Shivani Vora, New York Times

NEW YORK » "Seven Nation Army" by the White Stripes was pulsating through the dimly lit Ab Attack class at Crunch gym near Union Square on a recent morning. Jewlie Williams, 25, a fashion stylist, was in front of the packed room, wearing a tank top and leggings and following the instructor in a 30-minute nonstop sequence of standing crunches with high knees, planks and toe touches with legs raised: all exercises focused on developing the muscles of the midsection.

Even for those oblivious to fashion, it’s hard to ignore that midriffs are suddenly in America’s face — in a way not seen, perhaps, since a young Britney Spears was in regular gyration-rotation on VH1. Crop tops were all over the spring runways, from Proenza Schouler and Dolce & Gabbana to more moderately priced lines like Tibi and Alice & Olivia. They are stocked several racks deep at Zara, H&M and Forever 21.

And they are a focal point of current red carpets: at The Metropolitan Museum of Art’s annual gala this month, to which, despite the white-tie dress code, Rihanna wore a daring backless top by Stella McCartney with matching long skirt; Anne Hathaway, a red Calvin Klein bustier with a skirt; Emma Stone, a similar ensemble by Thakoon but in two shades of pink; Imogen Poots, a Proenza bra-dress; and model Cara Delevingne, a black halter over white pants (also by McCartney). Each woman had at least an inch of belly proudly exposed.

Perhaps these stars were emboldened by other recent ab-skimmers, like Taylor Swift’s short-sleeve white silk crepe J. Mendel at the Academy of Country Music Awards in early April, or the green Topshop number that Mindy Kaling sported at PaleyFest 2014, or the oasis of midriffs at Coachella, including Emma Roberts’.

Mary Alice Stephenson, a fashion commentator, thinks the look now evokes refined elegance rather than the overt sexiness or exoticism it used to signal (see: "I Dream of Jeannie"). "The stomach is the new erogenous zone, but not in a vulgar sort of a way," she said. "Yes, you can show your whole midsection in a bra top, but most of the styles only give you a peek. Regardless, it is making women frenzied about shaping up their abs."

The 5-foot, 115-pound Williams, for example, spends more than two hours most days of the week exercising in stomach-centric classes like Ab Attack (Crunch offers three others as well), running and dancing. Her day begins and ends with 100 crunches, she said, and she has drastically cut down on her sugar intake.

"I bought six crop tops, but I felt like I needed tighter and flatter abs to feel good wearing them," she said, "so I’ve been working really, really hard to get them better-looking."

At SLT, a studio with six locations in New York and New Jersey that offers Pilates-style workouts, attendance was up 20 percent in March and April. "The buzzword in all the classes is crop top," said Amanda Freeman, SLT’s founder.

Physique 57, the ballet barre-based workout studio with three locations in Manhattan, has 25 percent more visits to its Mat 57 classes, which target the middle, than last year; many classes have waiting lists of 20 and more have been added to meet the demand, said Jennifer Billett, a company spokes­woman.

And Josh Holland, the Technogym "fitness ambassador" who runs the fitness center at the members-only Core club in Midtown Manhattan, says that women constantly ask for classes and personal training sessions that work just the middle.

Midori Repp, 33, a social worker, visits Physique 57 daily, motivated in part by the three crop tops in her closet. "When I saw that they were going to be a trend I was into, I decided I wanted more visible stomach definition," she said. "I wear them all the time, and because of all the work I’ve put in, I feel good about how my middle looks."

Ever since the advent of the bikini, women have experienced seasonal anxiety about their stomachs. And some feel that in more formal situations, full disclosure is viewed with a less forgiving eye.

Sandra Ciconte, 37, an advertising executive, has no hesitation about wearing a two-piece hanging out by a pool, but she became self-conscious about the red Isabel Marant crop top she recently bought.

"I never felt like I had to have the perfect abs in a bikini," she said, "but a crop has different pressure."

Ciconte, an already-slender 5-6, has embarked upon a six-day-a-week routine that includes two private sessions with Holland at the Core club and a 20-minute floor routine by herself on other days.

Dr. Michele Olson, an exercise physiologist at Auburn University, stressed that cardio intervals such as alternating sprinting with walking are the first step to a six-pack. "You need to have less fat overall to have firm abs," she said, "not do hundreds of crunches or situps."

Neither approach was enough for Nicole Abrahamson, 24, of Camarillo, Calif., a nursing student who spent more than $6,000 in March on a minimally invasive fat-removal technique called Airsculpt to address 4 pounds of unwanted belly. She says that even though she exercised and dieted, her middle was "doughy." "I was really intrigued by crops," she said, "and wanting to wear one gave me the push to get this procedure done."

But Kate Young, who helps to outfit Miranda Kerr, Natalie Portman and Michelle Williams, stressed that being movie-star slender is not a criteria for the style. "If you have a lot of weight around your middle, then it’s probably best to avoid the crop, but showing a peep of skin works for many bodies," she said, suggesting balancing the sexiness of a crop with a high-waist pant or pencil skirt as Kaling did.

Amy Smilovic, the founder of Tibi, says that the 10 crops in her spring line are meant to be balanced with fuller bottoms. "I wear them but never without a blazer and billowy skirt or wide culotte," she said.