By Michael Edgar
If you have read any of our articles up to this point, you’ll know we’re typically fans of the tried-and-true basics related to guideline adherence. Our three pillars of bias are exercise, reassurance and education. Today however, we’re going to enter the realm of passive modalities, specifically neuromuscular electrostimulation (NMES). The evidence, the utility, and practical applications.
What is NMES?
NMES uses electrodes placed on the skin using adhesive pads around a region of focus, like the quadriceps.(1) The sticky pads are then attached to a device by wires to send an electrical impulse to the motor nerves. (Muscles go zappy) (1) This is aimed at causing a muscular contraction without the individual needing to actively engage the area.(1) This is distinguished from transcutaneous electrical nerve stimulation (TENS) which works through the same mechanism to stimulate sensory nerves.(1) You can think of TENS as a low powered NMES.(1) Both TENS and NMES can be categorized under the category of electromuscular stimulation (EMS).
Physiological Mechanism
NMES is typically mediated through muscular and neural mechanisms, the same mechanism as physical exercise.(1) Tetanic contractions are produced through high intensity and short duration pulses, following the same principles used in strength and hypertrophy training.(1) This has led some individuals to believe NMES can be added as an adjunct to resistance training to increase the training stimulus.(1) For example, placing NMES pads on someone's quadriceps while they perform a leg extension to theoretically have a summative effect.
Beliefs
By causing muscular contraction, NMES is thought to induce high metabolic stress in the muscles engaged.(1) It is also believed to facilitate adequate motor unit recruitment to improve maximal capability of the neuromuscular system.(1) This is speculated to increase force-generating capacity of the muscle through intensified voluntary contraction, as mentioned above, using the leg extension example.(1)
Parameters
NMES parameters typically involve electrode placement, limb position, NMES waveform, frequency, pulse duration, current amplitude, work-rest cycle and session frequency.(1) We don’t need to worry about this for this article.
Healthy Populations
As mentioned, NMES is sometimes used by individuals to try to increase the stimulus to muscles, in order to improve training adaptations. Research has found this to be a bit messier than we would like. Individuals using NMES with resistance training versus voluntary resistance training alone typically show the same magnitude and time course for improvements.(1,4) There does seem to be some conflicting findings for hypertrophy and strength adaptations in untrained individuals though. Stevenson et al. showed benefit in time course but not magnitude of adaptation (they adapted quicker but ended up in the same spot) while Ruthers et al. showed an increased magnitude of hypertrophy in the NMES group.(2-3) Both studies had been done in untrained populations with one glaring flaw, they had no description of volume or load used between groups. (2-3)
As mentioned in my previous article on hypertrophy, volume is one of the major drivers of muscular adaptations and as such, this offers a large confounder in their results. Did they simply have the participants perform additional resistance training with the NMES device? Bezerra et al. highlights my worries with these two studies, as they found no difference between groups in strength or hypertrophy response when training parameters were controlled.(4)
What About Clinical Populations?
Well, the short answer is...it depends. The long answer is...it really does depend.
For example, in knee osteoarthritis, NMES is typically aimed at improving strength and musculature of the quadriceps, improving functional capacity and endurance and reducing pain and stiffness.(1) This sounds great in theory, but what about practice. In several reviews looking at a variety of outcome measures, like strength, quadricep cross-sectional area and self-reported and objective function, they found no difference when compared to standard resistance training.(1, 5-8) However, one systematic review did find an improvement in pain and no adverse effects when using NMES.(8) Maybe there is some promise to NMES!
When we look a little deeper at this review, we see they included four studies in their meta-analysis.(8) Two of the studies showed a significant effect driving the cumulative effect of the meta-analysis to favour NMES for pain.(8) That being said these two studies did not use resistance training as a comparator.(8) Following a similar theme, the two studies that did not show a positive effect for NMES used resistance training as a comparator.(8) So what can we say? I think it is fair to say that in most populations with the ability to perform active resistance training, NMES shows no inherent benefits compared to resistance training, and does not have a summative effect.
Do I think NMES has zero merit? No. As mentioned, it depends why we’re using it. It has been shown to have some benefit if exercise or active muscular recruitment is not possible. We just need to be cognizant of the population we’re using it in. Individuals who have suffered from a stroke and are unable to actively engage their muscles may see a benefit from usage.(9) The story is also quite different for preserving muscle mass or avoiding muscle atrophy in partially or totally immobilized patients.(10-13) This can be seen as an important context for patients in critical care or a post-operative setting, in which recruitment of muscles may be impaired.(10-13)
Limitations of NMES Research
Research has given some clarity to the efficacy of NMES, but there are still some glaring issues with the research collectively. Typically, randomization methods are not fully described, sample sizes are not calculated and observed power is not reported (learn more on that here) and assessors and participants are not blinded.(1, 5-8) An inherent limitation to NMES is that a sham limitation is not really possible given that a ‘below threshold’ NMES device is simply a TENS device.(1, 5-8) TENS is believed to be a therapeutic intervention, with its own unique benefits. (I will save getting into the limitations of TENS for another day).
