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European perspective: a comparison between TENS and MET.

Physical Therapy Products. September 2000

By Patrick De Bock, Physical Therapist
 
“MET has a different mechanism than TENS. It works on a cellular level and has its effects much closer to the cause of the injury.”
 
While everyone talks about back pain, many doctors often question whether patients really have it. Clinical and technical examinations such as X-rays, scans, MRIs and kinesiological evaluations may not always reveal a clear cause. Classic treatments such as nonsteroidal anti-inflammatory drugs, epidural infiltration and even surgery do not always solve the problem.

However, physiotherapists already have a better solution-one that is pleasant and may competently control the pain without much interference with daily activities.

In America, the use of a transcutaneous electrical nerve stimulation (TENS) device is common. On the other hand, microcurrent electrotherapy (MET), even though its not that new, may not be common enough. In this article, we will compare TENS to MET, using the eight-parameter law, and see which gets the most credits.
 

1. Eight-Parameter Law

Bringing together every possible influence in electrotherapy leads to a list of eight parameters.

1.1 Waveform Polarity

Biphasic waves are much smoother to the skin than monophasic.  Monophasic waves are known to cause skin burns in long treatments.  This will be no problem for TENS or MET, because they are both biphasic or can be switched to it. On some occasions, the polarity is important to get a specific effect such as on wound healing or on bone.  In this matter, we are dealing with back pain so that these effects are complementary.

 

Howson believes that human skin resistance will deform and decrease the initial wave.  Maybe this indicates that the waveform is not of primal importance as long as it is biphasic.  Keep in mind, however, that these conclusions were made using TENS devices!  They may not apply to the more sophisticated MET technology.  MET stimulators differ through  their waveforms. Though treatment circumstances were comparable, pain-reducing effects were not always the same with different types of stimulators. This is an open field for investigation.

1.2 Frequency

Theres no evidence for an overall efficient frequency.  Most studies indicate that TENS frequencies in the 50-200 Hz range are used often with good effects.  MET uses other frequencies. Empirically extreme low frequencies in the 0.3-3 Hz range have shown the best results. It seems that 0.5 Hz is especially good for pain relief. In some cases, 1.5 Hz may give better results.

To release endorphins, one should choose frequencies less than 8 Hz. At this time it isnt completely certain that MET triggers the endorphin release system with electrodes placed on the trunk. However, MET, being of lower current by definition, is sometimes used on the head where the mechanisms are different and where endorphins are more clearly implicated.

1.3 Pulse Width

Howson, Li and Bak investigated stimulation thresholds for afferent nerve fibers.  Their findings show how A and C-fibers can be stimulated with pulse widths larger than 200 S. According to many studies on the effect of electrical stimulation and endogenous opioids, these fibers should be stimulated to start endorphin release.  On the other hand, but still according to Howson, Li, and Bak, pulse widths in the 60 S range should be used only to affect A (-fibers and thus use Melzack and Walls gate control) to achieve the effect of pain decrease.+

It may be clear that in this latter situation, optimal use of the gate- control mechanism should not trigger any small, pain transporting A (-and C-fibers). These two mechanisms of controlling the pain can be used by adjusting the pulse width of the TENS device. MET, however, uses much larger pulse widths of 0.1 2.0 seconds. Due to the much lower intensity, it is believed that afferent fibers will not be affected in the same way. Cheng, etc. reported a significant increase in ATP at intensity levels below 750 S. It is probable that these large pulse widths are also necessary to achieve effects on ATP production.

1.4 Pulse Train

Interrupting the stimulus is used to avoid adaptation of the central nervous system. Both TENS and MET pulses use this technique. Some authors believe that pulsed painful stimuli will trigger the endorphin release. TENS has an extra possibility to interrupt the current by switching it into burst-mode causing the patient to report an on-off feeling. A series of pulses alternating with pauses known as the burst-mode is the real pulse train technique in TENS.

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Physical Therapy Products Sept. 2000

Used with permission of Electromedical Products International, Inc.

©2006 by
 

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European perspective: a comparison between TENS and MET