EMS Research Update - CNS Fatigue/Recruitment Order

EMS Research Update

Hi everyone, now Xmas is here and I have time off of work I can finally get around to writing up a few things for the Forum. Starting with this very interesting talk I attended back in March… :eek: TC

This is a review of information presented by Nicola A. Maffiuletti of the University of Burgundy at Bijon, France during the UK Strength and Conditioning conference in May 2005. I found his talk quite interesting because he suggests some different opinions to those most of us on the CF Forum currently hold on EMS.

Just for reference Maffiuletti uses the terms ES = Electro stimulation and EMS to = Electromyostimulation. I can’t remember exactly what the difference is but apparently it was important to make the distinction!

Motor Unit Recruitement

Firstly, Maffiuletti suggests that while it was originally believed that motor unit recruited during ES occurs in the opposite order [1] to voluntary contraction (e.g. large/fastest motor units first down to smallest/slowest motor units last), in reality this is not the case. He believes the motor unit recruitment is affected by:

  1. Axon diameter – the largest are recruited first
  2. Anatomical positioning – the largest superficial axons are recruited first
  3. Cutaneous receptors – the recruitment order is altered [2]

One study suggests that while voluntary contraction appears to show that 6% of motor units are recruited in reversal during voluntary contraction, during ES this increases to about 30% [3]. From this and his own work Maffiuletti suggests that during ES motor unit recruitment doesn’t occur in reversal but it does occur in an order that is different to voluntary contraction.

When comparing the acute effects of ES contractions with voluntary contractions it has been shown that the drop off in the force of contraction during an ES session varies between 10-20% (depending on the muscle group) during 30 isometric repeated contractions [4]. This is pretty similar to what occurs during voluntary muscular contractions.

Metabolic Cost of EMS

The same study [4] also showed that anaerobic glycolysis also increased during ES sessions (basically the subjects started to develop lactic acid as a result of ES).

From several studies the metabolic cost of ES in comparison to Voluntary contraction can be summarised as:

02 consumption is believed to be some 500-900% less than voluntary work
Pi:Pcr ration is some 300-400% less than voluntary work
ATP turnover decreases by 200-500% compared to voluntary work

It is believed the increased metabolic effects associated with voluntary contractions result because of the rotation and need for synchronisation of the fastest motor units during voluntary contraction.

[Please note that this part of my notes are not very clear so it might be the other way around! – I just can’t tell!]


Interestingly it is also suggested that muscle soreness (DOMS) is greater and peaks more abruptly on the second day after training with ES than with voluntary contractions. This increase in DOMs seems to correlate with an increase in muscle damage (CK activity) [5].

Personally I havn’t found ES with Compex to create any soreness what so ever! Though a dull ache (feeling of fatigue) does occur but it is nothing like DOMS.

CNS Fatigue!

MRI scans have shown that there is significant activation of the CNS during ES [6]! This has been shown to lead to a drop off in activation of around 4.5% and contractile force is reduced by about 6% [7].

Maffiuletti was pretty decided on the issue that EMS does not bypass the CNS and creates central and peripheral fatigue!

Practical training applications

In terms of the basic question: should the athlete voluntarily contract the muscle during EMS or not? It has been shown that voluntary contraction increases the force output and reduced the pain perceived by the user. However, voluntary contraction during ES makes it hard to define intensity.

Strength Increases

For healthy populations research suggests that ES on its own is not as effective as voluntary contraction for increasing strength but in unhealthy populations ES is better. However, he suggest the two are complementary because of the different motor unit recruitment patterns.

In terms of neural adaptations, ES seems to increase surface EMG, Activation Level and H reflex, while reducing coactivation of antagonists.

In terms of muscular adaptations there appears to be conflicting ideas with some studies showing an increase in the cross sectional area and other not. However, High Frequency activation via ES seems to lead to a shift from type IIx > IIa and I > IIa which is similar to voluntary resistance training.

In short Maffiuletti believes ES does increase muscle strength and the neuromuscular effects are very similar to voluntary contractions.

EMS as a practical strength training device for Team Sports

Having described a number of studies relating to the use of EMS as a method of strength training for athletes (basketball, volleyball, soccer, tennis) the conclusions were that EMS increased muscular strength but did not translate to sport specific performance improvements (specific power).

