I needed a break from fasting papers and, while I’m a big, BIG fan of EMS, and to a substantially lesser extent TENS, I am underwhelmed by high “kilohertz-frequency” alternating type currents as is commonly used in Russian stimulation for strength and interferential currents (IFC), sometimes used for both strength and pain. However, I thought this paper was a good review on both and concludes that neither Russian stimulation are ideal and that there are better forms of “kilohertz-frequency” electrical stimulation. I don’t doubt his conclusions but I think biphasic square wave currents are better still, such that this paper was almost academic for me.

Electrical stimulation using kilohertz-frequency alternating current. Ward AR. Physical Therapy. 2009 Feb;89(2):181-90. [FREE FULL TEXT]

Abstract
Transcutaneous electrical stimulation using kilohertz-frequency alternating current (AC) became popular in the 1950s with the introduction of “interferential currents,” promoted as a means of producing depth-efficient stimulation of nerve and muscle. Later, “Russian current” was adopted as a means of muscle strengthening. This article reviews some clinically relevant, laboratory-based studies that offer an insight into the mechanism of action of kilohertz-frequency AC. It provides some answers to the question: “What are the optimal stimulus parameters for eliciting forceful, yet comfortable, electrically induced muscle contractions?” It is concluded that the stimulation parameters commonly used clinically (Russian and interferential currents) are suboptimal for achieving their stated goals and that greater benefit would be obtained using short-duration (2-4 millisecond), rectangular bursts of kilohertz-frequency AC with a frequency chosen to maximize the desired outcome.

My comments:

The above abstract and the paper were interesting to me because, the author does a good job of reviewing the research putting the slam on IFC, both in theory and in practice. However, the part I thought was fascinating and got me thinking about my use of electric stimulation for headaches was his section titled High-Frequency Fatigue:

“When electrical stimulation is applied to elicit a motor response using PC frequencies higher than physiological or at the high end of the physiological range (ie, greater than about 50 Hz), it is possible to produce a blockage of muscle activity due to propagation failure or neurotransmitter depletion…This is responsible for the phenomenon of “high-frequency fatigue…”

“…whether propagation failure or neurotransmitter depletion, a blockage of muscle contraction at stimulation frequencies around and above about 50 Hz is the result, and the effect is described as high-frequency fatigue.”

So that got me thinking. In the above quotes, when talking about PC frequencies, the PC stands for ‘pulsed current’ for which the the biphasic square wave current I use is an alternating form of PC. I set all my machines at 120 hz, which is higher than most recommend for muscle strengthening, which I got from something the famous sprint coach Charlie Francis said he used with his sprinters (and unfortunately I can’t remember where I read it. Anyway, I have been programming my EMS machines with 120 hz ever since and it seems to work great. When I first tried EMS on my headache patients I used 120 hz (10 seconds on, 50 seconds off for 10-12 minutes) the same as I did for muscle strengthening and it worked great with my results being better than reported with the Cefaly studies. There are a few reasons why I think my results are better (use of much larger electrodes, use of rubber carbon electrodes) and I thought my higher frequency stimulation (120 hz vs 60 hz with the Cefaly) felt better. I tried 60 hz and thought it felt ‘prickly’ while the 120 hz felt like a comfortable buzz, and particularly when I started stimulating the supraorbital nerves, as the Cefaly device does. I wasn’t intending to strengthen any muscles, and utterly fatiguing out the sensory nerves sure is what it feels like was happening. So maybe that’s part of what’s going on, the higher frequency stim, which might be overkill (though still plenty effective) for muscle strengthening is fatiguing out the nerves eliminating headache pain. It also makes me wonder if the 120 hz frequency I use for core strengthening in low back pain patients explains why my patients get such a remarkable amount of immediate pain reduction. I always thought it was from the higher amount of current delivered which is implicit with EMS vs TENS, but maybe it’s higher than normal rate of delivery of said current. Reading this paper also makes me wonder if the faster muscle fatigue with the higher rate of stimulation leads to long term performance gains, and if the quicker muscle fatigue lets you tolerate higher amplitudes of stimulation, thus better exploiting gate control theory for pain reduction.

So I don’t know, this paper gave me some ideas for which there aren’t yet definite answers. I like a higher frequency EMS than most think is ideal, and my reasons have been because I subjectively liked them better, and because that’s what Charlie Francis used. However, now I think I might be making better use of high-frequency fatigue, which might be at least part of the reason behind my (and my patients) subjective preferences.