What are TENS units primarily used for?

TENS units, or Transcutaneous Electrical Nerve Stimulation units, are primarily designed to alleviate pain through a method that involves applying low-voltage electrical impulses to the skin. The key principle behind their operation is the transcutaneous electrical nerve stimulation, which helps to modulate and reduce pain signals sent to the brain. By stimulating specific nerves, TENS can interfere with the transmission of pain signals and promote the release of endorphins, the body's natural pain-relieving chemicals.

While the other options mention various electrical applications, they do not accurately describe the primary function of TENS units. For instance, generating high voltage electrical currents is not characteristic of TENS units, as they operate at low voltage. Similarly, testing electrical conductivity and initiating muscle contraction pertain to different applications and devices, such as electromyography units or muscle stimulators, rather than TENS therapy. Thus, the primary focus of TENS units is indeed on providing effective pain relief.