However, the effects of EMS are more specific if combined with plyometrics. From this Maffiuletti’s general recommendations were that ES should be used during GPP and not the competitive season and should not be used as the sole method of strength/power training.


[1] Hennemann et al. J Neurophysiolo 1965
[2] Enoka. Sports Med 1998
[3] Feiereisen et al. Exp Brain Res 1997
[4] Hamada et al. J Appl Physiol 2004
[5] Moreau et al. J Sport Sci 1995
[6] Han et al. Am J Phys Med Rehab 2003
[7] Boerio et al. Med Sci Sport Exerc (in press)

Perhaps if we could know a bit more about the protocols in some of the studies…
E.g., reference 4 in the production of lactic acid and references 6 & 7 on CNS, although the latter seems a fundamental difference.
Did most of the studies referenced involve voluntary contractions?
We have to keep in mind that the conclusions drawn are always specific to the conditions under which certain protocols were indeed employed.

PS perhaps someone who has employed a combination of an EMS -or ES- device and the OW system could bring some data into the light. Pakewi? Charlie?

This is true, I havn’t had a chance to look at the actual studies yet. This is just a quick overview of the major points. However, the two points that:

  1. CNS and Peripheral fatigue do occur with ES
  2. Recruitment does not occur in reverse order but the order is different to voluntary training.

do appear to be well supported by science.

The DOMS and metabolic stuff is (my instinct tells me) related to the protocol used - as from personal experience I find no DOMS with Compex ES but then Compex are clever and pulse the muscles being contracted at varying frequencies between maximal contractions. This seems to both increase tollerence to pain and help the body recover from the work.


So it’s all supported by “science” but the one experience you’ve had contradicts the study. I see much to rebut through my practical experience here and will do so when I have a bit more time.

I would like to get more information on the studies and protocols used, particularly how they:

  • Determined CNS fatigue with an MRI
  • Determined recruitment order with EMS (how did they measure it?)

I know I’ve fiddled with EMS and Heart Rate Variability (Measuring R-R intervals via ECG). From my informal tests on myself, I’ve found no significant differences in HRV (parasympathetic nervous system response) in pre-EMS vs. post-EMS. I’ll keep monitoring it with different protocols to see if I get any significant results. I would think if there was any difference, it would primarily be from my body’s response to the intensity (i.e. pain) from the EMS contractions - higher sympathetic response - as opposed to a reduced parasympathetic response (CNS fatigue).

Here are the abstracts for the two references on CNS Fatigue…

Functional Magnetic Resonance Image Finding of Cortical Activation by Neuromuscular Electrical Stimulation on Wrist Extensor Muscles.
American Journal of Physical Medicine & Rehabilitation. 82(1):17-20, January 2003.
Han, Bong Soo PhD; Jang, Sung Ho MD; Chang, Yongmin PhD; Byun, Woo Mok MD; Lim, Seung Kwan MS; Kang, Duk Sik MD

Han BS, Jang SH, Chang Y, Byun WM, Lim SK, Kang DS: Functional magnetic resonance image finding of cortical activation by neuromuscular electrical stimulation on wrist extensor muscles. Am J Phys Med Rehabil 2003;82:17-20.

Objective: To investigate the effects that neuromuscular electrical stimulation on the wrist extensor muscles have on the cerebral cortex.

Design: A functional magnetic resonance imaging study was performed on eight normal volunteers. The activation task was the maximum wrist extension by neuromuscular electrical stimulation, applied through a two-channel electrical stimulator. Monophasic square-wave pulses were used. The activation maps were generated by the threshold t test maps. The level of primary motor cortex and primary sensory cortex activations was estimated.

Results: Among the eight subjects, seven showed significant activation on contralateral primary sensorimotor cortex by neuromuscular electrical stimulation on the wrist extensor muscles. In these seven subjects, additional bilateral or contralateral supplementary motor area activations were also observed. The number of activated pixels on the primary sensory cortex was slightly greater than that on the primary motor cortex.

Conclusion: Neuromuscular electrical stimulation, when applied to the peripheral muscles, seems to have a direct effect on the cerebral cortex.


Central and peripheral fatigue after electrostimulation-induced resistance exercise.

