Nilima,
There are many misconceptions about interferential current (IC) which have
been perpetuated by the manufacturers of this equipment over many years.
The traditional theory that we have all been taught is that when we apply
two slightly different medium frequency currents to the body, these somehow
miraculously mix in the tissues and produce a new amplitude-modulated
current that rhythmically increases and decreases in amplitude at a fixed
and predictable frequency (the 'amplitude modulated frequency'). The actual
evidence for this, however, has never been produced. Even in a homogenous
water medium, the AMF is set up not just where the two currents mix, but
also directly under the electrodes (Treffne 1983). Another study presented
at the 1995 WCPT conference (Demmink 1995) found that, in tissue, the
pattern of modulation was completely haphazard and unpredictable. The AMF,
therefore, is unlikely to produce a predictable pattern within the tissues.
Another misconception is that medium frequency current is incapable of
stimulating excitable tissues but requires to be amplitude modulated at low
frequency. At a frequency of 4,000Hz, each pulse of an evenly alternating
current will have a phase duration of 125microsec and at 2,000Hz it will be
250microsec. These are well within the range capable of stimulating nerve
and muscle. We have carried out work comparing pure 4,000Hz current (0Hz
AMF) with a range of other AMFs and found that there was no clear difference
in the effects that these settings had on excitable tissues (Palmer et al
1999). The medium frequency (4,000Hz in this instance) was therefore the
dominant stimulating parameter and not the AMF (contrary to traditional
theory). In fact no IC setting was shown to be superior to the stimulation
possible with TENS (a much cheaper alternative of course).
I agree that different AMFs do feel differently, and may also produce
different patterns of muscle activation (tetanic responses at high frequency
and twitch responses at low frequency). Amplitude modulation, however, is
simply akin to turning the medium frequency current on and off very quickly
(even though the pattern may be somewhat unpredictable). At frequencies
above 40-50Hz, the current intensity becomes more constant and the body is
no longer able to distinguish between separate 'pockets' of current. In
terms of comfort, most individuals prefer higher AMFs (50-100Hz), but
justification for different AMFs stimulating different tissues (as
traditional theory would suggest) is difficult to sustain.
Our work, and that of others, would suggest that IC is an inefficient method
of stimulating tissues. TENS can probably achieve the same results more
efficiently and at a fraction of the cost. There is more research being
published in this area now, so views on the distinctive nature of IC as a
treatment modality will hopefully change over time. Some references that you
may want to chase up regarding the mechanisms of action of these currents
(some in abstract form) include:
ALON G. (1987) Interferential current news. Physical Therapy 67(2),
pp.280-281.
DEMMINK J.H. (1995) The effect of a biological conducting medium on the
pattern of modulation and distribution in a two-circuit static
interferential field. In: Proceedings of the 12th international conference
of the world confederation for physical therapy, Washington D.C., pp.583.
JOHNSON M.I. (1999) The mystique of interferential currents when used to
manage pain. Physiotherapy 85(6), pp.294-297.
KANTOR G., ALON G., HO H.S. (1994) The effects of selected stimulus
waveforms on pulse and phase characteristics at sensory and motor
thresholds. Physical Therapy 74(10), pp.951-962.
MARTIN D.J., PALMER S.T. (1996) The effect of beat frequency on perceived
comfort during stimulation of healthy subjects with interferential current.
Physiotherapy 82(11), pp.639.
PALMER S., MARTIN D., STEEDMAN W., RAVEY J. (1999) Interferential current
and transcutaneous electrical nerve stimulation frequency: effects on nerve
excitation. Archives of Physical Medicine & Rehabilitation 80, pp.1065-1071.
PALMER S., MARTIN D. (2002) Chapter 18: Interferential current for pain
control. In: Kitchen S (Editor) Electrotherapy: evidence-based practice 11th
Edition. Edinburgh: Churchill Livingstone, pp.287-300.
TREFFENE R.J. (1983) Interferential fields in a fluid medium. The Australian
Journal of Physiotherapy 29(6), pp.209-216.
I hope you find this information helpful.
Kind regards,
Shea
Dr Shea Palmer,
Lecturer & Joint List Owner,
Department of Physiotherapy,
Queen Margaret University College,
EDINBURGH,
EH6 8HF.
Tel 0131-317-3356
Fax 0131-317-3815
e-mail mailto:[log in to unmask]
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Date: Thu, 18 Jul 2002 20:51:06 +0530
From: Shirish Bedekar <[log in to unmask]>
Subject: Re: PHYSIO Digest - 16 Jul 2002 to 17 Jul 2002 (#2002-38)
Hello members,
writing after a long time.
I am working as lecturer at Pune, India
I teach part of electr- therapy. I have a doubt. If we say that laser
energy is been given in pulses, still the wand/output gives a continuous red
glow (wave length is in IR range, it is diode applicator.)
Second doubt about Interferrential therapy. In theory we apply two medium
frequency currents and interference is inside the tissues. It is still
medium frequenct just under the electrodes. In practice we do experience the
effect of Amplitude modulated frequency (low frequency effect) just under
the electrodes also. How can this be explained? Does a frequency range of
2000 or 4000 Hz stimulate peripheral sensorimotor nerves?
Thanks,
Nilima Bedekar
MScPT MCSP
