TENS units are not designed to build muscle and won’t produce meaningful muscle growth. TENS (transcutaneous electrical nerve stimulation) targets your nerves to manage pain, not your muscles. The device you’re probably thinking of, one that actually causes muscles to contract, is an EMS (electrical muscle stimulation) unit. The two are often confused because they look nearly identical and both use electrode pads on the skin, but they work in fundamentally different ways.
How TENS Units Actually Work
A TENS unit sends small electrical impulses through your skin to activate large-diameter nerve fibers. These signals travel to your central nervous system and essentially interfere with pain messages before they reach your brain. Think of it like static on a phone line: the electrical input makes it harder for pain signals to get through clearly. This is based on what’s known as the gate control theory of pain.
At the frequencies and intensities used in standard TENS therapy, the current stays shallow. It reaches sensory nerves in and just below the skin, activating receptors that detect pressure and touch rather than driving deep into muscle tissue. There is one exception: when TENS is turned up well above the motor threshold (roughly 25% or more past the point where muscles start twitching), it can increase local blood flow and cause some involuntary muscle contractions. But that’s not how TENS is typically used, and those contractions aren’t strong enough or sustained enough to trigger muscle growth.
EMS Is the Muscle-Building Device
EMS (sometimes called NMES, for neuromuscular electrical muscle stimulation) sends impulses directly into muscle tissue at intensities above your motor threshold. This forces the muscle to contract in a way that mimics what happens during exercise. The stimulation typically runs at around 35 Hz with a wider pulse width than TENS, which is enough to recruit both slow-twitch and fast-twitch muscle fibers.
Below the motor threshold, electrical stimulation only activates skin receptors and sensory nerve fibers. Above it, the current also recruits proprioceptive receptors inside the muscle, including muscle spindles and Golgi tendon organs. That deeper recruitment is what creates the forceful contractions needed to stress muscle fibers and potentially stimulate growth.
What the Evidence Says About EMS and Muscle
Research published in the Journal of Cachexia, Sarcopenia and Muscle found that EMS can increase muscle mass by about 1% and improve muscle function by 10 to 15% after five to six weeks of regular treatment. Those numbers are modest compared to traditional resistance training, but they’re meaningful for people who can’t exercise normally due to injury, surgery, or chronic illness. EMS has also been shown to counteract the fast-twitch muscle fiber loss that comes with aging and prolonged inactivity.
The most effective approach appears to be combining EMS with voluntary exercise. A 1998 study on female track and field athletes found that pairing dynamic contractions with EMS produced greater strength gains than either EMS alone or weight training alone. However, for people who already have significant muscle loss from conditions like heart failure, adding EMS to an existing exercise program didn’t provide additional benefit in a large randomized trial of 91 patients.
The FDA has cleared EMS devices for prescription use in muscle re-education, preventing muscle atrophy, relaxation of muscle spasms, and increasing range of motion. Only one consumer device, the Slendertone Flex, has been cleared specifically for toning and strengthening abdominal muscles. No EMS device has been cleared for weight loss, reducing body measurements, or producing “rock hard” abs.
Can TENS Help With Recovery?
While TENS won’t build muscle directly, some athletes use it after workouts hoping to speed recovery. The theory is that increased blood flow from electrical stimulation helps flush metabolic waste products from fatigued muscles. In practice, the evidence is thin. Reviews of the research have found limited support for TENS improving physiological recovery markers or meaningfully reducing delayed-onset muscle soreness after exercise. It may take the edge off post-workout discomfort through its pain-blocking effect, but that’s a perception change rather than a physical recovery boost.
Safety Considerations for Both Devices
TENS and EMS share several important restrictions. Neither should be used by people with pacemakers, implanted defibrillators, or other electronic medical devices, since the electrical current can cause malfunctions. Both are off-limits over the chest in people with heart disease or arrhythmias, over the front of the neck, near the eyes, and over areas of known or suspected cancer.
EMS carries additional restrictions because it produces stronger contractions. It should not be used over areas with impaired circulation, near recent fractures or surgical sites, over osteoporotic bone, or on the chest and lower abdomen. Pregnant women should avoid EMS entirely. TENS is also contraindicated in areas where you have reduced sensation, since you can’t accurately gauge whether the intensity is too high.
Which Device You Actually Need
If your goal is building or maintaining muscle, a TENS unit is the wrong tool. You need an EMS device, ideally one with adjustable frequency and intensity settings that let you reach above your motor threshold. Many consumer units sold online are combination TENS/EMS devices with separate modes for each function, so check the product specifications carefully before buying.
For healthy people looking to add muscle size and strength, EMS works best as a supplement to regular resistance training, not a replacement. The 1% mass gain and 10 to 15% function improvement seen in studies are a fraction of what consistent weight training produces. Where EMS really shines is in rehabilitation: maintaining muscle during periods when you physically can’t train, recovering from surgery, or counteracting age-related muscle loss when traditional exercise is limited.