Shocking your body with electric currents might seem like a sweat-free way to tone your abs — but the fitness trend is starting to worry experts.

The process is called whole body electrical muscle stimulation, often abbreviated as ES or EMS. Electrodes stuck on the abdomen, chest, and legs deliver a current that shocks the muscles into contracting, similar to how they’d tense up during a workout.

Sounds simple, but there are hidden risks: Doctors writing in a recent BMJ editorial describe a 20-year-old man who needed hospital treatment for five days after suffering severe rhabdomyolysis, or breakdown of muscles. He’d undergone a session of whole body electrical stimulation under the supervision of a fitness professional.

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“Moderate muscle damage can be positive for strengthening muscles,” said Nicola Maffiuletti, a rehabilitation specialist at the Schulthess Clinic in Zurich and one of the editorial’s authors. But with DIY electrical shocking, “there can be very heavy muscle damage,” he warned.

Electrical stimulation has long been used in physical therapy to prevent muscle atrophy in patients who can’t exercise. In the last two decades, personal devices have promoted the same concept as a shortcut to a toned physique. The devices can be pricey — the Compex Muscle Stimulator can run up to $1,149. But there are also options for bargain hunters, like the Portable Dual Channel Muscle Stimulator that’s under $25.

The FDA regulates individual devices that make specific health claims. But the agency doesn’t oversee the procedures used with whole body electrical stimulation, because that falls under the “practice of medicine,” which the FDA legally cannot regulate, spokesman Eric Pahon said.

That’s dangerous, said Maffiuletti, because it means there’s no regulatory authority keeping tabs on how strong the electrical currents are, or how much of a dose is acceptable. Finding the proper dosage isn’t easy, either.

“If we don’t give enough current to the muscles, we don’t get any benefit,” he explained. “But for people with a high pain threshold, we are overdosing.”

Delivering too strong an electrical current can cause muscle damage and even, in some cases, destroy muscle cells. Maffiuletti equated the potential muscle damage to what would happen if a person ran for 10 hours straight without any endurance training.

“It doesn’t make any sense,” he said. “It’s oriented to healthy people for treatment, but for healthy people we know normal exercise is much more effective than whole body electrical stimulation.”

Few rigorous studies have been done on EMS. A small 2002 study gave 16 college students sessions with “body-shaper” devices, then compared them with 11 students who didn’t get the treatment, to see if EMS could increase muscle strength, decrease body fat, and boost muscle tone as touted. There was no evidence that EMS was effective.

The author of that study, John Porcari, a physiologist at the University of Wisconsin, La Crosse, said the devices were too painful for people to use for prolonged periods, so the muscle contractions weren’t strong enough to create any results. “It’s like going to the gym and lifting a 2-pound dumbbell. Are you going to get stronger?” he said.

Porcari later started using EMS devices on his own and conducted additional research — paid for by makers of EMS devices — that he said made him a believer.

After using one product for two months, he said, “I doubled how many curl-ups I could do, with no additional abdominal work.”

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