By Samuel Burke, CNN Business

(CNN) -- "Keep your elbows as close to your sides as possible," the personal trainer tells me as I start my last set of push-ups.

My arms feel like they're about to buckle onto my hotel room floor, but Mike pleads with me not to give up. "Only a few seconds left!"

I can hear Mike cheering me on, but I can't see him. In fact, I've never even met the guy.

I'm working out through Aaptiv, an app with dozens of coaches who lead pre-recorded, audio-only workouts. The app, which costs $14.99/month or $99.99/year, features a variety of beginner, intermediate and advanced workouts.

About 200,000 users work out with Aaptiv, and the startup is growing fast. Last week the company announced an international expansion of the English-language app to 20 countries, and it plans to move into local languages in 2019.

Aaptiv is valued at $200 million and is backed by investments from Amazon, Disney, Warner Music and Bose. But the industry is crowded: An endless amount of workout apps and YouTube videos target people who don't want to fork over the cash for a personal trainer.

So Aaptiv hopes to stand out with its audio-only format. Founder and CEO Ethan Agarwal came up with the idea after identifying a need in his own workouts.

"I needed some kind of coaching that would work while I was running," Agarwal told CNN Business. "The problem with videos is that I can't run down the street staring at my phone. That's why I started Aaptiv."

The app goes far beyond running: Courses are available for everything from yoga to strength training, voiced by a coach with current music playing in the background. The routines are designed for different settings like the gym, outdoors and home, and some require equipment like weights or a treadmill while others need no props.

The convenience and variety are big draws, but Aaptiv's audio-only format poses a major challenge: Because users can't see the exercises, they must rely solely on the trainer's verbal explanation to ensure they're doing the workout correctly.

I experienced this problem whenever I tried a new workout. I understood the vast majority of the exercises, but about once a routine I would find I wasn't doing the movements the coach intended. And occasionally I was way off.

Correct form for a workout is key to avoid injury, noted Dr. Daniel Yadegar, a cardiologist at Weill Cornell Medical College who also uses Aaptiv. But he added, "there are many people who work out on their own at the gym, without supervision, who take a similar risk. If used properly and with consistency, apps like this can be an great tool in achieving fitness goals."

The app's "very vocal online community" in its 32,000-strong Facebook page helps mitigate the risk of confusion and improper form, Aaptiv trainer Ackeem Emmons told CNN Business.

"If one person has a question about a particular exercise, then almost always lots of other users have the same question," Emmons said. "And that tells us how we need to improve."

Users can also weigh in at the end of each exercise, as the app prompts them to rate the workout and leave feedback for Aaptiv's trainers.

Those 22 trainers -- five full-time employees and 17 contractors -- develop their workouts with a quality-control team and record the audio sessions in a New York recording studio.

About 4,000 personal trainers have applied for positions at Aaptiv, said CEO Agarwal.

"I heard about Aaptiv from a friend, and I thought these guys might put me out of business one day," said Emmons. "So instead of waiting for that to happen, I applied to work with them."