What are the physiological foundations of muscle stimulation?
In this article you can learn the theoretical background of EMS — electrical muscle stimulation: how stimulator parameter settings determine which muscle fibers are activated and how EMS bypasses the natural regulation that cannot be voluntarily overridden. The related clinical evidence is summarized in the electrotherapy evidence article; the technical details for sports and rehab applications are in the NMES rehab+sport piece.
Key idea
The human skeletal musculature is composed of three main fiber types (I = slow/endurance, IIa = intermediate, IIb/IIx = fast/power). During voluntary movement, the Henneman size principle recruits the small (type I) motor units first, then progressively larger ones. EMS partially overrides this recruitment order: with appropriate frequency, pulse-width and amplitude settings, IIa and even IIb fibers can be reached without the athlete working to maximal exhaustion. Genetic fiber-ratio can only be modified within limits, but IIa “intermediate” fibers can be shifted toward I or IIb — the training goal (endurance / strength / hypertrophy) determines this.
Muscle fiber types — what makes them different?
Skeletal muscle fibers form a functional and metabolic spectrum: from slow, oxidative (type I) fibers to fast, glycolytic (IIb/IIx) fibers. Between them lie the intermediate, IIa fibers — these are the most adaptable. The 2019 Moreillon study (PMID 30907516) reported that roughly 5% of fibers in the human vastus lateralis are “hybrid,” containing multiple myosin heavy-chain isoforms — evidence of fiber type plasticity.
Function: endurance, sustained low-intensity work (walking, postural maintenance, respiration). Metabolism: aerobic, many mitochondria, high myoglobin content (hence red color). Natural firing frequency: low (10–25 Hz). Fatigue: slow. EMS compatibility: low and medium frequencies (10–30 Hz) effectively target these fibers; suitable for recovery and postural-strength programs.
Function: intermediate — both endurance and strength. CrossFit, combat sports, canoe/kayak, middle-distance running. Metabolism: mix of aerobic and anaerobic. Natural firing frequency: 25–50 Hz. Plasticity: this is the most important group — with appropriate training they can shift toward I or IIb; the main substrate for modifying genetic fiber-ratio. EMS compatibility: medium frequency (25–50 Hz) targets these fibers; they are most commonly trained during sports preparation.
Function: explosive power, short-term maximal performance (sprint, weightlifting, jumping). Metabolism: anaerobic, rapid glycolysis, high glycogen. Natural firing frequency: high (50–80 Hz). Fatigue: rapid. EMS compatibility: high frequency (50–80 Hz) targets these fibers; used in strength and hypertrophy programs. The 2025 Fessard et al. animal study (PMID 39910613) showed that 12 weeks of NMES training induced myonuclear accretion and hypertrophy selectively in IIB fibers — an important explanation for why athletes use EMS for maximal strength development.
Henneman's size principle — why does it matter?
The Henneman size principle (Elwood Henneman, 1965) is the central physiological rule of voluntary muscle activation. Its essence:
- The nervous system recruits motor units by size — first the smallest (type I, slow) units, then progressively larger ones (IIa, and finally IIb).
- Recruitment increases in proportion to required force: the greater the force demand, the more (and larger) motor units are activated.
- Practically: IIa fibers typically begin activating above approximately 50% of maximal voluntary contraction (MVC); IIb fibers around ~75% MVC.
- This is an energy-efficient system — during natural movement always the smallest necessary muscle mass works.
The Henneman principle applies to voluntary movement. It also imposes an important limitation: you cannot voluntarily activate only IIa or only IIb fibers. If you want to sprint, your slow fibers engage first — the fast ones only join when force is high enough. That is why maximal strength development requires heavy loads and low repetitions: only then is the ~75% MVC threshold reached.
How does EMS bypass the Henneman principle?
EMS's most valuable property is that the electrical impulse directly stimulates motor nerve fibers, bypassing the central nervous system's “economy” regulation. Thus, parameter selection determines which fiber types are activated:
| Frequency (Hz) | Targeted fiber type | Physiological effect | Typical application |
|---|---|---|---|
| 1–10 Hz | Type I (short contractions, circulation) | Increases microcirculation, removes metabolic waste, recovery | Post-workout recovery, chronically fatigued muscle |
| 10–20 Hz | Type I (below tetanic range) | Slow, sustained muscle tone, postural muscle activation | Deep back muscles, thigh and abdominal muscles — posture improvement |
| 20–50 Hz | Type I + IIa | General muscle strengthening, tone increase | Home muscle strengthening, body shaping |
| 50–80 Hz | IIa + partially IIb | Hypertrophy, explosiveness, neuromuscular adaptation | Sports preparation, professional athletic training |
| 80–100 Hz | IIb / IIx | Maximal force, neural development | Only briefly, on well-trained muscles |
The 2022 Borzuola systematic review (PMID 35856620) confirmed that combining NMES with voluntary contractions (NMES+) produces greater long-term strength gains than passive stimulation alone or traditional strength training — precisely because the two activation pathways complement one another. The 2020 Murach review (PMID 32673155) treats NMES as a controlled model for inducing muscle hypertrophy, particularly because of its effects on IIB-type fibers.
