So what exactly is interval training? Well ultimately, you are performing intervals whenever you do something for a set period of time that pushes you to your max energy expenditure (high heart rate) followed by a rest or lower-intensity exercise for a set period of time. Intervals come in many shapes and sizes and can include resistance training, dancing, or running. Whatever will get your heart rate soaring within a set period of time in considered an interval.
Keep in mind that not all interval workouts are the same. For example, running to catch a bus would be considered an interval as you are sprinting at a high pace to catch the bus, then once on it, you are resting. The problem is, that is only 1 interval and definitely not enough to be considered a 'workout'. The duration and intensity of each interval is what defines it as a great workout, good workout, or no workout at all!
On the flip side, aerobic training - such as jogging - is a great way to improve your aerobic capacity, but does little to improve your body.
At this point, some of you may be thinking the same thing, what about the "fat-burning zone"? These posters are plastered in virtually every gym. They show a graph that indicates your theoretical fat-burning zone based on age and heart rate. Many trainers may even tell you to reduce the intensity of your workout so that you stay within your zone so that you burn more fat.
On paper this makes sense and sounds good, and here is the science behind it. When you eat too much or take in too many calories (including sugar), your body makes stores of energy throughout your body called adipose tissue, or fat. This is an evolutionary thing that your body does for a multitude of reasons like insulating your body, protecting organs, energy storage, etc. Adipose is like diesel fuel for your body. It is used as your primary fuel source during low-intensity activities such as breathing, digestion, walking, etc. The problem is, fat (diesel fuel) burns at a very slow rate. That is why diesel fuel is not used in race cars.
So, staying within a certain (low) heart rate means I burn more fat right? Well, that is only part of the equation. Yes, your body burns a higher ratio of fat during low-level activity as another product of evolution. It is trying to conserve your blood glycogen (sugar) that is stored mainly in your muscles and liver for high-intense activity, such as running from a predator. Your glycogen is your jet fuel and is used for things like lifting weights, jumping up and down, sprinting, and pushing your body to maximum energy expenditure, say, when you're doing intervals!
Ok, so this still doesn't explain why intervals are better, but I am getting there. Our bodies are always adapting, for good or for bad. For example, sitting on the couch all day is sending a message to your brain and body that your muscles are not needed, so if they're not needed, then dispose of them and remove them. This is called atrophy. How do we combat this? You have to push your body to a level of exhaustion. When you do so, your are damaging body tissues. Yes, in this case, damaging your tissues is a good thing.
By pushing your body to exhaustion, you are damaging tissues and telling your brain and body that not only are you using your body, but you need a better body. By tearing down the walls of your muscles and other tissues in your body your brain releases waves of hormones in order to rebuild your better body. The process of reconstructing your body tissues requires a lot of energy so your metabolism is naturally increased.
This is known as progressive overload. Your body has to adapt to the damages you created so that the intense exercise you just did won't be as difficult next time. But, if you keep returning and pushing yourself a little further each time, then you will continue to create dramatic change in your body, constantly bumping up your resting metabolism, or the amount of energy your body needs daily to repair damaged tissues as it continues to adapt in a positive way. All of this positive adaptation is causing your body to burn fat at a faster, more efficient rate!
The thing about intervals is that most people like to exercise in their comfort zone. As long as they go to the gym and do their 30mins on a treadmill, they feel good about themselves for exercising, but also feel fine because they didn't push themselves too far. That is fine and dandy, but it does not dramatically change your body. Yes, people that run or cycle great distances have lean bodies, but that is because they exercise for very long periods of time covering great distances.
Resistance training can technically be considered interval training, but generally you see body builders doing intense sets followed by long rest periods. Circuit training, on the other hand, is a highly effective form of resistance training, especially for the recreational exerciser, because you can train alternating body parts with little rest in between to keep your heart rate peaking up and down with short breaks in between sets.
Interval training should be a mainstay in everyone's workout program, not just for athletes. You need to push yourself to your limits, allow your body to spend all day burning fat like crazy repairing your tissues, then come back and do it all again next time by pushing yourself a little further. That is how you create dramatic change in your body!