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Why high-intensity interval training is best for fat loss

Warum hochintensives Intervalltraining am besten für den Fettabbau ist

If you want to lose fat (not muscle) fast without having to do hours of cardio every week, then high-intensity interval training could be just the thing for you.

Most of us learned at a young age that promises of "more for less" are usually empty promises. This is especially true when we talk about fat loss. If we believed dubious supplement companies, we would simply have to take their pills and powders to get hard and defined in no time.

Unfortunately, that's not how it works. No supplement can give you the body you want. In truth, most supplements can't even help you develop the body you want - they're completely worthless.

And half-baked exercise gurus tell us that all we need to do to get the look of a Greek statue is to spend a few hours a week doing their workouts.

Well, that's not true either. Achieving great shape may not be as complicated as many people think, but it does require you to do a lot of little things right, which include your calorie intake, proper macronutrient balance, progressive overload, proper training frequency and more.

"7 minute workouts" and fancy fad diets won't get you there.

If you've heard anything about high-intensity interval training - HIIT for short - then you've probably heard a similar story: that it has almost magical fat-burning powers - that you only need to do a few minutes of HIIT a day and you'll see your fat just melt away.

Well, that's not true either. HIIT is not the alpha and omega of fat loss...but it can be a powerful fat loss tool if you know how to use it properly. And that's exactly what we're going to talk about in this article:

  • What is HIIT - and what isn't it?
  • Why is it great for fat loss?
  • Why is it superior to cardio training with moderate, consistent intensity for optimizing body composition?
  • How to do it properly?

By the end of the article, you'll know how to get the most out of your fat loss efforts.

What is HIIT - and what is it not?

High-intensity interval training - also known as HIIT - is a style of training in which you alternate between periods of maximum effort and periods of low effort.

The high-intensity intervals - where you try to give it your all - push your body to its metabolic limits and the low-intensity phases allow you to recover.

You probably already knew this, but here are some more specific questions to answer:

  • How intense do the high-intensity intervals need to be?
  • How hard should you push yourself and for how long?
  • How exactly do the rest intervals work?
  • How long should your HIIT training sessions be?
  • How often should you do them?

Basically, the question is how to get the most out of your HIIT workouts and your training program as a whole. Well, let's find out...

How intense should your high-intensity intervals be?

If you look at scientific research on high-intensity interval training, you'll find a lot of talk about something called VO2max. Your body's VO2max is a measure of the maximum volume of oxygen it can use and this volume is a primary determinant of your endurance.

This is relevant to HIIT for the following reasons:

Studies show that you need to reach 80 to 100% of your VO2max during high-intensity intervals to reap most of the benefits of HIIT training (1).

That's nice to know, but not particularly practical because it's hard to estimate your VO2max during training. There are simply no reliable indicators that you can use to accurately estimate your VO2max.

Fortunately, you can also work with a useful measure: the Vmax. Simply put, you have reached a Vmax level of exertion when you feel that you cannot inhale as much air as your body wants (if you can carry on a conversation while exercising, then you have not yet reached this point.

For most people, this is 90% of maximum effort.

1. your goal during high-intensity phases is to reach and maintain your Vmax.

To do this, you need to move fast and long enough to get out of breath and you need to maintain this speed for a while. As you can imagine, this means hard work - think sprinting instead of jogging.

2. your goal during HIIT workouts is to repeatedly reach and maintain this Vmax level of effort

This may seem obvious, but it's important to keep in mind that the total amount of time you spend at a Vmax level of effort determines the overall effectiveness of the HIIT training session (2).

In other words, a HIIT session that totals one minute of exercise at a Vmax level will be far less effective than a session that adds up to several minutes at that level.

Fortunately, this is simply a question of planning your training sessions correctly and not something you need to worry about during training. We will go into this in more detail shortly.

So that's the 'basics' of HIIT.

Next, let's take a look at why you should choose this type of training over lighter, less stressful forms of cardio.

High-intensity interval training and fat burning

Most cardio machines have pretty good graphic guides that recommend you keep your heart rate in the mid "fat burning zone". If you do this, they claim, you should maximize the amount of fat (compared to the amount of carbohydrates) you burn.

Well, there is a grain of truth in this statement. You burn both fat and carbohydrates while you exercise, and the ratios vary with the intensity of the workout. Scientific research shows that as the intensity of exercise increases, the body relies more and more on carbohydrates and less and less on fat as an energy source.

This is the reason why low-intensity activities such as walking rely mainly on fat stores, while high-intensity sprints draw more heavily on carbohydrate (glycogen) stores. These are also the main reasons why many people think that cardio training at a constant, low intensity is best for fat loss.

