Remember the buzzy boutique fitness classes that soared in the mid to late-2010s? There was a time when you couldn’t skim a stone down 23rd Street in Manhattan without it ricocheting off three different studios, each offering their unique spin on high-intensity interval training, or HIIT.

I pretty much lived in those studios, hitting three or four classes a week—oftentimes more. But now? Those classes just feel like, well, a lot. Honestly, I get tired just thinking about it. And it turns out, I’m not the only one.

According to a 2025 report from the Sports and Fitness Industry Association, participation in high-intensity interval training (HIIT) and bootcamp-style workout classes has been slipping since 2019, while functional fitness and low-impact workouts like Pilates and yoga have been gaining ground.

“You’re definitely hearing it and seeing it, along with the rise of strength and Pilates,” says Sam Tooley, a performance coach and the co-founder and CEO of group fitness studio Alpha Fit Club. “I think they’ve given what appears to be the same customer base that would have attended HIIT classes, which were way more prominent, a different alternative that is resonating.”

While it’s normal for fitness trends to come and go, industry experts think there’s something else going on when it comes to the drop-off of interest in HIIT workout classes.

“HIIT sessions tax both our aerobic and anaerobic capacity, while also putting a significant physical load on multiple muscle groups at the same time,” says Jesse Shaw, DO, USAW, sports medicine physician and associate professor of sports medicine at the University of Western States. “Due to the unique stress and the often heavy and intense movement required, people burn themselves out.”

But there’s nothing inherently wrong or unsustainable about HIIT workouts. In fact, with benefits to V02 max, cardiovascular health, and overall athletic performance, it’s one of the more valuable components of any well-rounded fitness regimen. So we talked to the experts to find out why people are losing interest, and how you can take advantage of everything these workouts have to offer—without burning out.

Make sure your ‘HIIT class’ is actually a HIIT class

Turns out, that boutique studio workout you’ve come to know as HIIT might not be HIIT at all, but rather a watered-down, less effective version that’s probably putting unnecessary strain on your body.

“What most workout classes are calling HIIT is not really HIIT,” says Danny King, director of performance and recovery at Life Time. “High-intensity interval training, in scientific terms, is short-duration, high-intensity intervals—with long rest periods,” the latter being a key component,” he says. “If you aren’t resting enough, the intensity drops, not allowing a person to push hard enough to create the stimulus we are looking for.” In many popular workout classes that advertise themselves as HIIT, rest periods are typically between 30–60 seconds long, with work periods sometimes lasting more than twice that.