Genetic predisposition and EMS — what can and cannot be changed
Your innate muscle fiber ratio is a key determinant of sporting performance. Dominant type I fibers favor endurance sports (marathon, cycling, triathlon); dominant IIb fibers favor strength and explosive sports (sprinting, weightlifting, throwing). This typology is partly hereditary — classic studies indicate the primary I/IIb ratio changes only within limited bounds across a lifetime.
But the system is not completely rigid:
- IIa fibers (intermediate) are plastic. Training direction (endurance vs strength) determines whether these intermediate fibers shift toward I or IIb. This is the main message of the Moreillon study mentioned above.
- With aging, a IIb→I shift is common. The 2022 Coletti review (PMID 35234025) reports that with age IIb fibers denervate and are reinnervated by slower motoneurons — resulting in sarcopenia and slower, more tiring movement. Regular exercise (and supplementary NMES) can significantly slow and partially reverse this process.
- EMS can selectively target IIa fibers, since frequencies between 25–60 Hz coincide with the natural firing ranges of IIa motoneurons. This means stimulation can elicit muscle behavior that would not be reached voluntarily without surpassing the ~50% MVC threshold.
What cannot be changed by EMS or any training:
- A purely type I fiber will not turn into type IIb, and vice versa.
- An elite endurance athlete will not become an elite weightlifter — and vice versa.
- Maximal muscle mass gain and maximal endurance cannot be developed simultaneously — priorities must be set at any given time.
The “goal trade-off” is real: endurance training slightly reduces strength and mass, while hypertrophy-focused training reduces endurance. Within this context EMS is a valuable fine-tuning tool: aligned with the chosen goal it can focus on the appropriate fiber type.
Structure of a well-designed EMS treatment
An effective EMS protocol consists of three phases — just like a conventional training session:
- Warm-up (3–5 minutes): low frequency (3–10 Hz), mild intensity. Goal: prepare circulation, gradual muscle activation.
- Main work phase (10–20 minutes): frequency and intensity according to the goal (endurance 25–35 Hz, strength development 50–80 Hz, hypertrophy 60–80 Hz).
- Cool-down (3–5 minutes): return to low frequency to support recovery and metabolic clearance.
A full session typically lasts 15–30 minutes. Weekly frequency: beginners 2–3 sessions per targeted muscle group; advanced users may progress to daily application on alternating muscle groups. A given muscle group requires 24–48 hours of recovery between repeated intense loading — same as traditional resistance training.
Electrode placement principle: place one electrode on the muscle belly (the most bulging point of the muscle) and the other near the muscle origin, aligned with the direction of the muscle fibers. Do not cross electrodes and avoid joint lines or the spine. Details: electrode polarity and pad selection.
When should EMS NOT be used?
EMS is generally well tolerated when using a CE/MDR-certified device and following the user manual. However, home use is not recommended in the following situations. The full list: electrotherapy contraindications.
- Implanted electronic device (pacemaker, ICD, nerve stimulator) — see: implants and electrotherapy
- Acute deep vein thrombosis, recent vascular inflammation
- Active or unknown tumor in the treated area
- Epilepsy — specialist consultation required
- Acute fever, infectious condition
- Dermatitis, wounds, fresh surgical incision in the treatment area
- Pregnancy (lower abdominal and lumbar regions) — consult a physician
- Anterior triangle of the neck (carotid line) — electrode prohibition
Summary — key takeaways
- Human skeletal muscle consists of three main fiber types: I (slow, endurance), IIa (intermediate, plastic), IIb/IIx (fast, power-oriented).
- Voluntary movement recruits fibers according to Henneman's principle: small (I) fibers first; IIa around ~50% MVC; IIb around ~75% MVC.
- EMS, via frequency selection, can partially override this order: 25–60 Hz targets IIa; 50–80 Hz reaches IIb fibers.
