A Transcutaneous Electrical Nerve Stimulation (TENS) unit is a non-invasive, battery-operated device designed for pain management. It delivers low-voltage electrical currents through electrodes placed on the skin, stimulating the nerves to provide temporary pain relief. A TENS unit can be used effectively for various types of knee pain, including discomfort from arthritis, sports injuries, or general joint strain. This drug-free method is a popular choice for individuals seeking an alternative or complementary therapy.
How TENS Therapy Relieves Pain
The pain-relieving effects of TENS therapy operate primarily through two neurophysiological mechanisms. The most immediate relief is explained by the Gate Control Theory of pain perception. This theory suggests the body possesses a neural “gate” in the spinal cord that controls which pain signals reach the brain. The TENS unit’s electrical impulses activate large, non-pain-carrying nerve fibers. This rapid input effectively “closes the gate,” blocking or reducing the transmission of slower-moving pain signals, thereby diminishing the perception of pain.
TENS also stimulates the release of the body’s natural pain-relieving chemicals. These endogenous opioids, such as endorphins, are released into the bloodstream and bind to opioid receptors. This provides a secondary form of pain relief that can last longer after the TENS unit is turned off. While the Gate Control mechanism provides fast-acting relief during treatment, the release of endorphins contributes to a more sustained analgesic effect, especially with lower-frequency settings.
Essential Steps for Electrode Placement on the Knee
Proper electrode placement is essential for maximizing the effectiveness of TENS therapy on the knee. The general principle is to position the pads so the electrical current “brackets” or surrounds the area of greatest pain. For generalized pain, a common setup involves placing two pads on the inner (medial) side and two pads on the outer (lateral) side of the knee, creating a box around the joint. This four-pad setup ensures broad coverage.
If the pain is focused below the kneecap, such as with patellofemoral pain syndrome, a different approach is used. One pad should be placed on the lower thigh, above the kneecap, with the second pad placed below the kneecap on the upper shin. Avoid placing the pads directly on the kneecap (patella) bone, as this can be uncomfortable and reduce the therapeutic effect. Never place electrodes directly into the hollow behind the knee.
Before applying the electrodes, the skin must be clean and dry to ensure optimal conductivity and adhesion. Lotions, oils, or moisture can interfere with the electrical current and may cause skin irritation. The pads should be placed at least one inch apart, preventing the current from jumping between them and causing an unpleasant sensation.
Understanding TENS Unit Settings and Session Length
TENS units offer adjustable parameters for customization based on the type of pain experienced. The primary adjustable settings are the Pulse Rate (Frequency, measured in Hertz or Hz) and the Pulse Width (measured in microseconds or µs). High-frequency TENS (80–120 Hz), often called “Conventional TENS,” is used for acute pain because it activates the Gate Control mechanism for immediate relief.
Conversely, low-frequency TENS (2–10 Hz), sometimes called “Acupuncture-like TENS,” is better suited for chronic pain. This lower setting stimulates the release of endorphins, leading to a slower onset of pain relief that lasts longer after the session ends. The Pulse Width determines the duration of each electrical pulse, typically ranging from 50 to 250 µs.
The intensity should be adjusted to produce a strong, yet comfortable, tingling sensation without causing muscle contraction. Users may need to slightly increase the intensity as their body adapts to maintain the sensation. A typical TENS session lasts between 30 and 60 minutes and can be repeated multiple times per day as needed.
Important Safety Warnings and When to Avoid TENS
While TENS therapy is safe for most users, strict contraindications must be observed. Individuals with a cardiac pacemaker, an implanted defibrillator, or any other implanted electronic or metallic device should not use a TENS unit. The electrical current could interfere with the function of these devices, creating a health hazard.
Never place electrodes on the front of the neck, especially over the carotid sinus, as this could cause spasms or affect blood pressure. Avoid placing pads over the eyes, on the head, or anywhere the current could flow across the brain. TENS should not be used over areas of broken, irritated, or numb skin, since a lack of normal sensation makes it impossible to safely judge the intensity.
Pregnant women should avoid using TENS, particularly over the abdominal and pelvic regions, as its safety during pregnancy has not been established. People with a history of epilepsy should use the device only with medical supervision. If experiencing chronic or severe pain, consult with a physician or physical therapist to ensure TENS is an appropriate part of a comprehensive pain management plan.