To the Heart of it

High Intensity Interval Training (HIIT) versus Steady State Cardio (SSC)

When most people decide to get into shape and become fitter they generally turn to heart healthy “cardio” exercise. Running, swimming, cycling, etc. In recent years the new craze in the fitness industry has been the insurgence of High Intensity Interval training (HIIT).

For years the fitness world has prescribed Steady State Cardio (SSC) as the preferred method for burning fat, losing weight, and getting into shape. But now it all seems to be about HIIT. I constantly get asked what is the best form of exercise for burning fat and losing weight? Is it HIIT or SSC? This can be a complicated question to answer, as it really is about application. So to help you understand cardio training more you need to know about fuel systems.

Intensity = Type of Fuel

The sources of energy you use when performing exercise varies depending on the intensity it is performed at. At low intensity (easy pace) your body should convert energy or fuel from predominantly fat stores and at high intensity from carbohydrates (stores as glycogen in the body). Your body can convert energy quicker from carbohydrates than from fats. Hence, as intensity increases the continuum of fuel source will switch to carbohydrates to provide the muscles with the fuel they require quickly. When there is a demand for quick energy carbohydrates are the preferred fuel source.

The intensity fuel relationship

The graph illustrates the continuum of fuel source where at rest and at low exercise intensities (expressed as %VO2max), fat is the predominant fuel. During moderate intensity exercise fat and carbohydrate contribute equally to fuel muscle contraction. However, as exercise intensity increases to moderately hard or hard a point is reached (ranging from 65-75% VO2max) where carbohydrate becomes the predominant, and then the exclusive, fuel for working muscles.

Carbohydrates can be considered the high power fuel as they are the primary fuel used by fast twitch muscle fibres and provide more calories per litre of oxygen consumed as well as being the only fuel source that can produce energy anaerobically. Where as, fat can be considered the low power fuel, as it is the primary fuel used by slow twitch muscle fibres and produces fewer calories per litre of oxygen consumed.

The fuel continuum is important to understand becuase at different intensities you will burn more or less fat even if the overall calorie expenditure is greater. The below diagram illustrates this by showing how many grams of fat are ‘burnt’ verses overall calories ‘burnt’ at different intensity levels.  At 25% VO2max the total calories expended is approx. 250kcal per hour with 24g of fat being utilised. Where as, at 85% VO2max and a total calorie expendature of approx. 1000kcal only 23g of fat were utilised for fuel. This is a great illustration of how ‘total calories out’ doesn’t necessarily equal fat loss and that the conventional wisdom of “eat less vs do more” is not the total picture for fat loss.

The Romijn study of % fuel vs. exercise intensity

The popularity of HIIT training has developed because it requires less time and can deliver great results. The real benefits of HIIT training are the facts that it releases beneficial fat burning and muscle building hormones. HIIT helps create muscle, ‘burns’ a good amount of calories, continues to burn calories after exercise (through thermoregulation) and elicits beneficial fat burning hormones all in a shorter period of time.  Not everyone has 1-2 hours a day to exercise, so HIIT is the great answer to this because studies have shown that as little as 10 minutes of HIIT can give you great fat burning benefits.

SSC needs to be performed at an intensity that meets your goals. Low intensity SSC is great for training your body to be an efficient fat burning machine and is an important part of a healthy existence. To burn fat you need to train your body to use fat at low intensities. If you always train at high intensities you are training your internal systems to use a high percentage of carbs at rest. In addition, if you consume a lot of starchy carbs and simple sugars you are also training your internal system to predominantly use carbs for fuel. Therefore, your body will not burn optimal amounts fat even at rest when there is no requirement for quick energy.

The basic truth is that we need to move. Where people go wrong is with the intensity at which they exercise. As you can see from the charts if you are exercising using SSC you need to choose your intensity wisely. SSC does not produce the beneficial fat burning hormones that HIIT does and you need more time to complete workouts.

My advice is to incorporate both HIIT and SSC into your schedule to maximise health, fitness and weight management. You need to train smarter by matching your intensity to the training goal and eliciting beneficial fat burning hormones.


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