- Genetic fiber-ratio underlies an individual's sport profile; IIa plasticity allows limited modification of the ratio.
- Session structure: warm-up (3–5 min) → main work (10–20 min) → cool-down (3–5 min). Aim for 2–3 sessions per muscle group per week.
- 2020+ clinical evidence (Fessard 2025, Borzuola 2022, Murach 2020) supports EMS-based mechanisms for hypertrophy and strength gains.
Device tip with wide program range: Globus Genesy 1500 — many preset programs, fine parameter control, suitable for sport and rehab indications. Find Globus devices dedicated to athlete preparation here. Full EMS portfolio: EMS main hub.
Frequently Asked Questions
Partly, yes. EMS acts directly on motor nerve fibers, so the central nervous system’s “small fibers first” rule does not fully apply. High frequency (50–80 Hz) directly stimulates IIa and IIb motoneurons, bypassing the voluntary force threshold. However, this does not mean that small (type I) fibers are not activated — typically all fiber types are involved during a stimulation session, but frequency selectively favors some.
Only to a limited extent. Pure type I fibers will not become type IIb, but IIa (intermediate) fibers are plastic — depending on training direction (traditional or EMS-assisted) they can shift toward I or IIb. Full transformation is genetically constrained; Coletti 2022 (PMID 35234025) also shows age-related fiber-type shifts (IIb→I, sarcopenia), and training mainly slows this process.
There are several reasons. First, fiber-type distribution differs between muscles (e.g., soleus ~80% type I, triceps brachii ~60% IIa+IIb). Second, muscle mass and subcutaneous fat vary, affecting electrode-to-muscle distance. Amplitude, electrode size and pulse width must be adjusted to the specific muscle for effective results.
In traditional training, due to Henneman's principle, activation always starts with small fibers and larger (IIb) fibers only engage under high force — requiring fatiguing work. With EMS you can select parameters to favor larger fibers: high frequency (50–80 Hz) can reach IIa–IIb fibers even at lower voluntary intensity. That is why Borzuola 2022 (PMID 35856620) supports the combined NMES + voluntary training model for consistent strength gains.
Yes — particularly so. Coletti 2022 (PMID 35234025) reports that with age IIb fibers denervate and are reinnervated by slow motoneurons, underlying sarcopenia. Regular NMES (combined with exercise and nutrition) can slow this process and help reactivate IIa–IIb fibers. Specialist consultation and personalized protocols are of course necessary.
Yes. The 2025 Fessard animal study (PMID 39910613) demonstrated myonuclear accretion and hypertrophy — specifically in IIB fibers — after a 12-week NMES program, without signs of overt muscle damage or regeneration. Similar mechanisms operate in humans: regular, appropriately intense and frequent NMES combined with voluntary training strengthens and increases muscle size. Passive EMS alone is less effective than the combined approach.
Related articles
Scientific sources (2020+)
- Fessard A, Zavoriti A, Boyer N, Guillemaud J, Rahmati M, Del Carmine P, Gobet C, Chazaud B, Gondin J. Neuromuscular electrical stimulation training induces myonuclear accretion and hypertrophy in mice without overt signs of muscle damage and regeneration. Skeletal Muscle. 2025 Feb 5;15(1):3. DOI: 10.1186/s13395-024-00372-0 · PMID: 39910613
- Coletti C, Acosta GF, Keslacy S, Coletti D. Exercise-mediated reinnervation of skeletal muscle in elderly people: An update. European Journal of Translational Myology. 2022 Feb 28;32(1). DOI: 10.4081/ejtm.2022.10416 · PMID: 35234025
- Borzuola R, Laudani L, Labanca L, Macaluso A. Superimposing neuromuscular electrical stimulation onto voluntary contractions to improve muscle strength and mass: A systematic review. European Journal of Sport Science. 2023;23(8):1547-1559. DOI: 10.1080/17461391.2022.2104656 · PMID: 35856620
- Murach KA, McCarthy JJ, Peterson CA, Dungan CM. Making Mice Mighty: recent advances in translational models of load-induced muscle hypertrophy. Journal of Applied Physiology. 2020;129(3):516-521. DOI: 10.1152/japplphysiol.00319.2020 · PMID: 32673155
This article provides general information and does not replace specialist medical consultation. Home use of EMS is recommended with CE/MDR-certified medical devices and according to the user manual. For sports preparation, use under sports medicine or physiotherapist guidance; in rehabilitation, apply under supervising physician oversight.
