Whatever your mile time and however much you bench, tiny arm circles are the true test of fitness. Jane Fonda taught me that. Her aerobic fireworks got the glory, but she also proved that tight, repetitive movement can be as tough and transformative as the kind that leaves you breathless. Tiny arm circles don’t just make you feel the burn; they make you beg it for mercy.

Barre3 takes that idea and runs with it, though there’s no running involved. Yoga, Pilates and the barre work that makes dancers so enviably lithe inspired founder Sadie Lincoln, and barre3 faithful—celebrities to fitness magazines to my own best friend—swear by its power to build lean muscle, energy and grace while improving bad posture, back pain and stagnant physiques. The secret is a sequence of low-impact movement, recovery stretching and “isometric holds … combined with small range of motion to get deep into the muscles until failure.”

Muscle failure doesn’t sound appealing, but the results sure look good. As Jackie Edlund signed me in for a morning class at her new Henderson barre3 studio, I shamelessly stared at her triceps. She fell in love with the practice in Seattle, so she brought it with her when her family moved to Vegas.

“The barre3 principle is really about balance. … As a busy working mother and business owner it is so important for me to be balanced so that I can be the best version of myself,” said Edlund, who opened her studio March 19 and teaches some of the classes herself. “There are muscles that I never knew I had before starting barre3. It is such a great stress reliever, as well, because you stay present for 60 minutes just focusing on yourself and tuning out whatever else is going on. I always leave class feeling strong, refreshed and focused.”

Edlund’s barre3 outpost is Nevada’s first, though franchises based on the Portland original span more than a dozen states from coast to coast. There is an online platform, but it lacks the minimalist-chic atmosphere and the accountability that comes with working out in view of an instructor (and, incidentally, the Whole Foods parking lot).

In one hour with instructor Meredith Still, I pushed my body harder than I have in years. Pumped up by the music of Flo Rida, Rihanna and David Gray, we targeted arms, legs, seat and core, each series demanding balance, strength and flexibility but very little amplitude. Using the barre, 1- to 2-pound free weights, core balls and canvas straps, we isolated muscles and kept them engaged, the burn needling deeper. One static pose got our quads shaking, and instead of giving them a break we then rose and sank by a single inch over and over and over. We lifted our legs high behind us and squeezed core balls in the crooks of our knees until I could actually hear my glutes screaming. We did tiny arm circles! (With weights!) The room was quiet, but the collective strain was giving off heat.

“I love the shaking! Gorgeous!” Still exclaimed. She stressed the importance of listening to our bodies and modifying to our levels, but she also empowered us to push past the point where challenge is comfortable—to go the extra inch. “It’s so small, but it hurts like nothing you’ve ever done. What you’re doing is getting into that deep layer of muscle, so you’re engaged the whole time,” Still said. “If you’re not shaking, go deeper. Take it to your edge.”

During a final core shred, with the ball under my lower back and my entire torso lit with pain, Still saw me cheating and gently adjusted my form. My abs finally just gave out. I closed my eyes, accepted that I went as hard as I could. I’m so used to my sweaty cardio routine that I forgot what that feels like. In fitness, as in life, when we get too comfortable we stop growing. If we run the same trails and kickbox the same invisible ninjas all the time, our bodies have no reason to change. With barre3, they have countless tiny reasons. And it’s a beautiful thing.