Abstract
Transcutaneous electrical nerve stimulation (TENS) is a noninvasive technique used for pain modulation. During application of TENS there is a fading of current sensation. Textbooks of electrophysical agents recommend that pulse amplitude should be constantly adjusted. This seems to be accepted clinically despite the fact that there is no direct experimental evidence. The aim of the current study was to investigate the hypoalgesic effect of adjusting TENS pulse amplitude on pressure pain thresholds (PPTs) in healthy humans. Fifty-six healthy TENS naïve participants were recruited and randomly assigned to 1 of 4 groups (n = 14 per group): control, placebo TENS, fixed pulse amplitude TENS, and adjusted pulse amplitude TENS. Both active and placebo TENS were applied to the dominant forearm. PPTs were recorded from 2 points on the dominant forearm and hand before, during, and after 40 minutes of TENS. TENS increased the PPTs on the forearm (P = .003) and hand (P = .003) in the group that received the adjusted pulse amplitude when compared to all other groups. The mean final pulse amplitude for the adjusted pulse amplitude TENS group was 35.51 mA when compared to the fixed pulse amplitude TENS group, which averaged 31.37 mA (P = .0318).
PERSPECTIVE:
These results suggest that it is important to adjust the pulse amplitude during TENS application to get the maximal analgesic effect. We propose that the fading of current sensation allows the use of higher pulse amplitudes, which would activate a greater number of and deeper tissue afferents to produce greater analgesia.
My comments:
The above was a good/strong study on optimizing the use of TENS for pain reduction. Patients were blinded and they had control, placebo, standard TENS (starting with a “strong but comfortable” current), and TENS of increasing intensity as tolerated every 5 minutes (but keeping the intensity “strong but comfortable”) over 40 minutes. The idea being that electric stimulation feels strong at first but then feels less intense as time goes on as the patients either undergo:
- adaptation: action potentials from the electric stimulation decreases over time
- habituation: where electric stimulation continues to cause action potentials but the central nervous system (CNS) response decreases
- accommodation: when a rise in membrane threshold potential that is associated with slowly applied depolarization current
In the review sections the authors noted electric stimulation machines frequently have included or allow modulation of parameters, frequency, intensity, pulse width etc., but noted that no studies had confirmed if that strategy was effective. They noted an interferential current treatment study that found that while the sensation of electric stimulation faded over time the analgesic effect did not. So that’s good news. However it has been noted in a lot of reviews of TENS treatments over the years that there was controversy as to TENS effectiveness to reduce pain. This is likely due to many of the early studies applying the TENS haphazardly, letting patients put electrodes where they want and turning up the electric stimulation to what felt good to them and in doing so it was hit or miss whether the TENS reduced pain. As such, later papers found that when TENS intensity was increased to what was “strong but comfortable” the pain reduction was increased and more reliable.
So this paper not only started with a strong and comfortable current, but instructed subjects to increase the TENS electricity every 5 minutes over the 40 minute treatment time, and the finding was that pressure pain thresholds (PPTs) tolerated both between and distal to the electrodes was increased significantly and fairly dramatically (in comparison to controls, placebo TENS and fixed pulsed TENS). It was also interesting that strong and comfortable fixed TENS resulted ~5-13% improvements in PPTs while the intensity increasing TENS increased pain in the range of ~25-50%, with PPTs staying elevated somewhat 60 minutes after the increasing intensity TENS PPTs were still elevated.
So the take home message with TENS is to start high and increase from there. I like TENS for pain reduction, but I can’t say I love it. Electric Muscle Stimulation (EMS) I have a lot of love for, because EMS has been shown to decrease pain better than TENS and increase muscle strength and endurance at the same time. The reason, I think, is largely because EMS is not a strong comfortable current but I tell my patients to take all they can take, which is often more than uncomfortable. Anecdotally I get pain reductions after the EMS is removed seemingly a lot longer than I used to get with TENS, so it would be interesting to see that tested.
Of the proposed mechanisms describing the lessening of sensation intensity to TENS over time, the authors thought habituation was the best descriptor and that increasing TENS intensity over time counteracted this by activating a greater number of afferent (nerves heading from the periphery towards the brain) fibers to enhance neurotransmitter release in the CNS. This also got me thinking about the on and off portions of EMS in comparison to TENS currents which are generally more constant. On as hard as you can take and completely off, is a pretty extreme form of current “modulation” and that might be another reason for the enhanced pain reduction seen with EMS currents. So still lots of questions, but a lot of work seems to be going into the use of TENS and EMS for rehabilitation, which dwarfs any other physical therapy modality.
Thanks for reading my blog. If you have any questions or comments (even hostile ones) please don’t hesitate to ask/share. If you’re reading one of my older blogs, perhaps unrelated to neck or back pain, and it helps you, please remember SpineFit Yoga for you or someone you know in the future.
Chad Reilly is a Physical Therapist, obtaining his Master’s in Physical Therapy from Northern Arizona University. He graduated Summa Cum Laude with a B.S. Exercise Science also from NAU. He is a Certified Strength and Conditioning Specialist, and holds a USA Weightlifting Club Coach Certification as well as a NASM Personal Training Certificate. Chad completed his Yoga Teacher Training at Sampoorna Yoga in Goa, India.
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