Like many group fitness workouts, at first this one sounds like a ridiculous idea: Take what is by far the least popular cardio machine in the gym — one that involves sitting down, endlessly sliding back and forth — and devote an entire class to it.

Yes, the dusty old rowing machine has been plucked from the far corner, emerging in group workouts and boutique studios across the nation, and drawing a crowd of fierce loyalists, many of whom never have and never will pick up an oar.

Ericka Sullivan, a fan of barre-method toning classes, resisted her advertising-executive husband’s pitch for a year that they try a class at the 18-month-old GoRow Training Studio in Hoboken, N.J.

“There’s just nothing enticing about being bored, and rowing sounded kind of boring,” said Ms. Sullivan, 35, who gave in as bathing-suit season neared. “But the intervals go by so fast.” Her fears of “huge rower shoulders” also haven’t materialized: “I’m longer and leaner,” she said.