TENS Electrodes Work Best Over Muscle or Soft Tissue

An investigation into the magnitude of the current window and perception of transcutaneous electrical nerve stimulation (TENS) sensation at various frequencies and body sites in healthy human participants. Hughes N, Bennett MI, Johnson MI.Clin J Pain. 2013 Feb;29(2):146-53.

Abstract
INTRODUCTION:
Strong nonpainful transcutaneous electrical nerve stimulation (TENS) is prerequisite to a successful analgesic outcome although the ease with which this sensation is achieved is likely to depend on the magnitude of current amplitude (mA) between sensory detection threshold (SDT) and pain threshold, that is, the current window.

OBJECTIVES:
To measure the current window and participant’s perception of the comfort of the TENS sensation at different body sites.

METHODS:
A repeated measure cross-over study was conducted using 30 healthy adult volunteers. Current amplitudes (mA) of TENS [2 pulses per second (pps); 30 pps; 80 pps] at SDT, pain threshold, and strong nonpainful intensities were measured at the tibia (bone), knee joint (connective tissue), lower back [paraspinal (skeletal) muscle], volar surface of forearm (nerve) and waist (fat). The amplitude to achieve a strong nonpainful intensity was represented as a percentage of the current window. Data were analyzed using repeated measures analysis of variance.

RESULTS:
Effects were detected for body site and frequency for SDT (P<0.001, P=0.018, respectively), current window (P<0.001, P<0.001, respectively), and strong nonpainful TENS as a percentage of the current window (P=0.002, P<0.001, respectively). The current window was larger for the knee joint compared with tibia (difference [95% confidence interval]=12.76 mA [4.25, 21.28]; P=0.001) and forearm (10.33 mA [2.62, 18.40]; P=0.006), and for the lower back compared with tibia (12.10 mA [1.65, 22.52]; P=0.015) and forearm (9.65 mA [1.06, 18.24]; P=0.019). The current window was larger for 2 pps compared with 30 pps (P<0.001) and 80 pps (P<0.001). Participants rated strong nonpainful TENS as most comfortable at the lower back (P<0.001) and least comfortable at the tibia and forearm (P<0.001).

CONCLUSIONS:
TENS is most comfortable and easiest to titrate to a strong nonpainful intensity when applied over areas of muscle and soft tissue.

My comments:
My observations working with TENS and EMS  are pretty much in agreement with this paper that both TENS and EMS are better tolerated if you place the electrodes over areas of muscle and soft tissue rather than directly over bone. For example, in tennis elbow it only took me a couple tries at placing an electrode directly over the painful epicondyle (pretty boney) to learn that patients just don’t like that, but if I put the electrodes over the muscle, they had a mild reduction in pain, but it also helped to increase strength. So I think the findings here of placing electrodes over areas of deeper muscle might partially explain why EMS seems to work better for pain than TENS does. With EMS, electrodes are (at least the way I do it) put over the area of greatest muscle mass, in the region I want to work, that I can find. It might also explain why my EMS protocols are well tolerated even though I don’t look for acupuncture points or motor points like some practitioners advocate.
 
A couple other things from the paper I thought worth touching on were that they found some subjects didn’t like the higher intensity TENS on their forearm muscles, not because of discomfort from the TENS itself but because of disconcerting contractions of their fingers. I felt the same thing with my year of EMS experiment, and found that problem went away if I gripped something firm with my hand like a rolled up towel, dowel rod, or in my clinic we now have short pieces (~5” long x 2” diameter) PVC pipe to hold onto while doing TENS or EMS to the forearm. So I expect the subjects in this study could have worked to higher intensities of electric stimulation on their forearm muscles, more comfortably, if they had done the same.
 
Last, the cited other papers indicating there were higher reductions in pain with greater current amplitudes, which agrees with a number of other papers indicating there is a dose response relationship with TENS for pain reduction, for which I have also commented on regarding advantages of EMS over TENS just due to the higher currents used. And in the same vein they also referenced 3 other papers (which I’ll be looking up) that found greater pain reduction in animal studies, where TENS activated deeper levels of muscle tissue. So the evidence continues to mount up that to get the most out of TENS you might want to make the parameters more EMS-like. Which generally isn’t too hard since better electric stimulation units have settings for both, with EMS just being a strong variant of TENS with some other simple parameter changes regarding rate, rest periods, etc.
 
The advantage that TENS still might have over EMS is that the on periods are usually on longer, or always on, while EMS has rest periods of varying length. In my office I don’t notice the rest periods to be a problem, and I get surprising pain reduction with EMS currents with a 10-50-10 (10 seconds on, 50 seconds off, for 10 minutes) protocol, but it does make me wonder, if pain reduction is a primary goal, if shorter off times and longer on times would be better. It might not though, as I did a trial of 5-15-10 for all my physical therapy patients for a couple months, which had a greater proportion of on periods in my office, but ended up going back to 10-50-10. Patients sometimes thought that the 50 seconds rest in 10-50-10 was too long and they wanted more stimulation. I would explain that the longer rest allowed for greater recovery and a stronger following contractions during the “on” times, which they accepted pretty well, but I thought the 5-15-10 might be better. With 5-15-10, I didn’t have as much explaining to do, but subjective pain reductions seemed about the same. On myself it felt like 10-50-10 was a better workout, and I could do something with the 50 seconds off. With 5-15-10 I was just waiting for the next blast and afterwords I didn’t feel like I had worked any harder or maybe not as hard.
 
As always, if you have any further questions or need for clarifications, please don’t hesitate to ask. Being aware that some of my blog ideas are contentious and occasionally a bit out of the field of my expertise, I encourage my readers to come forth with any questions/comments that are of interest or concern. Your comments are valued and welcomed.
 
Chad Reilly is a licensed physical therapist, located in North Phoenix, practicing science based medicine with treatment protocols unique and effective enough to proudly serve patients from Phoenix, Scottsdale, Mesa, Chandler, Tempe, Peoria, and Glendale.

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