RunnersJames Tang discusses the merits of high-intensity interval training for time-poor dental professionals.

Dentists are acutely aware of the importance of regular exercise for healthy living and we are well informed about the adverse effects of obesity due to a sedentary lifestyle. We also know that prolonged sitting is related to a host of musculoskeletal disorders, such as lower back pain and neck pain.

Surely we all wish to enhance our cardiovascular fitness and well being through regular exercise; the problem is, for most people, fitness training comprises a workout involving long, continuous exercises at a constant intensity. In this day and age when dentists are so busy – and bearing in mind that we do not always have time to spend hour after hour in the gym – what is the most effective way of burning fat and improving overall fitness?

According to the American College of Sports Medicine, more calories are burned in short, high-intensity interval training (HIIT) routines. Not only does this type of training improve your overall cardiovascular fitness and save time, it also helps you to increase your metabolism and burn calories for hours after you have completed your workout. Let’s take a look at how this can be achieved.

What is HIIT? Why use it?

Athletes have used interval training to build fitness for many years. This type of training combines short, high-intensity bursts of speed with slow recovery phases (active rest), repeated during one exercise session.

It was believed that a long, steady-paced cardio workout is superior for fat loss because relatively more fat is burned via aerobic glycolysis at lower exercise intensities than at higher intensities. The so-called ‘fat burning zone’, which is around 60-65% of maximum heart rate, is really not optimal for burning fat. It is true that you burn more fat relative to glycogen, but the main concern is the aggregate amount of fat burned. At higher intensities, you burn more fat in total, even though the fat/glycogen ratio is lower.

So, the more intense your training, the greater the duration and the greater the frequency, the larger the number of calories expended. In that case, why don’t we just push hard the whole time instead of mixing in periods of active recovery/rest? The problem is that you can only maintain peak intensity for a very short period of time; you must use intervals where you train at peak intensity in short bursts followed by moderate-paced cardio (active rest), and then start all over again.

The physiology of HIIT

Do you know what happens to a car’s engine at the end of a long journey? It gradually cools to a resting temperature. The same happens to your body after exercise; once a workout is over, your body’s metabolism can continue to burn more calories than when at complete rest. This physiological effect is called excess post-exercise oxygen consumption (also known as EPOC or oxygen debt), and this is the amount of oxygen required to restore your body to homeostasis. Let us take a look at how oxygen debt is achieved and how it is beneficial to us in terms of fat burning and weight loss.

  • Intensive phase of a workout: the body is most efficient at producing adenosine triphosphate (ATP – a molecule that transports chemical energy within cells for metabolism) through aerobic metabolism. However, during the high-intensity phase, it cannot supply oxygen at a rate that is fast enough to fuel the muscles so the anaerobic system kicks in, using your intramuscular glycogen store to ‘assist’ in providing energy to the muscles for short bursts of activity. This type of training depletes the intramuscular glycogen within a couple of minutes. Recovery should be active (eg walking between running intervals) to aid the return of blood to the liver where glycogens are stored. The active recovery period allows aerobic glycolysis to produce and replace ATP in the involved muscles
  • Recovery phase: the byproduct of anaerobic glycolysis is lactic acid and, as this builds up, you enter oxygen debt, and it is during the recovery phase that your heart and lungs work together to ‘pay back’ this debt and break down the lactic acid. In other words, after completion of your intense training, your body has to repay that ‘borrowed energy’ it owes to return your body to homeostasis. The more energy your body borrowed during the intense effort, the more oxygen it owes.

The relevance of HIIT to fat loss

The body expends approximately five calories of energy (a calorie is the amount of energy required to heat one litre of water to 1ºC) to consume one litre of oxygen. Therefore, increasing the amount of oxygen consumed both during and after a workout can increase the amount of net calories burned.

So, after an intense effort, you continue to breathe hard and your heart rate is elevated; your metabolism is operating at a higher rate to repay the oxygen it borrowed. The larger the oxygen debt created by your workout, the longer it will take to repay it, with the benefit of more calories being burned for a longer period of time after your training. So, what is the best way to create a large oxygen debt?

