How to Compare TENS Units – A Review of Therapeutic Functions

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Deciding which TENS unit is the “right” choice for pain relief and/or muscle rehabilitation can be a daunting task. There are a multitude of options ranging from prescription-only units offering high-frequency signals of 5,000 Hz to over-the-counter units offering low-frequency signals of 50 Hz. Lacking a single guide to distinguish devices, it can be challenging to know when to pick one over another.

This blog series explores several ways to consider as a basis for comparing TENS units. In an earlier post, a review of the signal types – i.e., Interferential Therapy, Galvanic Stimulation, etc. – was used as a means to distinguish the beneficial value of one device over another. Another post reviews devices by price and performance.

In this post, our attention shifts to another method of evaluating units – therapeutic function. That is, which natural body processes can be triggered by the use of TENS. Having this information offers a way to begin discerning the best choice for the patient.  

TENS Units – One Name, Many Different Devices

Before diving into an analysis, it’s important to clear up a common misperception – that all TENS devices are the same. This is like saying one pill is just like any other. Categorically, these devices are used to treat chronic and acute pain, and, in some cases, muscle conditions.

The one attribute all transcutaneous devices do have in common is that they transmit electrical signals through the skin by way of electrode pads that adhere on top of the treatment area. The challenge – the confusion, really – is that the category itself and a specific device type within the category both get referred to as TENS.

It’s a bit like facial tissue being called Kleenex®. There is a specific brand named Kleenex within the facial tissue category, but many people have gotten used to calling all facial tissues “Kleenex.” In this article, any mention of TENS will be referring to the TENS category unless stated otherwise.

Therapeutic Functions Offered by TENS

The ability of any TENS unit to stimulate a natural process when the body fails to do so on its own is key to the effectiveness of electrotherapy. Knowing that it takes a specific signal to trigger a desired bodily response – for instance, causing a muscle to contract or preventing a nerve from transmitting pain signals – presents a guide to understanding the therapeutic value of each.

Pain Relief

The primary objective of electrotherapy treatment (also called e-stim) is often acute and/or chronic pain relief. TENS units achieve fast-acting pain relief by hyper-stimulating nerves near the injury site in a way that causes a slowdown or stoppage of pain signals traveling to the brain. This happens though a process called the Gate Control Theory of Pain.

To achieve this fast-acting pain relief over an extended period, devices with low-frequency signals (50 Hz or less) typically must be worn on a continual basis. By comparison, devices with high-frequency signals (4,000 to 5,000 Hz) that penetrate deeper into tissue need only be used for 30 minutes once or twice per day to provide relief that lasts for a period of several hours.

While it sounds incredible, science shows that this longer-lasting pain relief is likely achieved through the release of endogenous opioids, the body’s natural painkillers. Higher-frequency signals stimulate the brain in a particular way, causing it to emit these neuropeptides that inhibit the communication of pain signals. Patient feedback indicates that endogenous opioid-based pain relief may last anywhere from 4-8 hours post-treatment.

Muscle Conditioning

There is a wide range of muscle conditions that can be effectively treated with TENS. Generally speaking, treatments apply electrical stimulation in order to mimic ordinary healthy bodily function and return affected muscles to normal. Conditions and symptoms include:

  • Muscle Spasm
    Spasms may occur when muscle is overused, tired, previously injured or strained. They may also occur when muscle has been overstretched or held in the same position for an extended period of time. A spasm may involve part of a muscle, the whole muscle or even adjacent muscles. It may take the form of a cramp or simply present as pain.

  • Muscle Atrophy
    Muscle atrophy occurs when muscles waste away primarily due to the lack of physical activity. This can happen when a disease or injury makes it difficult or impossible to move an arm or leg. A symptom of atrophied muscles is an arm or leg that appears smaller than its opposite, for example, after being in a cast or brace for many weeks. Atrophy can also occur in larger muscle groups, including areas of the back, hips and upper legs. Its symptoms are typically muscle weakness and pain.

  • Poor Blood Circulation
    A wide range of symptoms can indicate poor circulation. They can affect a person's quality of life and may even cause severe complications. Common symptoms include numbness and tingling in the hands and feet, cold hands and feet, swelling in the feet, ankles, and legs, fatigue, joint and muscle cramps.

In each of the above cases, patients can achieve therapeutic benefit by causing muscle contractions within the affected area(s). A TENS unit that is “tuned” to stimulate muscle – often called NMES or a neuromuscular electronic stimulation device – is the ideal candidate for this purpose.

The low-frequency signals (<100 Hz) offered by these devices cause skeletal muscle to contract and release in a pattern that acts as a pump – moving metabolic waste out of the muscle and drawing in regenerative blood and lymphatic fluid. The end results of this contract-and-release pattern are preventing disuse atrophy, increasing circulation and re-educating tissue.

The most advanced NMES units offer a waveform that can affect both slow- and fast-twitch muscle. Recruiting fast-twitch muscle, commonly found in extremities like hands, arms and feet, can be accomplished by way of a quick, spiked waveform. However, these waveforms don’t stay on long enough in any cycle to trigger contractions in larger, slow-twitch muscles that provide stability and support. This requires another waveform – a wider waveform – to accomplish the goal. Combining both into the same program offers the best likelihood of results for the patient, where relief can be elicited from both muscle types within the same session.

Next Steps 

Knowing that all transcutaneous electrotherapy devices are not the same is the first step in understanding the TENS category. Taking a deeper dive into what makes them different reveals that there are varying therapeutic distinctions between devices, and these distinctions can be used to hone in on which device is the best for addressing a patient’s particular condition.

Some questions to consider when reviewing different devices:

  • What are the indications for use? If a device is indicated only for pain, it isn’t a good choice for muscle treatment. If muscle therapy is the goal, make sure it includes indications like relaxing muscle spasms, preventing disuse atrophy and re-educating muscle.

  • Does the device work on all muscle types? Recruiting both slow-twitch and fast-twitch muscle fibers is key to treating most muscle conditions. Be sure to review the waveforms before making a device selection. 

  • Is the patient able and willing to wear a device constantly to receive pain relief? If not, a low-frequency TENS device probably isn’t the best choice.

  • Does the device offer high-frequency signals of 4,000 to 5,000 Hz? If so, these signals are best for achieving an endogenous opioid release to relieve pain.


Do you have experience with TENS electrotherapy devices? If so, feel free to add your questions or comments.

Related Resources

Blog: Deciding Which Electrotherapy Device will be Most Effective for You
Blog: How to Compare TENS Units by Price


This blog provides general information and discussion about medicine, health and related subjects. The words and other content provided in this blog, and in any linked materials, are not intended and should not be construed as medical advice. If the reader or any other person has a medical concern, he or she should consult with an appropriately-licensed physician or other healthcare professional.