Boerio D, Jubeau M, Zory R, Maffiuletti NA.

INSERM ERM 207, Faculty of Sport Sciences, University of Burgundy, Dijon, France.

PURPOSE: To investigate central and peripheral fatigue induced by a typical session of electromyostimulation (EMS) of the triceps surae muscle. METHODS: A series of neuromuscular tests including voluntary and electrically evoked contractions were performed before and immediately after 13 min of EMS (75 Hz) in 10 healthy individuals.

RESULTS: Maximal voluntary contraction torque of the plantar flexor muscles significantly decreased (-9.4%; P < 0.001) after EMS, and this was accompanied by an impairment of central activation, as attested by twitch interpolation results (P < 0.05), whereas soleus maximal Hoffmann reflex and tibialis anterior coactivation did not change significantly. Contractile properties associated with paired stimuli and maximal M-wave amplitude for both soleus and medial gastrocnemius muscles (-9.4 and -38.7%, respectively) were significantly affected by EMS (P < 0.05), whereas postactivation potentiation did not change.

CONCLUSION: A single bout of EMS resulted in fatigue attributable to both central and peripheral factors. The most obvious alteration in the function of the central nervous system is a decrease in the quantity of the neural drive to muscle from the supraspinal centers. On the other hand, neuromuscular propagation failure was more evident for the muscle with the higher percentage of Type II fibers.


First off: Their definition of CNS fatigue is the classic one (reduction via various means in the ability to carry on at a set intensity during the activity) not the definition of CNS fatigue being used by us to describe delayed recovery from intense activity, as opposed to muscular recovery. Derek’s informal check via Omega Wave would be the reasonable way to assess that.
Second: The conclusion that Type 11 is fatigued most would appear to contradict the non-reversal of recruitment order suggested by the study.
I could go on with my objections but anyone with any reasonable equipment will know that after max voluntary contraction is achieved in the quad, the stim can lift it higher. Maybe the muscle chosen and the type of equipment buggered the results. Regardless of whether you want to contract/respond or not, when the stim gets going- you will anyway! (hence the Cerebral cortex findings).

Yes, that would be interesting!!

Since I had a discussion with Tom, I thought I should add any info I can to this…

Some info about the presenter first -it’s always important, I think, to understand where everyone is coming from…

Dr Nicola A. Maffiuletti received a Licence degree (1996) in Physical Education (ISEF) at the University of Milan (Italy) before completing his Master (1997) and PhD (2000) at the Faculty of Sport Science, University of Burgundy at Dijon (France). He is currently a “Maitre de Conferences” (i.e., researcher and lecturer) at the INSERM ERM 207 Laboratory (Motricity-Plasticity), in the same Department. Nicola is fascinated by the way in which neuromuscular mechanisms mediate acute adjustments and chronic adaptations in response to physical activity (e.g., fatigue, resistance training) in healthy individuals.

Presentation Synopsis
Title: Acute and Chronic Effects of Electrostimulation Resistance Exercise.
Due to the beneficial effects of neuromuscular electrical stimulation (NMES) in rehabilitation medicine, this technique has become a complement and sometimes even a substitute for the ordinary resistance training programme for many athletes, particularly in strength and power events. The effectiveness of NMES training for the enhancement of maximal strength and power has been demonstrated in numerous sports, such as weightlifting, tennis, volleyball, basketball, swimming and track and field. The presentation addressed the physiological basis and methodological considerations of NMES, the acute and chronic effects associated to the use of NMES as well as the potential risks and applications for different sport activities.

Before any comments I need to make clear that I wasn’t present at this presentation.

  1. Although I get what the first reference claims (Hennemann, 1965) and how the first two factors (i.e., axon diameter and anatomical feature) can affect MU recruitment, I can’t understand the third one, i.e., cutaneous receptors, which apparently alter recruitment order and this might be a crucial point. I can’t argue with the reference, of course (Enoka, 1988) and I suppose it’s something I just don’t get… At least this is how Maffiuletti interprets the study.

The first sentence is absolutely clear from the graph and in the following order: Vol < ES @1ms < ES @0.1ms. The mean values are significantly different (i.e., Vol sig. diff. vs. ES). However, this doesn’t justify why the order wouldn’t be reversal -axon diameter will always play its role, no? Are the Cutaneous receptors to alter the order?
Tom, do you remember anything specific said about this factor?

