For an optimal site experience, we recommend using a different browser.
Using Internet Explorer may prevent you from accessing, and some site features may not function as expected.


The Science Behind HIIT

Created 7/9/2020
Updated 6/30/2021

Allison Walsh


If you’re into fitness at all, you’ve likely heard the term high intensity interval training (HIIT) over the past several years. These intense but effective workouts have been well-loved for years now – and for good reason.

During HIIT workouts, participants take part in repeated, brief rounds of high-effort exercise and rest periods. Because of the intensity required for the high-effort periods, these workouts tend to be shorter in nature than traditional exercise formats. The period of intense exercise is followed by a short period of rest.1 An entire workout may be as short as 15-20 minutes total. For time-crunched folks, HIIT fits nicely into the day, and the gains prove impressive.

HIIT how-to

In HIIT workouts, intense periods of work, ranging from 5 seconds to 8 minutes long depending on exercise difficulty and fitness level, are performed at 80-95 percent of maximum heartbeat. (Here's how to find yours.) 2 A recovery period follows each burst and involves low intensity moves performed at 40 to 50 percent of maximum heart rate.

The main goal during these workouts is to do as much work as fast as possible.[1] Many HIIT workouts use traditional exercise equipment such as cardio machines or weights, but many can be done using only your own body weight making them easy to maintain during times of travel and perfect for a short break during the workday. Perhaps the most famous of the HIIT workouts is the New York Times 7-Minute Workout which was originally published in 2013. Eight years later and it has its own app and a loyal following.3 The workout asks users to do 30 seconds of 12 high intensity moves such as jumping jacks, planks, and lunges, with a 10 second break in between.

How it feels.

Simply put, HIIT is hard. When your body is going all-out during true HIIT, it relies on your anaerobic pathways (breaking down glucose without oxygen) to produce the energy it needs to fuel you. This provides an immediate supply of energy, but the amount is very limited—which means the length of time you can sustain that max effort is quite short.4

The benefits of true HIIT are performance-based, which include an increase in VO2 max (how much oxygen you can use during exercise) and improvements in insulin sensitivity (how responsive your cells are to insulin), blood pressure, and cardiovascular function.4

How it works.

So what’s the scientific secret behind HIIT? It’s in the details behind that cell stress. During high intensity exercise, chemical channels inside muscle cells that manage calcium variances become confused. This confusion creates a cellular-level change that requires energy production to become more efficient.5

To exercisers, this translates into increased endurance and faster muscle gains.4 And the benefits of HIIT go beyond the workout. Overall calories burned are higher on average than traditional workouts because excess post-exercise oxygen consumption, or the post-exercise period where the body is in restoration mode, uses more energy after HIIT workouts, burning more calories than other types of exercise.6 The American College of Sports Medicine reports overall benefits that include improvements to:

  • Aerobic and anaerobic fitness
  • Blood pressure
  • Cardiovascular health
  • Insulin sensitivity
  • Cholesterol profiles
  • Abdominal fat and body weight6

How to get started.

There are risks to consider with any new exercise program, so discuss HIIT with your doctor prior to starting. Once you have the go-ahead, do your research. The internet, app world, and streaming television are full of sample HIIT workouts to try, that can be easily modified to fit your fitness level. Start slowly, maybe once a week, and increase to two to three workouts each week.6



2- Creveling, M., & Mateo, A. (2020, November 10). 15 HIIT workouts to get in the best shape of your life. Retrieved June 23, 2021, from

1- High intensity interval training (hiit): Benefits and how to start. (n.d.). Retrieved June 23, 2021, from

6- High-intensity interval training: For fitness, for health or both? (n.d.). Retrieved June 23, 2021, from

5- Park, A. (2015, November 03). Interval training: Scientists explain why it works. Retrieved June 23, 2021, from

3- Reynolds, G. (2013, May 09). The scientific 7-minute workout. Retrieved June 23, 2021, from

4- Sgobba, M. (n.d.). Here's what a hiit workout is-and why you should try it. Retrieved June 23, 2021, from