TENS Under the Hood: Stimulation Pulse
This is the second post in a series exploring how TENS (Transcutaneous Electrical Nerve Stimulation) devices work. Our intention is to go “under the hood” of these valuable pain relief devices to help you understand their operation. We will explore key technical specifications and operating principles. We hope this information is useful when deciding on a TENS device for your particular needs.
In this post we cover the stimulation pulse, which is central to how TENS devices activate nerves. Read on to learn what it is and why its characteristics are important to effective pain relief.
What is the stimulation pulse?
TENS provides pain relief by passing an electric current between electrodes placed on the skin. The current flows across the skin underneath the electrodes and through the body where it stimulates nearby nerves. This current is not applied continuously such as would be the case with a battery, but rather in discrete packets called stimulation pulses. The reason is that nerves transmit information such as the sensation of touch or pain using impulses called action potentials. In this respect, nerves are like communication lines in computers. However, unlike computers that transmit millions or billions of impulses per second, human nerves communicate with only a few hundred impulses per second.
Stimulation pulse characteristics
A stimulation pulse has several key characteristics. To help explain, we will refer to the figure below that schematically represents the Quell® stimulation pulse.
The pulse is comprised of two phases, a positive phase and a negative phase, with a short gap between them. Charge in the positive phase activates nerves underneath one of the electrodes and charge in the negative phase activates nerves underneath the other electrode.
An essential feature of the stimulation pulse is the height of each phase, called amplitude. It is measured in milliamps, which are one-thousandth of an amp (a unit of electric current). In typical use, the amplitude is between 5 and 100 milliamps. As a point of reference, this is far less current than common appliances such as light bulbs or computers require.
Another important feature of the stimulation pulse is the duration of the phases, which is usually between 100 and 200 microseconds (millionth of a second, abbreviated µs). The intensity of nerve stimulation is a function of charge, which is equal to the amplitude multiplied by the duration (i.e., shaded area in figure). The greater the charge the stronger the nerve stimulation. However, the form in which the charge is delivered matters. A large amplitude and short duration (i.e., narrow and tall) is more effective than a low amplitude and long duration (i.e., wide and short). The reason is that short pulses pass across the skin more easily and more effectively stimulate nerves. The technical challenge for TENS devices is that short pulses require greater amplitude and most OTC TENS devices are limited to about 50 milliamps. Quell generates up to 100 milliamps and therefore uses efficient short pulses.
As a final point, the gap between the phases is important because, in its absence, the two phases interfere with one another to reduce the effectiveness of nerve stimulation. Quell is one of a few TENS devices with this feature.
If you would like to read more posts in the TENS Under the Hood series, please click here.
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