Conclusion
Currently NMES is not a device I would hang my hat on for the majority of contexts I experience in a clinical or fitness setting. If you enjoy being hooked up to a bunch of wires in a gym setting though, as a fashion statement, I won’t fight you on that. Let’s just remember that more doesn’t always mean better, and that the context and goal for an intervention really does matter. I will also state that if we look at things from a standard cost-benefit analysis, it’s typically a lot cheaper and easier for individuals to self-manage using a self-directed exercise plan.
If you are curious to read more about NMES and its use in various clinical settings, I would recommend reading the review by Nussbaum et al. 2017. It reviews the literature for NMES from a clinical perspective related to various clinical conditions.(1)
I’d also like to thank Nicola A. Maffiuletti, PhD, head of the Human Performance Lab, for his help finding some of the research cited in this article.
References
Nussbaum EL, Houghton P, Anthony J, Rennie S, Shay BL, Hoens AM. Neuromuscular electrical stimulation for treatment of muscle impairment: critical review and recommendations for clinical practice. Physiotherapy Canada. 2017;69(5):1-76.
Bezerra P, Zhou S, Crowley Z, Brooks L, Hooper A. Effects of unilateral electromyostimulation superimposed on voluntary training on strength and cross‐sectional area. Muscle & Nerve: Official Journal of the American Association of Electrodiagnostic Medicine. 2009 Sep;40(3):430-7.
Stevenson SW, Dudley GA. Dietary creatine supplementation and muscular adaptation to resistive overload. Medicine and science in sports and exercise. 2001 Aug;33(8):1304-10.
Ruther CL, Golden CL, Harris RT, Dudley GA. Hypertrophy, resistance training, and the nature of skeletal muscle activation. The Journal of Strength & Conditioning Research. 1995 Aug 1;9(3):155-9.
Bruce-Brand RA, Walls RJ, Ong JC, Emerson BS, O’Byrne JM, Moyna NM. Effects of home-based resistance training and neuromuscular electrical stimulation in knee osteoarthritis: a randomized controlled trial. BMC musculoskeletal disorders. 2012 Dec 1;13(1):118.
de Oliveira Melo M, Aragão FA, Vaz MA. Neuromuscular electrical stimulation for muscle strengthening in elderly with knee osteoarthritis–a systematic review. Complementary therapies in clinical practice. 2013 Feb 1;19(1):27-31.
Novak S, Guerron G, Zou Z, Cheung G, Berteau JP. New Guidelines for Electrical Stimulation Parameters in Adult Patients With Knee Osteoarthritis Based on a Systematic Review of the Current Literature. American journal of physical medicine & rehabilitation. 2020 Aug 1;99(8):682-8.
Giggins OM, Fullen BM, Coughlan GF. Neuromuscular electrical stimulation in the treatment of knee osteoarthritis: a systematic review and meta-analysis. Clinical rehabilitation. 2012 Oct;26(10):867-81.
Stein C, Fritsch CG, Robinson C, Sbruzzi G, Plentz RD. Effects of electrical stimulation in spastic muscles after stroke: systematic review and meta-analysis of randomized controlled trials. Stroke. 2015 Aug;46(8):2197-205.
Maffiuletti NA, Roig M, Karatzanos E, Nanas S. Neuromuscular electrical stimulation for preventing skeletal-muscle weakness and wasting in critically ill patients: a systematic review. BMC medicine. 2013 Dec;11(1):1-0.
Wageck B, Nunes GS, Silva FL, Damasceno MC, de Noronha M. Application and effects of neuromuscular electrical stimulation in critically ill patients: systematic review. Medicina intensiva. 2014 Oct 1;38(7):444-54.
Kim KM, Croy T, Hertel J, Saliba S. Effects of neuromuscular electrical stimulation after anterior cruciate ligament reconstruction on quadriceps strength, function, and patient-oriented outcomes: a systematic review. journal of orthopaedic & sports physical therapy. 2010 Jul;40(7):383-91.
Stevens-Lapsley JE, Balter JE, Wolfe P, Eckhoff DG, Kohrt WM. Early neuromuscular electrical stimulation to improve quadriceps muscle strength after total knee arthroplasty: a randomized controlled trial. Physical therapy. 2012 Feb 1;92(2):210-26.