What it used to be: Lotte Berk Method. Originally developed by the once famous European dancer Lotte Berk in the late ’40s, the Lotte Berk Method is a unique fusion of ballet, Pilates, and sculpting. The technique made its way to the U.S. in 1971, when an American named Lydia Bach opened the first studio in Manhattan’s upper east side.

After studying with Berk in London for a year, Bach was so taken by the method’s unique results that she purchased the rights to Lotte Berk’s name and technique to bring the ’70s workout to the States. Now the class described as “an all-round fitness program that will yield muscular strength, beautifully sculpted bodies, flexibility, and caloric burn” is at the root of popular workouts like Pure Barre, Exhale’s Core Fusion, and other barre-inspired trends.

Why is it still a hit? Because it works (and it can make you seriously sweat!), says Suzanne Bowen, a certified Lotte Berk instructor and creator of BarreAmped. With a heavy focus on the abs, hips, thighs, and glutes, this high-repetition, small-pulse method of movement is a ’70s workout that stands the test of time. It burns calories, builds lean muscle, and improves balance, coordination, and flexibility.