A short walk across 57th Street from Destination Maternity, where shoppers can find designer jeans and wrap dresses to fit their baby bumps, is Physique 57 — an exercise studio that has built its reputation, in part, on helping new mothers get back their pre-pregnancy bodies.

Less than two years ago, a former investment banker, Jennifer Vaughan Maanavi, together with a fitness instructor, Tanya Becker, opened Physique 57 on West 57th Street. There, they began offering classes that borrowed heavily from the Lotte Berk method, a strengthening and stretching regimen devised decades ago by its ballet-dancing namesake. Thanks to word-of-mouth marketing, the Physique 57 concept was a near-instant success. Ms. Maanavi and Ms. Becker have since launched two additional exercise studios, one in Bridgehampton — open between Memorial Day and Labor Day — and another on Spring Street in Lower Manhattan.

Together, the two city-based locations offer about 140 hour-long classes a week. They attract students in their 20s, as well as those in their 70s, but are particularly popular with young mothers. Devotees reportedly include boldface names such as actress Sarah Jessica Parker and talk-show host Kelly Ripa.

What does the much-hyped workout entail? Unlike other now popular exercise regimens that tout their Zen or meditative qualities, Physique 57 is a fast-pace, high-energy endeavor. Workouts are done to rhythmic music — during a recent class I took alongside 15 other women and one man, we worked out to boom-boom remixes of “Let the Sunshine In” and “Drift Away”— and led by vivacious instructors who wear microphone-enabled headsets.

Exercises are performed on a studio floor covered in shag carpeting, and at a ballet bar that surrounds the mirrored studios — illuminated by natural and track lighting. The basic routine begins with arm- and shoulder-toning motions, executed with 3-, 5-, or 8-pound weights. This is followed by five minutes of stretching, including gymnastics-style splits, and then about 20 minutes of exercises, done in quick succession, targeting the legs and rear. The lower-body workout is made up of classic squats and standing leg lifts, adapted to include Lotte Berk staples, such as a ballet bar, a rubber ball, and a cotton strap that resembles a karate black belt.

The abdominal work that rounds out the workout also incorporates those props and is the most taxing part of the hour; the exercises require you to hold your back off the floor at a 45-degree angle for minutes at a time.

Almost all of the classes follow the same format, though advanced classes involve less explanation and resting time. Ms. Maanavi said that most clients begin to see a real change in their bodies after about 10 classes taken over the course of about three weeks. But a Physique 57 devotee since November, Sarah McTeigue, 45, said she saw results after the first class, and after five weeks had dropped two jeans sizes — and inches from her thighs, hips, waist, and arms. “You walk out of a class, and you feel like you’ve been vacuum-sealed,” she said.

In addition, Physique 57 recently began offering two supplementary classes: “Mat 57,” a mat-based, Pilates-like workout, and “Physique Yin,” a stretching class. For $235, clients can purchase a “Sexy in the City” package that incorporates five basic classes, three mat classes, and one stretching class — and comes with a goody bag full of Bliss- and Becca-brand beauty supplies for $235. Prenatal classes are also on offer at the studios.

Physique 57, 24 W. 57th St., between Fifth and Sixth avenues, 212-399-0570; 161 Sixth Ave. at Houston Street, 212-463-0570; group classes, $33 each, packages available; annual membership, includes unlimited classes, $4,100.