This therapy takes its name from the abbreviation of the English terms Transcutaneous Electric Nervous Stimulation. This is because the method is based on the application, by means of special electrodes, of appropriate currents whose micropulses excite only the nerve fibers of tactile sensitivity located under the skin.

The nerve influences thus produced on the sensory nerves go up to the spinal cord, blocking at this level “the gateway to pain” (gate control). In practice, electrodes are placed at the level of the painful region and cover as much of the skin above the affected area as possible. Electrical intensity is adjusted to produce a distinct tingling sensation below the electrodes.

In this case there is not and should not be any motor stimulation of the muscle: if this happens (movement of a limb or shoulder) it is a case of misapplication and the current should be reduced until the muscle is in total stasis again. Rather, during the course of the session, habituation to the stimulus may occur, reducing the tingling sensation: in that case the current should be increased to reactivate the stimulus itself.

Interferential currents are alternating currents of medium frequency that interact at the point where they cross; they are obtained by using two alternating current generators of medium frequency, connected to two pairs of electrodes arranged orthogonally, so that the lines of force of the two electric fields overlap in the desired area. Since the two currents have different frequencies, it happens that at certain times the two positive and negative half-waves add together, giving rise to a half-wave of greater amplitude, while at other times they cancel out.

A new alternating current is thus created, the frequency of which is related to the phase shift of the two applied currents. Such a new current is an endogenous low-frequency current.

The reason for seeking the formation of a low-frequency current instead of applying it from the outside, as would seem simpler, is easily guessed by recalling the different bioelectric properties of low-frequency and medium-frequency currents. In fact, medium-frequency currents encounter less resistance because skin impedance decreases with frequency. They can therefore penetrate more easily without yielding energy to the skin and without causing uncomfortable sensations.

Electrophysiological effects

The electrophysiological effect varies with the frequency used, although there is no clearly differentiated action, since, depending on the type of current, both excitomotor and antalgic effects are found simultaneously, to a lesser or greater degree, in each treatment. In general, frequencies of 50-100 Hz have predominantly an antalgic effect, while frequencies below 50 Hz have an excitomotor effect that becomes increasingly intense below 25 Hz. Medium-frequency interferential currents are thus able to penetrate deeply where they have a higher intensity than that existing at skin level. Neither current, in itself, presents an intensity sufficient to excite nerve fibers and, since the threshold of sensitivity is not reached, is undetectable by the patient. The subliminal intensity at the surface, the decrease in skin impedance, and the absence of the electrolytic effects all combine to make these currents better tolerated than others. The excitomotor action, considering the very short duration of the pulses, occurs only in normally innervated muscles for which these currents find use in traumatology for the purpose of maintaining muscle trophism and reducing osteoporosis.