Interferential currents are medium-frequency alternating currents that interact at the point where they cross; they are obtained by using two medium-frequency alternating current generators connected to two pairs of electrodes arranged orthogonally so that the lines of force of the two electric fields overlap in the desired area.

Because the two currents have different frequencies, at some times the two positive and negative half-waves add together, resulting in a half-wave of greater amplitude, while at other times they cancel out.

This creates a new alternating current whose frequency 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 as skin impedance decreases with increasing 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 present a higher intensity than that existing at skin level. Neither current, in itself, presents such intensity as to excite nerve fibers and, since the threshold of sensitivity is not reached, they are imperceptible to the patient. The subliminal intensity at the surface, the decrease in skin impedance, and the absence of electrolytic effects combine to make these currents better tolerated than others.

The excitomotor action, given the very short duration of the pulses, occurs only in normally innervated muscles, so these currents find use in traumatology for the purpose of maintaining muscle trophism and reducing osteoporosis.