By Steve Edwards

There are a lot of contradicting ideas—many of them misconceptions—about the best way to reshape your body. This is largely because there are a lot of trainers out there, each seemingly espousing a different fitness philosophy. In this article, we'll examine an assortment of training strategies, bust a myth or two, and explain why interval training is generally the most efficient way to improve your fitness level.

Having just launched TurboFire, we dusted off this article that was first published when INSANITY came out. Since most Beachbody programs use some type of interval training, the information we lay out here is also a perfect lead-in to TurboFire. We'll go into more depth about that program next week. Consider this Part I, your introductory course on interval workouts.

What is interval training?

Basically, you're interval training any time your workout includes a set where you perform at your maximum level for a given amount of time, followed by a lower-intensity set; this sequence is then repeated to achieve a cumulative effect. An interval can be a set of curls, a dance move, or anything that tires you out over a given length of time. Intervals can be short and hard, or long and easy, but they're all intervals, just as long as there's some cumulative effect—you get more tired as you go. All interval workouts aren't the same, though; the duration and intensity of the intervals are what define the workout.

Conversely, aerobic training is when you maintain a steady output at a low-intensity level over the course of the workout. Beachbody does offer some workouts that do this, but they're generally either for recovery or for the second daily workout of a doubles program. This type of workout helps your aerobic efficiency, but does very little for changing your body.

The myth of the fat-burning zone

It's impossible to approach this topic without debunking the term "fat-burning zone." You often hear uninformed trainers recommend that their clients reduce the intensity of their workouts so their bodies will burn more fat. In reality, all these trainers are doing is lowering the overall effectiveness of their clients' programs.

Here's a quick explanation of the fat-burning zone. At an aerobic pace (see above), your body utilizes stored body fat as fuel to save its preferred fuel (stored blood glycogen) for more pressing matters. This sounds great, because you're burning body fat. However, you're burning it at a very slow rate.

During higher-intensity work, your body turns to a limited supply of blood glycogen (often called blood sugar) for energy. While your body is burning glycogen rather than fat during this more intense period, it's also breaking down more body tissue. "Breakdown" is a negative-sounding word for a good thing, because while it's happening, your body produces more hormones and increases its metabolism to repair the breakdown. As the tissue repairs itself, it builds more muscle, so the next time you do a stressful workout it won't be so taxing. This process of adapting to intense exercise is where your body makes the most rapid change.

Continually building on this process is called "progressive overload." By continually adapting to stress, then adding more stress, either with weight, speed, or intensity, you increase your body's fitness so it's actually burning body fat for fuel as you rest. Interval workouts should be a key component in every phase of your training.

Techie science made simple

Asked what separates serious athletes from recreational athletes, author and fitness trainer Steve Ilg replies, "Intervals." But because "intervals" is an umbrella term for training that targets many different energy systems, Steve's answer is somewhat cryptic and requires further explanation. At the same time, however, it's pretty accurate. Recreational athletes like to train within their comfort zones. Interval training, regardless of the targeted intensity level, always forces you out of your comfort zone. And you have to be willing to leave your comfort zone if you want to see significant changes in your fitness level.

Interval levels can change dramatically. For example, HIIT (High Intensity Interval Training) workouts are very short, sometimes lasting only seconds, and are completely anaerobic. Marathon runners will often run for one- or two-mile intervals, which can take many minutes and are obviously somewhat aerobic. The reason for the varying intensity of intervals is to train different energy systems in the body. These are defined by terms you may have heard, like anaerobic threshold (AT), VO2/max, etc., but we don't need to go into that level of detail yet. For our purposes here, we'll give you the "101"-level course in what you need to know:
  1. LSD. Not the hippie drug from the 60s, but rather "long slow distance." This isn't an interval; it's a term you're especially likely to hear if you know or are a runner or cyclist. The purpose of LSD is base-level aerobic conditioning. As I said above, this approach isn't really applicable for making significant body changes, unless you do it for a very long time. Yet many trainers still recommend it. I think this is primarily because their clients won't complain about doing 30 minutes of easy exercise.
  2. Sports-specific intervals. These can be anything, like the two-mile example above. Interval training exists for all athletic endeavors. Since it's targeted for sports performance, we won't discuss it. You'll learn plenty about it when you join a local group to train for a triathlon, or other activity or event.
  3. Weight training intervals. All weight training could be considered interval training, but traditionally, you rest so long between sets you don't get a cumulative effect. All Beachbody weight training is done interval-style, which we call circuit training. During these workouts, you move from body part to body part without much rest between sets, so the workouts don't just target muscle building, but also improve your cardiovascular fitness. P90X and ChaLEAN Extreme are good examples of this kind of training.

    What defines these circuits is your targeted number of repetitions. A low target using more weight will create muscular hypertrophy, or growth. A higher number of reps limits muscle growth (although you do still get some muscle growth) and gives you more cardiovascular improvement.
  4. Cardio intervals. These are what most of you probably think of when you think of interval training. First, let's define the difference between "cardio" and "aerobic." Cardio means heart, while aerobic means oxygen. Aerobic training is most easily defined by the word "easy." (It's really defined by training below your anaerobic threshold, but let's dispense with the science talk.) Cardio, however, is any and all training that affects the heart, so it can include aerobic training, but also all the high-intensity training associated with intervals.

    High-intensity cardio intervals are performed in something we call "heart rate training zones." Cardio intervals target these heart rate training zones for various periods of time. When you design your own interval workouts, you have to do this for yourself. When you have a trainer, he or she does it for you. This is why we at Beachbody always have test groups to make sure our workouts train you in your proper zone. That way, all you need to do is follow along.
Interval lengths

In general, the longer the duration of the interval, the easier the workout. Some interval sessions have long and moderate intervals with short aerobic breaks. Others have short, difficult intervals with long aerobic breaks. Two interval systems that buck this trend are HIIT and INSANITY's MAX Interval Training. Both feature high-intensity intervals with short breaks, and both systems are very effective for creating rapid body composition changes. But due to their intensity, it's vital that you only do HIIT or MAX Interval Training for short periods of time as part of an overall program—like INSANITY, or Beachbody's new program TurboFire—that includes other types of workouts as well.

How to incorporate intervals into your workout program

As with every other aspect of fitness training, the type of interval training you start with should be based on your current physical condition. If you aren't very fit, you'll want to start with a very basic interval program (which will still feel plenty hard). Workouts like Slim in 6's Start It Up! or Power 90's Sweat Cardio 1–2 are good introductory interval sessions. If you're in doubt, start slowly. It's easier to increase your workout's intensity than to go backward. Intervals are the most effective way to see quick results from a workout program. If you're not doing intervals in your current workout regime, try adding some. If you're already doing intervals, maybe it's time to step up to the next level. You might just be amazed by the results.