On a recent Sunday, the fitness instructor Carl Hall was at the Crunch gym in the Fort Greene neighborhood of Brooklyn leading his regular 10 a.m. spin class. The dark, stuffy room held 53 stationary bikes, every one of which was occupied by a sweating cyclist in workout clothes.

“Move it up, move it back. Move it up, move it back,” Mr. Hall ordered calmly but firmly through a headset over a dance remix of “Rolling in the Deep,” by Adele. “Lower, lower, lower, lower.”

He was presiding from a tiny stage with a bike on it at the front of the studio, sometimes pedaling along, sometimes jumping off to patrol the digital monitors affixed to participants’ handlebars. As his glow-in-the-dark baseball cap loomed up suddenly behind them, they would pedal with renewed fervor. Despite, or perhaps because of, their simplicity, Mr. Hall’s mottos — “Today!”; “Life, not life support!” — have been known to elicit grateful tears.

“That’s fat knocking,” he may say, banging on a wall. “Are you going to let fat knock on your door?”