Interferential Therapy
Interferential Therapy (commonly referred to as IFC) has been used for decades in the physical therapy setting.
Although similar to TENS and NMES stimulation, Interferential outputs are at much higher frequencies (4000Hz+) and allow for better tissue penetration as well as increased comfort, even at quite high current settings.
It is common to use various "frequency scanning" protocols in order to treat difficult pain and other rehabilitation problems: ligament strains, many neuralgias, intractable chronic pain syndromes, muscle strengthening, etc.