Those results are likely caused by the type of contractions the HIFEM machine triggers. The device throws magnetic impulses that stimulate your nerves, causing all your deep gluteus muscles to shorten. And just like with a workout, that causes fibers to break down and rebuild bigger and stronger. But these aren’t like regular contractions. They’re supramaximal contractions, which create more tension and require more energy than a normal booty squeeze, according to Bruce Katz, MD, clinical professor of dermatology at the Icahn School of Medicine at Mount Sinai and director of the Juva Skin and Laser Center in New York.