However, numerous studies show otherwise (3 - 6). Specifically, these studies show that high-intensity cardio workouts result in greater fat loss over time than longer, lower-intensity cardio workouts.

Why is this?

Well, let's start with the obvious: the total amount of calories burned during exercise.

High-intensity exercise can burn a lot more calories than low-intensity exercise, and since fat loss is dictated by energy balance, the benefit of this should be obvious. Let's say you jog several times a week and burn 200 kcal per session, of which 100 kcal comes from fat stores. If you combine this with a suitable calorie deficit, these workouts will help you get leaner faster.

However, high-intensity workouts of the same duration would be better, burning say 400 kcal per workout, of which 150 kcal comes from fat stores. Regardless of diet, the workouts that burn the most energy will result in the greatest fat loss.

However, energy expenditure during exercise alone does not fully explain why high-intensity interval training is better for fat loss. A study conducted at the University of Western Ontario gives us an indication of how much more effective this type of training really is (7).

The authors of the study had 10 men and 10 women train three times a week, with one group doing 4 to 6 30-second sprints on the treadmill (with 4 to 6 minutes rest between sprints) and the other group doing 30 to 60 minutes of cardio training at a moderate, consistent intensity (running on the treadmill in the "magic fat loss zone" at 65% of VO2max).

The result? After 6 weeks of training, the subjects who completed the intervals had lost more fat.

Yes, 4 to 6 30-second sprints burn more fat than 60 minutes of walking on an incline treadmill.

The exact mechanisms underlying the superiority of HIIT are not yet fully understood, but scientists have already isolated a few factors (8):

  • An increased metabolic rate at rest for up to 24 hours after exercise
  • Improved insulin sensitivity of the muscles
  • Higher levels of fat oxidation in the muscles
  • Significant spikes in growth hormone levels (which aid fat loss) and catecholamines (chemicals your body produces to mobilize stored fat for burning)
  • A suppression of appetite after exercise
  • And much more...

The science is clear: if your goal is to burn as much fat in as little time as possible, HIIT is the workout of choice.

High-intensity interval training and your muscles

In the minds of most strength athletes, cardio training and muscle building are two opposing things. You can either have one or the other. There is some truth to this, but it is an oversimplification.

For example, scientific research has shown that a combination of strength and endurance training can hinder your strength and muscle gains compared to strength training alone (9). Studies have also shown that the longer your cardio training sessions are, the more your strength and muscle gains are hindered by cardio training (10).

However, this does not mean that cardio training directly impairs muscle growth - which is not the case. Too much cardio training can do this.

The right amount of cardio training can even accelerate muscle growth. But what is the right amount? Well, there are two factors you need to consider:

  1. The duration of each workout
  2. The total amount of cardio you do per week

And if your goal is to optimize your body composition (which requires progress in the weight room), then you need to keep your individual cardio workouts short and your total weekly cardio time relatively low.

Only HIIT allows you to meet these criteria while burning significant amounts of fat.

How to create an effective HIIT training program

So there's a good chance you could use some HIIT in your life. Well, there are 5 things you should consider when creating a HIIT program:

  1. The type of cardio workout.
  2. The length of the workouts.
  3. The frequency of the workouts
  4. The duration and intensity of the high-intensity intervals.
  5. The duration and intensity of the low-intensity intervals.

Let's look at these points separately.

The best types of HIIT cardio

HIIT principles can be applied to any type of cardio workout, but some forms are more practical (and effective) than others. In general, the three best types are as follows:

  1. Cycling
  2. Rowing
  3. Sprinting

Cycling and rowing are my favorites because sprinting is hard on the legs and can interfere with deadlift and squat workouts.

The reason I prefer these three forms over other forms of cardio training is because scientific studies show that the type of cardio training you do has a significant impact on your ability to build strength and muscle through weight training (10).

In short, the following applies: The more a cardio exercise mimics the exercise used to build muscle - such as squats or barbell rowing - the less it will hinder strength and muscle gains.

This makes sense, because the important part of building strength is simply practicing a movement pattern repeatedly (the more times you perform a movement, the better you get at performing that movement).

However, if you can't cycle, row or sprint, then don't shy away from other forms of cardio training such as swimming, jumping rope, boxing or whatever. These won't make your muscles shrink either.

To reiterate - the biggest mistake with cardio training is doing too much of it.

How intense should your intervals be?

The goal with HIIT is to train fast and hard - not slow and hard. This means that if you're using a machine like an ergometer or rowing machine, you need enough resistance to work against, but that resistance shouldn't be so high that the exercise becomes resistance training.

