Celebrity trainer reveals a common mistake people make while working out

A celebrity trainer says the boutique fitness trend is causing many people to waste their time.

"There's just so much expansion happening in that area of fitness and not everyone is doing it well, because it's hard to create a comprehensive program," Anna Kaiser, train to stars like Kelly Ripa and Sarah Jessica Parker and founder of boutique studio AKT inMotion told Business Insider. "It's easy for people to get addicted to one thing, and that's where they can go wrong."

"All of these companies were really focused in one modality, so you have to go to three different studios to get a comprehensive workout," she said. "It's really frustrating and kind of a waste of time."

She says her studio aims to rectify that with a mix of classes, from dance cardio to interval circuit training. Further, she's pretty clear that if people do the same thing over and over again, it's not going to benefit them - and it's a waste of their time, especially if they're forking over lots of money.

For instance, she advises against cycling every day.

"Now that people are doing it [indoor cycling] five or six times a week or more ... the majority of the work is being done in their quads and hamstrings, their knee is constantly in a state of flexion, they're never straightening behind their knee, there's no core or arm work that's really doing anything for your body," she said. "You're just working [the same] muscles over and over again."

Kaiser also emphasizes the importance of wearing a heart rate monitor and getting one's heart rate up; that, ultimately, is how you know if you're getting a solid work out or not. (To emphasize the importance of this, Kaiser sells and lets people rent heart rate monitors at her New York studio, and has a leaderboard - much like Flywheel - that tracks clients' heart rates.)

It's "really important if trying to figure out - 'how hard am I working?' Get a heart rate monitor," she said. You "have to work hard enough for your body to sweat."