering the amplitude.
Size of the electrodes for neuromuscular electrical stimulation
The following applies to electrical stimulation on the skin, also referred to as Transcutaneous Electrical Neuro Stimulation (TENS). ES can also be applied internally, information about this will be added later.
The electrodes you need depend on the size of the muscle you are going to stimulate. When you want to stimulate a muscle that is small, e.g. one of the arm/hand muscles, you will also need small electrodes in order to not also stimulate the nearby muscles. For bigger muscles on the other hand, it is very useful to use bigger electrodes. On the one hand, a big part of the muscle can be stimulated, involving as much muscle fibres as possible in the stimulation.
Furthermore, the applied electricity can be spread better in a bigger electrode. This leads to a more comfortable feeling when bigger electrodes are used compared to small electrodes, while the intensity is the same. This is because the electricity is spread over the surface. That’s also why a stimulation can become more painful when an electrode partly comes off. The same amount of electricity has to go through the electrodes, and increased electricity thus goes through every stimulated piece of skin.
How can electrodes best be attached, used, and maintained?
In TENS, you can use self-adhesive or rubber electrodes. Non-adhesive electrodes can be attached to the skin by making use of a tight flexible belt or tape and possibly lubricating the electrodes with contact gel. For hygienic reasons, electrodes are usually used for one patient. Self-adhesive electrodes may lose their adhesive power pretty quickly. For all electrodes, it is important you wash these with water and place them back on the foil they belong to.
Position of the electrodes
In TENS, the precise position of the electrodes is of great importance, especially in the smaller muscles. When the muscle belly is not or not entirely stimulated, a greater intensity is needed in order to reach the same effect, if this effect can even still be reached. Therefore, you should always make sure that the electrodes are placed correctly by trying to stimulate as much of the muscle belly as possible without stimulating on the tendon.
You can often find suggestions for electrode placement in the instruction manuals of stimulators. I am also planning to add information about electrode placement per mucle to the website at a later point. The precise perfect stimulation location may differ per person, as muscles and nerves can run a bit differently through the body among people, but also because of damaged nerve paths or other conditions related to diseases.
Other important things concerning Neuromuscular Electrical Stimulation (NMES)
You should keep in mind that suggestions for electrode placement, for example in the manual of your stimulator, are often based on the anatomic position, unless specified differently. The anatomic position is the position in which the person is standing upright in neutral position, with the palms of the hand to the front. This is one important thing to keep in mind for the attachment process. Another thing is to make sure that the conduction between the electrode and the skin is good. You should make sure that there are no substances on the skin. Excessive hair may counteract ES, but in many cases it is not necessary to remove hair. When you use ES a lot, however, it is often worth shaving.
Muscles that do not respond to electrical stimulation
It may occur that electrical stimulation has no effect on some muscles. This could be due to the decay of nerves. When this is the case, it is often still possible to innervate these muscles, but only by directly applying ES to the muscle or by using triangular pulses. More information about this will be added later.
Neuromuscular electrical stimulation can also be triggered by brain signals. In this case, it is called a Brain machine interface. Hit the link to read more about this exciting technology.
If you have any questions, remarks, tips, or other words about neuromuscular electrical stimulation left, feel free to leave a comment!