Interferential therapy with medium-frequency currents.
Interferential therapy uses medium-frequency interference currents. These are applied in a 2- or 4-pole configuration to produce a low-frequency stimulation effect.
Interferential therapy was introduced in the mid-1950s; prior to that, low-frequency stimulation was used for pain relief, muscle rehabilitation after sports injuries, etc. However, low-frequency currents have the disadvantage that normal human skin has a relatively high impedance at these frequencies.
Therefore, significantly higher voltage must be used in low-frequency electrical stimulation to achieve the desired current intensity. Only then can skin impedance be overcome.
However, this can result in an uncomfortable treatment for the patient. Furthermore, the penetration depth of low-frequency currents is very shallow and is further limited by the discomfort many patients feel with this form of therapy.
The problem with skin impedance does not occur with interferential therapy. At 50 Hz (faradic current), the impedance of a 100 cm2 skin area is approximately 3000 ohms. At 4000 Hz (medium-frequency current), the skin impedance for the same area is only about 50 ohms.
As a result, a much lower voltage signal is required to generate the necessary current intensity. This leads to significantly less skin irritation and a much more comfortable EMS session. However, this medium frequency is far outside the normal biological frequency range (0.1 to 250 Hz).
To nevertheless produce the desired electrical stimulation, two medium frequencies are used. A constant frequency of, for example, 4000 Hz is applied to one pair of electrodes. The other pair of electrodes, however, is controlled with a slightly different frequency, e.g., 3900 Hz.
These two frequencies now "interfere" with one another to produce an amplitude-modulated medium frequency (beat frequency) in the tissue. The tissue reacts to the cyclical rise and fall of the current intensity. It is the amplitude modulation frequency (AMF) that lies within the normal biological frequency range, not the medium frequency (carrier frequency).
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