Which online fitness challenges are the best? Personal trainers review the 80-Day Obsession, the Sweat app, Nike Training Club and more.

As a personal trainer and health coach who helps people learn to live their healthiest lives, I've seen a lot of what works and what doesn't. There are tons of online fitness challenges out there, but which ones actually work? How do they engage people to stick with it? Which ones are focused on overall health, and which plans are focused on simply a good workout?

I talked with some of the top fitness professionals who have experience with online challenges to see what they had to say. Here are some notes on the most popular online challenges that you can start today:

1. 80-Day Obsession

The 80-Day Obsession has taken the internet by storm. It's full of high-intensity workouts plus a meal plan that doesn't focus on counting calories. Instead, it tracks macronutrients like protein, fat and carbohydrates.

It's important to note these workouts are tough! This plan is fit for people who've been exercising for a few months or have a baseline level of fitness, although there are modifications for all levels. Trainer and creator Autumn Calabrese (who's also the creator of the 21 Day Fix) leads all of the workouts.

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"They're designed to be intense, you have to be uncomfortable to make progress," Calabrese told TODAY. "Expect a variety of things, expect that your heart will be pumping, your muscles will be burning, and you'll be sweating and laughing, too."

There are various packages, ranging from $39-$160.

2. Nike Training Club

Nike Training Club creates amazing online fitness challenges that are led by professional athletes and trainers. Brooklyn-based personal trainer Lita Lewis said these challenges are her favorite.

"I particularly love the workouts created by my favorite professional athletes such a Serena Williams and Allyson Felix. I love functional movement that focuses on core strength, power and explosiveness for my lower body," she noted.

These challenges are for all levels, but they're more demanding than workout classes that you'd find at a traditional gym. You can choose from 100+ workouts, some are only 15 minutes long.

The app is free to download, and some workouts are free, but you can pay for more.

3. The Sweat App

Samantha Cutler, the creator of the health and fitness blog @TheFitFatale, is based in Montreal and Toronto, Canada, and loves the circuit workouts in this app.

"Every time I do them, I am left sore and drenched. To me, that proves it works efficiently!" she shared.

The Sweat App features workouts from trainers Kayla Itsines, Kelsey Wells (post-pregnancy fitness) and Sjana Elise Earp. People love Itsines' workouts because they're only 28 minutes — and really get your heart pumping. Check out the video below to try one of her circuits!

The Sweat App is $19.99 per month, or $119.94 a year.

4. The Plank Challenge

Joey Thurman, a Chicago-based celebrity trainer, said he encourages ALL of his clients to do planks, which is one reason why he loves the 30-Day Plank Challenge. It encourages people to think about their core, trunk and spine health. And, it's free!

Thurman has even put himself to the test.

"Every day, I had to do at least one 2-minute plank and after two weeks, I had more core control, body awareness and my lower back felt amazing. You can modify this and complete 2 minutes total every day, just get it in!" he said.

He does caution that more intense fitness challenges can sometimes lead to injuries by encouraging someone who is sedentary to do too much, too fast. Always talk to your doctor before starting a new fitness or diet plan.

5. Feel Great in 8

Another top trainer shared Thurman's sentiment. Hector Bones, an exercise physiologist and certified personal trainer who owns The PrivatGym in Philadelphia, loved this challenge.

"It's the most rounded challenge for a healthier life. It is not asking for people to go from '0-100 in 3.2 seconds,'" Bones noted. "It's an eight-week challenge, where the participant puts a little skin in the game with $50, and you can earn $25 back by living the healthy lifestyle."

This challenge focuses on many lifestyle areas, including diet, physical activity, sleep, stress and even gratitude.

"Of course, if you're doing all the right things, you'll see some pounds come off, and more importantly, you'll feel great about yourself," he added.

Feeling good about yourself is the biggest accomplishment after completing any of these challenges. So whichever one you choose, make sure it's one that you look forward to doing most days. While you may not be enthused to do something every single day, the point is to feel motivated, committed and excited to be in a challenge that has structure and yields results that make you feel good!

Stephanie Mansour is a health & fitness expert and weight-loss coach for women. Join her complimentary health and weight-loss challenge here!