The Science Behind HIIT + Today’s Mini Challenge!

The Science Behind HIIT - Tone It Up Summer Tone Up

When we’re craving an energy boost, there’s one workout that always hits the spot…HIIT! And today in your Tone It Up App we have a brand new HIIT workout that does all that and then some: from boosting your metabolism, to increasing your endurance, to sculpting lean gorgeous muscle from head to toe! 

Keep reading for the science behind HIIT, why it works, and today’s quick HIIT challenge! (HINT: it’s from your all new Supercharged HIIT workout with K&K!) 

PS If you haven’t joined the Summer Tone Up series yet, there’s still time! Sign up HERE, or join the program directly in your Tone It Up App! 

The Science Behind HIIT - Single Arm Swing - Summer Tone Up

HIIT Me Up: The Benefits Of HIIT

High-intensity interval training, or HIIT, is one of the most efficient ways to get a great burn. The technique is simple: Go all-out with quick, intense bursts of exercise (at least 80% of your max heart rate, for anywhere from 15-60 seconds), followed by a short recovery period. Towel off, sip some water, and repeat! 

Feel your heart rate spike almost immediately? Yep, you’re doing it right! HIIT workouts, which typically last just 15-30 minutes, have been shown to boost your metabolism, better your endurance (VO2 max), and burn more fat than basic cardio workouts twice as long. Plus, if your HIIT routine includes bodyweight strength exercises like squats and push-ups, you’re building lean muscle, too! (More on alllll the benefits of HIIT here!) 

What’s The Science Behind HIIT?

So what’s going on in that gorgeous bod to make all of these benefits come true? Because HIIT is so intense, it requires more oxygen than your body can produce. So unlike a slow, steady jog (which uses your aerobic system), HIIT forces your body to tap into its anaerobic system to meet the demands. Without oxygen present, your muscles begin to break down sugars, resulting in the production of lactic acid. This triggers the release of adrenaline, a hormone that helps transport and accelerate fat burning. 

And those rest periods? They’re key too! As you recover between all-out rounds, your body switches back to the aerobic system, using carbohydrates and fat as your main fuel sources to energize you. Best of both worlds in one quick and efficient workout!

The magic of HIIT doesn’t end there, though. You’ll continue burning calories even after your workout ends. Why? A little something called the afterburn effect. Remember the oxygen your body couldn’t produce quickly enough? By the end of your HIIT sesh, your body has an oxygen debt that requires even more energy to pay back. So as you’re taking that sweaty selfie in your TIU app, or blending up your post-workout shake, you’ll continue burning anywhere from 6 to 15 percent more calories. This is called excess post-exercise oxygen consumption, or EPOC. Not bad for just 20 minutes of work! 

How Often Should I HIIT It?

So now you know why HIIT is one of our favorite ways to give that metabolism a boost. It’s also a whole lot of stress on that gorgeous body of yours! That’s why we recommend sticking to 2-3 times per week, ideally with at least 48 hours between HIIT sessions for adequate recovery. We love alternating HIIT with low- to moderate-intensity yoga, strength, or barre days. It’s all about variety babe! Check out the Summer Tone Up program in your Tone It Up App to see how we program each results-driven week! 

Summer Tone Up - Jump Squat Challenge

Your Quick HIIT Challenge!

With your knowledge fired up…let’s get that body burnin’ too! Your challenge today: Do as many jump squats as you can in 60 seconds! You’ll find this HIIT finisher at the end of your Supercharged HIIT workout in the TIU App. (You can find it On Demand or in every week of your Summer Tone Up program!) 

Write down how many reps you get — this is your Week 1 benchmark. Now imagine what you’re capable of by the end of the series! Yes, we’re doing this workout every week for all six weeks of your Summer Tone Up! Share your progress and let’s cheer each other on using the hashtag #SummerToneUp! We can’t wait to see how strong, confident, and accomplished you feel each and every week. Let’s HIIT it!!!

Join us for the 6-week Summer Tone Up! You’ll get daily at-home workouts, motivation & connection, mindful meditations, and healthy recipes!


  1. i only got 30 and my form wasn’t great and my hip injury was hurting. my goal is at least 50 by the end!

  2. I did 29 and had to switch to normal squats 5 and finished with another 4 jump squats. Hoping by the end of the challenge to at least do 60 seconds without having to take a rest.

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