High intensity interval training (HIIT) is a form of exercise that involves short bursts of high-intensity activity followed by periods of low- or no-impact activity. HIIT workouts are often used to improve fitness levels, but they have been shown to increase blood glucose levels and insulin resistance in some individuals. Some studies suggest that HIIT may cause harmful effects such as increased heart rate variability, which could lead to arrhythmias and even sudden death.
The purpose of this post is to provide you with information about HIIT and its potential risks. If you’re interested in how HIIT works, please read our previous post: High Intensity Interval Training (HITT).
What Is HIIT?
In general terms, high-intensity interval training (HIIT) refers to activities performed at intensities higher than 60% VO2max for 30 seconds or less. The term “interval” is sometimes used instead of “high.” For example, a 10-second sprint at 80% VO2max would be considered an interval workout.
The primary goal of HIIT training is to produce greater metabolic stress and muscle damage than continuous moderate-to-vigorous aerobic exercise. This, in turn, leads to greater adaptations (or improvements) in cardiorespiratory fitness.
Aerobic exercise is generally considered anything that’s done at less than 80% of VO2 max. This would include activities like jogging, biking, or swimming for an extended period of time. These types of exercises typically use the same movements and intensity level throughout the entire workout.
One HIIT session can provide the benefits of 6 to 10 hours of continuous aerobic exercise. It is typically used by endurance and team sport athletes to improve performance.
HIIT can be practiced by individuals of all fitness levels, but it’s not recommended for those untrained in exercise.
You can perform HIIT by:
Performing sprint intervals (running, biking, or swimming). You can vary the time periods for high-intensity and low-intensity work depending upon your fitness goals and level of conditioning.
Using an exercise bike (either a stationary or a regular road bike).
Benefits of High-Intensity Interval Training
The main benefit of performing HIIT is that it allows you to get more fit in a shorter amount of time. Compared to other types of exercise, HIIT produces different changes in the body. These changes may be responsible for improving endurance more than steady-state aerobic exercise.
One advantage of HIIT is the improved ability of the body to remove waste products from muscle tissue, such as lactic acid. This can help you to perform better in your workouts and recover in between intervals.
Research has also shown that HIIT may also help you to lose weight and body fat, especially if combined with a calorie-controlled diet.
Potential Dangers of High-Intensity Interval Training
While many people experience health benefits from performing HIIT, some individuals experience health complications. The most common side effects are muscle strains and fatigue. These problems are usually minor and don’t cause long-term issues, but everyone responds to exercise differently.
In some cases, more serious side effects can occur. These can include rhabdomyolysis, compartment syndrome, and burnout. To prevent these complications from occurring or becoming worse, you should be slowly introduce your body to HIIT by starting with low-intensity intervals and working your way up.
Rhabdomyolysis is a condition that causes the breakdown of skeletal muscle tissue—especially when this occurs in a person who isn’t used to exercising. The skeletal muscle tissue breaks down into the bloodstream and is then removed by the kidneys. This can cause kidney damage in a process called renal failure.
Symptoms of rhabdomyolysis include:
Nausea or vomiting.
Swelling (especially in the face or belly).
If left untreated, this condition can be fatal.
This condition is usually caused by physical trauma to the body. It can also be caused by exercising in a new or intensive manner. With compartment syndrome, the increased pressure in muscle compartments can reduce blood flow and cause nerve damage. This may require surgical treatment.
Burnout is when you become mentally fatigued from exercise. Even though you’re still participating in the activity, your motivation is starting to wane. This may cause you to stop, but in some cases, over-zealous athletes can push themselves to the point where they experience physical burnout. If this happens, you should take time away from your sport or activity and focus on other aspects of your life.
Don’t Ignore Warning Signs
If you start experiencing pain during your exercise sessions, you should stop immediately. Continuing to exercise can lead to more serious and permanent injuries. You should also visit your doctor to make sure everything is okay.
In some cases, you may have an underlying health condition that’s causing the pain or discomfort. You may need additional treatment to alleviate these symptoms. If you’re really worried, you should stop exercising altogether and consult your physician.
Knowing the benefits and risks of HIIT is just the beginning. You should also understand how your body reacts to this type of exercise. Listen to your body and stop if you feel any pain or discomfort. If possible, see a physician before you start this type of training.
If you over-do it, you could be off of your feet for an extended period of time, which may prevent you from working out at all. To avoid long-term injuries and health complications, make sure you’re fully prepared before participating in high-intensity interval training.
A version of this article appeared in August 2015.
Photo courtesy of Shutterstock.
Sources & references used in this article:
High intensity interval training improves glycaemic control and pancreatic β cell function of type 2 diabetes patients by SM Madsen, AC Thorup, K Overgaard, PB Jeppesen – PloS one, 2015 – journals.plos.org
High intensity intermittent exercise improves cardiac structure and function and reduces liver fat in patients with type 2 diabetes: a randomised controlled trial by S Cassidy, C Thoma, K Hallsworth, J Parikh… – Diabetologia, 2016 – Springer
Effect of elliptical high intensity interval training on metabolic risk factor in pre-and type 2 diabetes patients: A pilot study by A Fex, JP Leduc-Gaudet, ME Filion… – … of Physical Activity …, 2015 – journals.humankinetics.com