Related: Choosing the Best Cardio Workout for You
Many people tend to have a negative attitude towards cardio because they think of long sessions on the treadmill which can get boring and be time consuming. Fortunately, there is a much more fun, time efficient and muscle sparing form of cardio: enter High Intensity Interval Training (HIIT).
What is HIIT?HIIT is a type of training involving repeated short bouts of high intensity cardiovascular exercise (80% or above of your VO2 max) followed by a recovery period of low to medium intensity steady state. Times of intensity/recovery and type of cardio used will depend on fitness and preference of user.
Sample HIIT cardio:
- 120 seconds - Warm-Up
- 15 seconds - Full-Out
- 45 seconds - Low-Medium Intensity Rest
High-impact exercises such as sprinting may be very taxing on your nervous and muscular system and hinder your ability to recover from weight training.
What are the benefits of HIIT (the science behind it)?It is important to understand that your body can produce ATP, or energy, in two ways: aerobically, meaning energy produced with oxygen, or anaerobically (produced without oxygen). When performing Low-Intensity Steady State (LISS) and Medium Intensity Steady State (MISS) cardio your body will rely on aerobic pathways for energy.
Aerobic pathways produce energy at a slower pace for longer-duration activities that need to be sustained for longer periods of time.
When you perform HIIT your body has a much faster demand for energy and will use anaerobic pathways to produce energy. Anaerobic pathways will produce energy right away in order to fuel quick, intense bursts of movement.
Anaerobic energy is the same type of energy that is used when you lift weights. The huge benefit of using anaerobic energy is that when this energy becomes depleted, your body will be in an "oxygen deficit" and it will work the entire day to replenish it.
Restoring this form of energy increases the famous and greatly desired "Excess Post-Exercise Oxygen Consumption (EPEOC)": in other words, your metabolism will be accelerated until the oxygen deficit is replenished (can be as much as 24 hours or more).
Your body is an incredibly adaptive organism and it will attempt to adapt to a state where the demand for energy becomes high very quickly. It will do this by increasing the number of mitochondria you have. 
Mitochondria are responsible for producing energy anaerobically in the form of ATP and they do this by using different energy stores from your body. By increasing the mitochondria density in the muscle, you will increase your oxidative capacity; in other words, you will convert more fuel to energy within a 24-hour period and increase your metabolism. 
To further the good tidings, HIIT is not affected by plateaus like traditional MISS or LISS cardio. Plateaus are dreaded; they can mean having to further reduce calories or add more cardio. Studies have shown that when steady state cardio is first introduced in a weight-loss diet, fat loss was increased.
However, this increase in rate of fat-loss seems to plateau as your body adapts to it.  This was not the case with HIIT, which shows to provide constant weight-loss.
HIIT has effects that will also benefit those who are not in a fat loss phase; it has been shown to increase insulin sensitivity, decrease central abdominal fat and resist decreases in leptin levels; which is a hormone that increases hunger and fat storage when it is decreased.4,5 Another huge bonus is that, this form of cardio is muscle-sparing (it does not consume muscle), as it gives your body a reason to maintain muscle mass so that it can power through this strenuous form of exercise.
If HIIT is so great, is there still a place for steady state cardio?The answer is yes. Although HIIT will increase the calories you burn post workout, 10-15 minutes of HIIT will not cause you to burn 400 calories like a step-mill session. The benefits of steady state cardio comes in when you need to create a large energy expenditure and do not want to do this by restricting calories any further.
Think of MISS and LISS cardio as a big blunt tool that is sometimes used to get the job done.
HIIT is also taxing on your body; if performed too frequently, it can interfere with your ability to recover from your weight training sessions; hence, LISS/MISS can be beneficial when you need to introduce more cardio but need to do so in a form that will not interfere with your ability to lift some heavy weight.
The Bottom LineHIIT cardio seems to be an effective and time efficient way to boost your metabolism by increasing oxidative capacity and increasing 24-hour energy expenditure. It may also help increase insulin sensitivity, decrease central abdominal fat, and resist decreases in leptin levels. However, it can be taxing on the nervous and muscular systems and if performed too frequently, it may interfere with your ability to recover from the weight room.
Additionally, in a fat-loss diet there may be a point where the "big blunt tools" of LISS or MISS are needed to create a greater energy deficit. An ideal combination, together with a weight loss plan, is to perform HITTs 2-3 per week on a low-impact machine and introduce LISS or MISS cardio as your body demands a greater energy deficit.
For more information like this, please check out my science-based fitness YouTube channel, Miguel Blacutt, by following this link: www.youtube.com/c/MiguelBlacutt