Medium-frequency stimulation
Interferential therapy employs medium frequency currents used in 2 or 4-
pole configurations to produce a low frequency stimulation effect.
Prior to the introduction of interferential therapy in the mid 1950s, low
frequency stimulation was used for pain relief, muscle re-education etc.
These currents, however, have the disadvantage that normal human skin
has a relatively high impedance at such frequencies. In order to overcome
the skin impedance a larger voltage has to be used to achieve the desired
current, resulting in a more uncomfortable treatment for the patient. In
addition, the penetration depth of these currents is poor and in part is limited
by the discomfort to the patient.
Interferential therapy overcomes the problem of skin impedance. At 50 Hz
(faradic current) the impedance for a 100 cm
2
of skin is approximately 3000
ohms. At 4000 Hz (medium frequency) the skin impedance of the same area
is around 50 ohms. This means that a much lower voltage signal can be used
to produce the desired current, resulting in less skin sensation and a more
comfortable treatment. This medium frequency is, however, well outside of
the normal biological frequency range (0.1 to 250 Hz). In order to produce
the required stimulation, two medium frequencies are used. A constant
frequency of, say, 4000 Hz is applied to one pair of electrodes and a slightly
different frequency of say 3900 Hz is applied to the other pair. These two
frequencies 'interfere' to produce an amplitude modulated medium frequency
(beat frequency) in the tissue. The tissue responds to the cyclic rise and fall
in the current intensity. It is the amplitude modulation frequency (AMF) that
is within the normal biological frequency range and not the medium
frequency (carrier).
Russian stimulation was developed by Dr Y Kotz, and uses 2.5 kHz
sinewaves pulse at a low frequency, typically 30 to 80 Hz, to produce
comfortable muscle contraction. It is similar to a surged, Interferential 2-pole
waveform.
Transcutaneous electrical nerve stimulation (TENS) refers to the application
of low-intensity, short-duration pulses for the purpose of relieving pain. The
Combination 860 provides up to two channels of asymmetric, symmetric or
sequential output with a wide range of pulse widths and repetition rates.
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