Can an EMS muscle stimulator build muscle?

Short answer

It can, and stronger stimulation grew more. 8 weeks of training made front thigh muscle 6% thicker, half-strength stimulation grew less, and a store-bought device used as directed changed neither strength nor shape.

Can an EMS muscle stimulator build muscle?
'Stick on the pads and sit still → more muscle' — does this formula hold?
Put pads on your belly or thighs and electric pulses make the muscle twitch on its own. Ads promise muscle and a flatter belly with no workout. We checked the research step by step to see whether muscles actually grow.
A device used as directed did nothing
Strength, body fat and looks all stayed the same
16 of 27 people used a store-bought device on their thighs, arms and belly 3 times a week, following the manufacturer's directions. Compared with 11 who got fake stimulation, nothing changed: not weight, body fat, girth, strength, or appearance in photos.
But strong, regular use did grow muscle
Front thigh muscle got 6% thicker in 8 weeks
12 people did 32 sessions of electrical stimulation training on their thighs over 8 weeks. Front thigh muscle size rose 6% and leg-straightening strength rose 27%. At week 4, muscle size had not changed yet.
The difference was stimulation strength
At half the intensity, muscles grew less
8 people stimulated one leg at the strongest level they could tolerate and the other at half that, for 8 weeks. The strongest level equaled an average 62.5% of their own max force. Both legs grew, but the weaker side grew less, and stronger stimulation meant more muscle and strength.
Ab belts built strength, not a flatter belly
Waist down 3.5 cm, fat thickness unchanged
24 people stimulated their abs 5 days a week, 20-40 minutes a session, for 8 weeks. Ab strength rose 58% and waist size dropped 3.5 cm. But body weight and the fat layer under the skin did not change. The belly got firmer; the belly fat did not go away.
How does it compare with lifting?
It can stand in, but it wasn't better
40 people did lifting, electrical stimulation, or both for 8 weeks. Upper-arm muscle thickness grew in all three groups, with no difference between them. An analysis that pooled 13 studies found that adding stimulation to lifting gave slightly more strength and muscle.
Bottom line: intensity is what builds muscle
As strong as you can tolerate, for 8+ weeks
An analysis of 26 studies and 1,183 people also found more muscle with whole-body electrical stimulation. But a store-bought device used as directed did nothing, and half-strength stimulation grew less. Think of it as a stand-in when you can't lift, or an add-on to lifting.