  1. By Lieber and Kelly (Phys. Ther., 1991) “the most important determinant of tension evoked by ES is not electrode size or stimulation current, or any other external controllable factor (underlined by presenter), but some intrinsic properties (same here) of the muscles, e.g., individual motor nerve branching”.

  2. Regarding the cases where a drop in Torque Level (as %MVC) was observed the muscle groups were Plantar Flexor Muscles (-10%) and Knee Extensor Muscles (-20%) in a “typical ES session (75Hz - maximum intensity- No of contractions 30)”. [Note: duration is not given, but from previous literature presented with sample protocols, for increases in force a ratio of 4:20-30 sec is given -with 15:50 sec for rehab at a lower frequency, of course…]

  3. Regarding the Metabolic Cost, there is no obvious difference in HR and VO2 from the graph provided with actually a drop from 20th minute -not contraction- onwards for both parameters and both conditions; however, the conclusion at the end was that there is an “exaggerated metabolic cost” during ES. I guess we have to take their word on this for correct statistical -or other- analyses… The same was true for lactate, which reached a peak at 5th minute (~3.0 mmol.l-1) with a steady decline thereafter reaching a non-significant difference vs. Vol at 30th minute, but still at a higher level. The same values during Vol Contractions were barely different from resting values throughout… (!!!). Same results were obtained for RER (Hamada et al., J. Appl. Physiology, 2004).

  4. Although no Muscular Soreness at all is observed for the next 3 days after Vol contractions, a peak in Muscular Soreness is demonstrated in Day 2 with all three days being higher based on a 0-20 Soreness scale. The latter is accompanied by increased CK activity for the same duration, where an actual reduction is observed after Vol contractions… For ES again a peak is observed in Day 2 (Moreau et al., Journal of Sport Sciences, 1995).

  5. CNS activation during ES revealed the following areas affected: Primary Motor area, Primary Sensory area, Cingulate Gyrus - Thalamus, Cerebellum (by fMRI --> significant activation) (Hanet al., Am. J. Phys Med. Rehab., 2003).

After these points we could go on presenting ES application and the rest of the presentation, if needed…

Thanks Nik, I should have been a sports scientist! What a thing to read on Xmas day!

In terms of recruitement order I can’t remember anything regarding the Cutaneous receptors all I can remember is that he talked about a number of things that affected recruitment order and when on to explain how they varied between persons and how the three overlapped so that while axon diameter is important, if the large axons are positioned deep in the muscle they might not be recruited first. Instead the superfical axons of a smaller diameter may be etc… Basically recruitment order was a trade off between the three factors and therefore we could not say that the recruitment order was reversed only that it was different to voluntary recruitment (size principle).

Like most scientists (who love to use words such as “probably” and “potentially”) he ummed and ahhhed with regard to the majority of possible conclusions he presented (if you pinned him down he said that more research is needed) but when it came to recruitment order and CNS fatigue he was a lot more convinced - as you can tell from his “Take Home Messages”.

Why don’t we email him and ask? Academics love talking about their work!


Maybe when it comes to recruitment order/rate, he’s right re proximity/size vs size alone, but the concept of reversability still seems intact.
Certainly it is true that the faster the subject is as a sprinter, the better the recruitment for a given input. in such a case, where most of my experience with high intensity EMS is in this group, the effect he has seen would be far less clear.

Charlie and Derek:first off I sincerely apologize for my disappearing,personal reasons kept me from getting the focus and time to get back to our original purposes.

Now back to training: my latest findings with the Omega Wave point out that CNS fatigue as reduced electrical activity at a cerebral cortex level and as a reduced signal potential does not correlate directly with the results of the Omega potential assessment . It does so only indirectly, through a secondary mean,as the Omega potential is strongly influenced by cerebral perfusion,which is influenced through a number of steps by the Motor Units involvement in any exercise,making it an excellent tool to determine (and manage) the relative importance of the Breadth dimension of the stimulus,as surely related but also opposed to the Height one.

Hope this information,which I surely wanted to share here sooner or later , fits this discussion properly.

My best Christmas wishes to You and all Members of the Forum.

Giuseppe Gueli