Electrical stimulation (EMS), also known as Neuromuscular Electrical Stimulation (NMES), can build muscle. This technology has moved beyond clinical rehabilitation into athletic and general fitness settings. EMS uses a device to deliver controlled electrical impulses directly to the muscle belly or motor nerve. While it is not a perfect substitute for traditional voluntary exercise, research shows that when used correctly, it can induce powerful muscle contractions that lead to physiological changes.
How Electrical Stimulation Works
Electrical muscle stimulation operates by bypassing the central nervous system (CNS), which initiates muscle movement. A device generates an external electrical impulse delivered through electrodes placed on the skin over a muscle group. This current directly stimulates peripheral motor nerves, causing muscle fibers to depolarize and contract involuntarily.
The resulting contraction differs from voluntary exercise because the fiber recruitment pattern is fundamentally different. Natural exercise follows the size principle, recruiting smaller, slow-twitch fibers before engaging larger fast-twitch fibers. In contrast, EMS activates motor units non-selectively and synchronously.
This non-selective recruitment often results in the immediate activation of larger, fast-twitch muscle fibers. This ability to recruit a greater number of high-force motor units simultaneously provides a potent training stimulus without CNS involvement.
Building Strength vs. Building Mass
The physiological outcomes of electrical stimulation show distinct benefits for both strength and muscle mass. Strength gains are consistently observed, especially in clinical populations where EMS prevents disuse atrophy following injury or surgery. In healthy individuals, EMS training alone can produce strength improvements comparable to traditional resistance training when volume and intensity are matched.
The mechanism for strength improvement is primarily neural, including enhanced motor unit activation and an increased firing rate. Building muscle mass (hypertrophy) is a more challenging outcome for EMS alone but remains possible with high-intensity protocols. Hypertrophy is driven by promoting muscle protein synthesis and potentially stimulating satellite cell regeneration.
Achieving significant hypertrophy is directly linked to the intensity of the induced contraction. Applied with sufficient current, EMS forces a muscle to contract with a greater magnitude than a person might achieve voluntarily. Combining electrical stimulation with voluntary exercise is often the most effective method for maximizing both strength and mass simultaneously.
Supplementing Voluntary Exercise
Electrical muscle stimulation is an effective tool for supplementing, rather than replacing, conventional voluntary resistance training. While it produces significant muscular adaptations, it lacks the systemic benefits that traditional exercise provides, such as improved cardiovascular fitness or enhanced coordination. The primary role of EMS is to enhance or target specific training adaptations.
Superimposing EMS onto voluntary contractions, such as performing a squat while muscles are stimulated, creates an additive effect. This combination leads to greater gains in strength and muscle mass than voluntary exercise alone because it maximizes fiber recruitment. EMS is also invaluable for targeted muscle groups or during recovery where heavy voluntary loading is not feasible. Its joint-friendly nature makes it an attractive option for high-intensity training without mechanical stress on tendons and ligaments.
Safety Considerations and Contraindications
While electrical stimulation is a powerful tool, users must be aware of certain safety considerations.
Contraindications
Absolute contraindications include the presence of a cardiac pacemaker or any other implanted electronic device, as the electrical currents can interfere with their function. Individuals with epilepsy, active infections, or thrombosis (blood clots) should also avoid EMS. Pregnant women are generally advised against using EMS, particularly on the abdominal area.
Side Effects
Minor side effects include skin irritation or burns at the electrode sites, which can be mitigated by ensuring proper electrode placement and skin preparation. Intense EMS protocols can lead to elevations in serum creatine kinase, an enzyme that indicates muscle damage, and in rare cases, may trigger rhabdomyolysis.
It is recommended to consult a physician before beginning any EMS training, especially for individuals with pre-existing medical conditions, and to only use devices cleared by regulatory bodies.