Electrical muscle stimulation (EMS), also known as neuromuscular electrical stimulation (NMES) or electromyostimulation, is the elicitation of muscle contraction using electrical impulses. EMS has received attention for various reasons: it can be utilized as a strength training tool for healthy subjects and athletes; it could be used as a rehabilitation and preventive tool for people who are partially or totally immobilized; it could be utilized as a testing tool for evaluating the neural and/or muscular function in vivo. EMS has been proven to be more beneficial before exercise and activity due to early muscle activation.^{[clarification needed]} Electrostimulation has been found to be ineffective during post exercise recovery and can even lead to an increase in delayed onset muscle soreness (DOMS).

The impulses are generated by the device and are delivered through electrodes on the skin near to the muscles being stimulated. The electrodes are generally pads that adhere to the skin. The impulses mimic the action potential that comes from the central nervous system, causing the muscles to contract. The use of EMS has been cited by sports scientists as a complementary technique for sports training, and published research is available on the results obtained. In the United States, EMS devices are regulated by the U.S. Food and Drug Administration (FDA).

A number of reviews have looked at the devices.

Electrical muscle stimulation can be used as a training, therapeutic, or cosmetic tool.

NMES works by delivering electrical impulses that cause involuntary muscle contractions, mimicking the effects of voluntary exercise. In addition to directly stimulating muscle fibers, recent research has shown that NMES activates corticomotor pathways, engaging both peripheral and central nervous system structures. This process helps activate fast-twitch muscle fibers and promotes neural adaptations similar to those seen with voluntary high-intensity exercise.

In medicine, EMS is used for rehabilitation purposes, for instance in physical therapy in the prevention of muscle atrophy due to inactivity or neuromuscular imbalance, which can occur for example after musculoskeletal injuries (damage to bones, joints, muscles, ligaments and tendons). This is distinct from transcutaneous electrical nerve stimulation (TENS), in which an electric current is used for pain therapy. "The main difference is the desired outcome. TENS unit is a medical device for pain relief. The desired outcome is to reduce pain by stimulating different nerve signals. EMS fitness is also an FDA-cleared medical device but meant for muscle development. EMS fitness is designed to stimulate all the major muscle groups to elicit strength and endurance adaptations."

For people who have progressive diseases such as cancer or chronic obstructive pulmonary disease, EMS is used to improve muscle weakness for those unable or unwilling to undertake whole-body exercise. EMS may lead to statistically significant improvement in quadriceps muscle strength, however, further research is needed as this evidence is graded as low certainty. The same study also indicates that EMS may lead to increased muscle mass. Low certainty evidence indicates that adding EMS to an existing exercise programme may help people who are unwell spend fewer days confined to their beds. Additionally, neuromuscular electrical stimulation (NMES) has been shown to improve functional capacity, walking distance, and muscle strength in patients undergoing hemodialysis for end-stage renal disease, with no major adverse effects reported.

NMES has been found to be effective in treating certain upper and lower extremity issues post-stroke, weakness following ACL repair and total knee replacement, muscle weakness in knee osteoarthritis, and debilitation and weakness after critical illnesses. However, the benefit of NMES for patellofemoral pain syndrome (PFPS) remains uncertain.