HIIT workouts are known to be an excellent way to burn fat in a short period of time and to help improve the physical performance of athletes of all kinds. (1)
For most people, because it’s one of the great metabolism boosters, the biggest draw to a HIIT workout is its ability to keep the body burning fat even after the workout is over.
What Are HIIT Workouts?
High-intensity interval training is a type of exercise that involves repeated short bouts of high-intensity, or “burst” exercises, followed by brief recovery periods. This sequence is repeated several times in a row usually for a duration between 20–30 minutes.
Considering the fact that many people use “not having enough time” as a popular reason to avoid regularly exercising, HIIT workouts are one of the best ways to overcome this block and get great results fast.
A HIIT protocol is a well-researched way to reap physical benefits as an alternative approach to steady-state exercise training but with less of a time commitment. (2)
For example, recently a study compared the impact of two different types of exercise training on body fat and muscle metabolism: HIIT workouts versus steady-state exercise.
The study investigated the effects of calorie expenditure and fat loss in young adults and found that although HIIT workouts actually burned fewer calories during the actual workouts than did steady-state cardio exercise (likely due to its shorter duration), the HIIT program produced more fat loss than steady-state exercise did overall.
Additionally, the study concluded that while the HIIT workout helped build muscle, the steady-state workouts actually broke muscle down. Researchers concluded that not only does HIIT burn more fat over the duration of the day, but it also builds more muscle and improves metabolic function. (3)
How to Perform HIIT Workouts … and Why
The exact type of exercise performed during the “intense” internal periods can vary, such as performing sprints or doing faster reps of a particular move. What stays the same throughout different types of HIIT programs? The act of performing these stop-and-go intervals, meaning alternating between periods of hard “work” and “rest” or recovery.
A popular example of a HIIT workout can be running on the treadmill, alternating between a very fast pace and one that is easier and slower. In order to follow an interval schedule, you switch between sprints that require roughly 90 percent of your energy, followed by walking or slowly jogging to rest and recover.
Steady-state exercises, on the other hand, usually stay within the same type of “work” zone over time, with the amount of effort needed remaining consistent. (4)
HIIT Workouts Provide Benefits Fast
HIIT workouts have the ability to transform your body and physical abilities due to their effect on at least three important systems within the body:
Your cardiovascular health and endurance
Your body’s ability to use oxygen
Your hormone levels
Studies have shown that resistance-based interval training specifically benefits blood flow and blood vessel dilation. A study, published in the American Journal of Physiology — Heart and Circulatory Physiology, found that resistance-based interval exercising improved endothelial function in individuals that previously exercised, those that didn’t and those with type 2 diabetes.
When researchers measured blood flow before, immediately following and at one and two hours after working out, participants with type 2 diabetes saw improvements at each time. The other two participant groups experienced improvements one and/or two hours after exercising. (5)
HIIT Workouts Boost Cardiovascular Health
Many studies are now showing that internal training — including HIIT workouts — promote greater improvements in VO2max and general fitness abilities than steady-state exercises do.
In fact, VO2max is considered the best indicator of cardiovascular endurance. This is the measurement most commonly used in fitness studies to show the effects that the exercise is having on the body. VO2 max is sometimes also called “max oxygen uptake” or “max aerobic activity” and is used to measure how well the body can use oxygen for energy.
Specifically, VO2max is defined as the maximum amount of oxygen (in milliliters) that a person can use in one minute per kilogram of their body weight. This measurement is important because the amount of oxygen that a person can utilize within one minute is an indicator of their overall fitness level and also their lung and heart health.
Due to the expansion of blood volume, the heart will undergo enlarging, or “hypertrophy,” during HIIT type of endurance exercises in order to allow the heart muscle to become bigger and stronger. (6)
By pushing your heart rate high during periods of intense work, you’ll be able to increase your cardiovascular ability and strengthen your heart. During the short rest intervals, you work on recovering more quickly and needing less time to rest. This is how you build stamina over time and increase your ability to perform physical exercises more effectively.
Performing short recovery segments in between the intervals where you’re working harder has the benefit of allowing you to keep the overall workout intensity high while still maintaining form.
While it’s hard to work very hard and maintain a high heart rate for an extended period of time because your body isn’t able to bring in enough oxygen, the rest/recovery periods of interval training allow you to catch your breath and for your heart rate to come down momentarily.
Knowing your VO2 max can help you to establish fitness goals to work towards and gives you a starting point as to how capable you are of maintaining a high level of effort over a period of time.
HIIT Workouts Trigger an Excess Post-Exercise Oxygen (EPOC) Effect
HIIT workouts also trigger something known as excess post-exercise oxygen consumption, or “EPOC.” EPOC is the phenomenon of an increased rate of oxygen that your body uses following strenuous activity (7).
Your body uses more oxygen after hard workouts like those performed during HIIT because it’s making up for the body’s “oxygen deficit” that occurred during the difficult “burst” periods of the exercise. EPOC has many functions for the body, including that following a HIIT workout, your body goes into a recovery phase.
A higher level of oxygen is needed during recovery in order to facilitate in the restoration of hormone levels, refueling your glucose stores, and repairing your muscle fibers and tissue.
The best part about EPOC? It’s accompanied by an elevated need for bodily “fuel” or energy in addition to more oxygen. After intense exercise, fat stores within your body are actually broken down and free fatty acids are released into the bloodstream. During the post-workout recovery phase, these free fatty acids become oxidized and your body uses them for energy.
As your body uses more oxygen to bring itself back into a resting state, more calories are burned in the process, even while you are done working out. This means you continue to experience benefits and fat loss during the remainder of your day following a HIIT workout.
Another benefit of EPOC that results from HIIT workouts? New ATP (adenosine triphosphate) — which is the fuel source or energy that your body works off of — is also synthesized. Additionally, post-exercise oxygen is used to reduce lactic acid.
Lactic acid is formed during exercise and is responsible for giving you the “burning” feeling in your muscles when they are working hard.
Lactic acid travels via the bloodstream to the kidneys, cardiac muscle and liver during workouts; then an increased amount of oxygen is necessary to convert the lactic acid back to pyruvic acid so that your pain subsides and body enters a resting state.
Yet another use of EPOC is to fuel the body’s increased metabolism that results from the increase in body temperature experienced during exercise. Due to all of these vital tasks that the body must undergo during a period of EPOC, you can see why HIIT workouts have such a huge effect on your strength, stamina and health.
HIIT Workouts Release Muscle-Growth and Fat-Burning Hormones
Intense interval training circuits also stimulate muscle-building hormones while simultaneously using up calories and burning fat. The body produces the growth hormone known as IGF-1 during HIIT, for example, which allows the body to build lean mass muscle.
Who Are HIIT Workouts For?
Certain studies have shown that high-intensity exercise can be potentially unsafe for sedentary middle-aged adults. It’s best performed by those who are already somewhat active and have a healthy cardiovascular system.
That being said, anyone can work towards practicing HIIT workouts for their multiple benefits. However, if you aren’t already exercising, then it’s best to start slowly to avoid injury or more serious problems.
In conclusion, an exercise plan that includes consistent high-intensity interval exercise has been shown to improve body composition, boost cardio-metabolic health, lessen the risk for heart disease, and help improve exercise tolerance, even in obese and overweight participants. (8)
Research has shown that HIIT workouts are safe, efficient, well-tolerated and could help to improve adherence to exercise training given the limited time commitment that they require.
As long as you practice HIIT workouts responsibly and ease your way into a HIIT program, you can experience great results using HIIT workouts in combination with other forms of exercise that you enjoy.