This is because the primary difference between high and low intensity intervals should be the speed and not the resistance used. This means that you should increase and decrease the resistance, but not nearly as much as you increase and decrease your speed.

As you already know, the key factor that determines the effectiveness of a HIIT training session is the total amount of time you spend at a Vmax level of effort. If you spend too little time at this level, it is not a true HIIT training session and if you spend too much time at this intensity level, you will burn out.

You will spend maximum time at Vmax level if you reach this level as quickly as possible during your sprints. This means that you should not increase the intensity slowly, but give it your all immediately during each sprint.

As for the duration of the high-intensity intervals, 50 to 60% of Tmax is sufficient if your goal is to burn fat and improve your metabolic health. Tmax is simply the amount of time you can maintain your Vmax speed before you have to stop.

So, for example, if you can sprint for 3 minutes at your Vmax on the bike before your heart feels like it's going to explode, then your Tmax is 3 minutes. So your high-intensity intervals should be between 90 and 120 seconds long (and yes, that's hard).

For your intervals, you can either test your Tmax (all you need is a stopwatch) or if you're new to HIIT, start with 30-second high-intensity intervals.

Your HIIT workouts should get progressively harder

The more HIIT workouts you do, the more your Tmax will increase. This means that you will also need to increase the duration of your high-intensity intervals if you want to keep your training maximally effective. As you can imagine, these workouts can get pretty intense for experienced athletes.

In three HIIT studies conducted with highly trained cyclists, the high-intensity intervals were 5 minutes long (and increased the athletes' performance) (11-13). However, other studies conducted with endurance athletes found that 2 and 1 minute intervals were not sufficient to improve performance (14, 15).

How "restful" should the recovery periods be?

There are 2 ways to make your HIIT training harder:

  1. Increase the length of your high-intensity intervals.
  2. Reduce the length of your rest intervals.

I generally recommend working on increasing the length of your high-intensity intervals first until they are in the 50-60% of your Tmax range. This will ensure that you are performing true HIIT workouts.

Once you have achieved this, it is up to you how you proceed.

I think it's reasonable to work your rest intervals down to a 1:1 ratio with the high intensity phases (e.g. 90 seconds of high intensity work followed by 90 seconds of rest) and then slowly increase the duration of the high intensity intervals and the low intensity intervals while maintaining the 1:1 ratio.

For example, let's say you start your HIIT workout with 30 seconds of high-intensity intervals followed by 60 seconds of rest intervals (1:2 ratio). As you progress, you get an idea of your Tmax and work up to the 50 to 60% range for your high-intensity intervals, which could be 60 seconds. You train at this level and maintain the 1:2 ratio of high to low intensity (120 second rest intervals).

Over time, you get the feeling that you can push yourself harder and keep the 60 second high intensity intervals while you start to shorten your rest intervals, starting with 90 seconds (1:1.5 ratio).

Eventually your body will adapt to this and you will be able to reduce the rest intervals to 60 seconds (1:1 ratio) and when this is no longer challenging enough, you will start to extend both the high-intensity intervals and the low-intensity intervals to 90 seconds. (And then continue in this way).

You should always be aware that your rest intervals serve as active recovery, which means that you keep moving and don't come to a standstill. Studies have shown that active recovery - as opposed to passive recovery - is beneficial when it comes to reaching your Vmax during the high-intensity phases and forcing the adaptive response we're after (16).

How long should your HIIT workouts be?

The great thing about HIIT is that you get a lot out of what feels like very little. There's no more efficient way to use cardio to drive fat loss and improve fitness.

The big downside, however, is that HIIT can be quite stressful for your body, which means you shouldn't overdo it. Do the following and you will be in the green zone:

  1. Start your training sessions with 2 to 3 minutes of low intensity warm-up
  2. Perform 20 to 30 minutes of HIIT training
  3. Perform 2 to 3 minutes of cool-down training.

There is simply no need to do longer HIIT workouts as long as the focus is on fat loss and not on increasing performance. If you feel that you need more HIIT to lose fat efficiently, then your diet is probably far from optimal.

How often should you do HIIT workouts?

The total amount of HIIT training you do per week will depend on your current goals and the rest of the training you are doing. If you are looking to lose fat fast, then you need no more than 4 to 7 hours of training per week and ideally you should do more resistance training than cardio.

In my training and diet programs, I only recommend 3 to 5 hours of training with weights and 1 to 2 hours of HIIT cardio per week. This way you lose fat instead of muscle and maintain a healthy metabolism.


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By Michael Matthews

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