High intensity interval training (HIIT) is a very popular training method. When used correctly it effectively improves cardiovascular conditioning, burns fat and promotes new muscle growth. In addition a relatively short HIIT session is sufficient to elicit substantial performance gains. Like any training method, understanding the basic physiological principles will make a big help to using it effectively. This article will explain a bit about this type of training and some of the pitfalls to watch out for.
HIIT is popular because it is time effective. An individual can burn a lot of calories in a short space of time. As the name implies it is an intense form of exercise. Our energy systems function on a simple mechanism of energy charge. The rate of energy (Adenosine Triphosphate/ATP) utilization in the muscle cell must be matched by an energy supply system. Slow rate of energy expenditure during low intensity work is supported by oxidation. Oxidation supplies a lot of energy but at a slow rate. High intensity work is supplied by the glycolysis and phosphate systems which have a much faster supply. Supply must meet the demand. There is often a slight lag between utilization and supply. This means that even during rest intervals and post exercise energy consumption is still elevated. In simple terms our metabolism is increased and we continue to burn more calories than at normal rest conditions. For this reason even though a 20min session burns, for example 500kcals, energy expenditure is raised throughout the day. A low intensity session lasting one hour may burn 800kcal with minimal elevation in metabolisms for the rest of the day. For this reason HIIT may actually burn more calories on a daily basis. This is why it is so effective at fat burning.
In addition, the power output which is produced during the work period of HIIT is high. Higher power output during work periods are often effective in improving your conditioning. It also helps maintain strength and power simply by utilizing larger motor units. The main issue to consider with HIIT relates to energy supply. If we cannot supply the cells with adequate energy then they become damaged. This is known as metabolic stress. A certain degree of metabolic stress or damage can be reversed. This is what promotes new muscle growth. Moderate metabolic stress during training can, at times, be quite effective for promoting hypertrophy.
If we place too much stress on the muscle cells the damage can be irreparable. The cells will begin to die. When this happens on a regular basis muscle wastage can occur. It also places the body under larger amounts of general stress which will begin to impact on our immune system. There is a large list of potential health implications that this can eventually lead to.
Preventing this scenario is relatively easy but not always something we think about. One of the determinants to energy supply is our energy store. In the case of HIIT we need adequate stores of glycogen for an adequate supply of energy. If we do HIIT in a fasted state we are putting ourselves under severe metabolic stress, as there is little energy supply to fuel it. In addition the lack of energy will dramatically reduce performance so conditioning benefits may also be lost.
The take home message is this. Fuel up for intense exercise! Low intensity exercise can be done in a fasted state as the oxidative system works effectively to provide fuel. With intense exercise such as HIIT style training, you must have some glycogen stores or glucose in the bloodstream. If you are in a totally fasted, glycogen depleted state then consume some simple sugars close to training. By doing so you can maintain high intensity and reduce cell stress. You will still achieve an elevated metabolism that promotes fat burning. You also place the cells under just enough stress to help promote hypertrophy
It is important to understand training methods as the smallest oversight can cause more harm than good. HIIT is an effective tool but if it is not adequately fuelled it loses a lot of its benefits. It is a popular successful way to train and should be used in any program. Like any training method the process is the important part. It needs to be considered and managed properly in order to see the full benefit.