As far as indoor cycling machines go, the $2,245 Peloton bike is nothing special. It has a sleek black and red frame. It has a big screen. It’s on Wi-Fi.

But a combination of aspirational infomercials (“This … is fitness evolved.”) and streaming classes taught by glamorous instructors has led Peloton to sell 577,000 of its bikes and treadmills in five years. Richard Branson is a fan. So are Jimmy Fallon, Kate Hudson and the Obamas.

Now as Peloton prepares to go public, the New York City-based company — which investors have privately valued at $4 billion — is facing questions about how long it can stay on top. Fitness is a historically faddish category. Exercise manias, from the Thighmaster to Tae Bo, have all come and gone. SoulCycle pulled its initial public offering entirely.

For Peloton, some troublesome signs have emerged. The company’s losses have more than quadrupled in the last year. It is embroiled in legal fights over music and patents. Competitors and copycats are moving in aggressively. And the boutique spinning craze has started to wane.