It must be noted that the level of oxygen debt is influenced by the intensity of the exercise, not the duration. In order to create an oxygen debt, the effort has to be intense enough to switch over to ‘borrowed energy’ or to an anaerobic mode.

Therefore, pushing hard all the way before catching your breath is the best way of increasing oxygen debt, and this also increases the production of lactic acid. You should perform another intense exercise before the body has recovered to assist it in accumulating more lactic acid in the muscles. With enough intervals, you will have a build-up of lactic acid and your body will in turn be able to burn more calories than normal after your workout has been completed.

When the production of lactic acid exceeds the body’s ability to disperse it, there will be a build-up of lactic acid – onset of blood lactate accumulation. Targeted interval training improves this lactate tolerance. By performing high-intensity intervals, the body adapts and burns lactic acid (ie, resynthesis of intramuscular glycogen from lactate) more efficiently during exercise. As a result of this adaptation, you are able to exercise at a higher intensity for a longer period of time before fatigue or pain kicks in and slows you down.

How to perform effective interval training

An appropriate interval training routine should include just enough rest so that you are able to push hard for the next intense effort, and you should be breathing pretty hard when the whole series of intervals is complete.

Sprinting and resistance training are the best ways to accomplish this. Resistance training with compound exercises (exercises that involve more than one joint) for large muscle groups, such as those in the chest, back, glutes and legs, places a greater demand on the involved muscles for ATP from the anaerobic glycolysis.

Examples of such exercises include chest presses, pull ups, deadlifts and squats. The increased need for anaerobic ATP also generates a greater demand on the aerobic system to replenish that ATP during the rest intervals (ie, you are performing the next intense effort before you catch your breath) and the post-exercise recovery process. Heavy training loads or shorter recovery intervals increase the demand on the anaerobic energy pathways during exercise, yielding a greater oxygen debt during the post-exercise recovery period.

Combining intense efforts while already breathing hard is the best way to create maximum oxygen debt. This is one of the reasons why interval training is ideal for boosting the metabolism.

Types of HIIT

Designing the correct interval training routine can be sophisticated or casual. Elite athletes may go to a sports performance lab to have their blood lactate and exercise metabolism tested to determine the optimum interval training routine. At the other end of the spectrum, casual ‘speed play’ interval training (‘fartlek’) can be adopted. You need to consult your exercise professional to select the most appropriate type of interval training to meet your goals.

There are different types of interval training, each designed to target the different energy systems mentioned above. Going into great detail about this is beyond the scope of this article.

Interval training programmes are now easily adapted to suit most sports. This is done by manipulating the intensity and duration of the work intervals, as well as the length of the rest periods, to create the desired adaptations.

Conclusions and safety tips

Keep in mind that interval training is extremely demanding on the heart, lungs and muscles, and it is important to have clearance from your doctor before you begin. You should also have a solid base of overall cardiovascular fitness before embarking on this type of training. You should engage an exercise professional to select the appropriate type of interval training for your goals and to give you guidance as to which cardiovascular equipment to use and how to perform the exercise safely.

Beginners should start with short intervals (under 30 seconds), fewer repeats and more rest. Elite athletes can up the intensity, time and frequency of training. Few athletes benefit from performing intervals more than twice per week.

In addition, don’t forget to incorporate regular stretching and flexibility training into your fitness programme. The added intensity of interval training requires your muscles and joints to be flexible and supple.

Please be reminded that losing weight and improving overall fitness is not just about exercising; it also involves other aspects of lifestyle, such as diet, work, rest and management of stress. Unfortunately, dentistry is well known for being a stressful job.

Cortisol, the body’s stress hormone, is a catabolic hormone that promotes the breakdown of carbohydrates, proteins and fats to provide energy for the body during stressful periods. It offers support during short-term bouts of stress. However, chronic stress and the resulting excess cortisol can lead to deterioration in health due to an unbalancing of the endocrine system.

Apart from causing muscles to break down, cortisol also boosts appetite and causes the body to hold on to fat. It encourages fat to be stored around the middle and it is known that fat in this area is associated with an increased risk of heart disease and diabetes.