High Intensity Interval Training is a form of cardiovascular exercise that alternates periods of high-intensity work with periods of rest or low-intensity activity.

The goal of HIIT is to push the body to its maximum capacity for short periods of time, leading to improved cardiovascular fitness, endurance and metabolism.

How does HIIT work, and what are its benefits?

The physiology behind HIIT is complex and multifaceted, but it primarily relates to the way the body utilises energy during high-intensity exercise.

1. When one works out at a lower intensity, such as a brisk walk or jogging, oxygen-dependent or aerobic metabolism predominates. The body uses oxygen to break down sugar and fat for energy. With that, the “system” is efficient but not intense. One can walk or jog for a long time before getting fatigued.

During HIIT, the body primarily relies on the energy systems of anaerobic metabolism, which does not require oxygen. The two main anaerobic energy pathways are the phosphagen system and the glycolytic system.

● The phosphagen system is the body's immediate energy source and relies on the breakdown of stored phosphocreatine to produce ATP. It kicks in the very first part of a high-intensity exercise.

● The glycolytic system uses glycogen as its energy source and generates ATP through the process of glycogen breakdown. It kicks in shortly after the above system.

These two systems are for “emergency” use and are inefficient when it comes to fuel expenditure.

2. Additionally, HIIT leads to spikes in the release of adrenaline and adrenaline-like hormones which play a role in the body's fight-or-flight response. These hormones increase blood pressure, vessels’ resistance and heart rate, leading to more energy consumption. Plus they increase the body’s ability to access stored energy, such as fat and glycogen.

The above mechanisms combined lead to a higher energy and nutrients use in a shorter period of time than in routine-type exercise.

3. The intense nature of HIIT also leads to an increase in lactate production and accumulation in the muscles. Lactate is a by-product of anaerobic metabolism and is typically associated with muscle fatigue and soreness. However, a recent look at previously dismissed research suggests that lactate may have anti-inflammatory and antioxidant properties.

4. HIIT also leads to the production of growth hormone and testosterone, which play a role in the body's muscle recovery and growth.

5. HIIT has been shown to improve insulin sensitivity and glucose metabolism, making it an effective tool for managing and preventing type 2 diabetes.

6. A large European study published very recently showed that HIIT can cause regression of coronary plaques, an effect only seen so far with drugs such as statins and PCSK9 inhibitors.

7. Regarding cognitive function: regular HIIT exercises of six to nine minutes have been shown to improve cognition, focus and mood.

What is an optimal HIIT regimen?

An optimal HIIT regimen will vary depending on an individual's fitness level, goals and any pre-existing health conditions. However, there are some general guidelines that can be followed to design an effective HIIT programme.

Frequency: for best results, HIIT should be performed two to three times a week, with at least one day of rest in between sessions.

Intensity: the high-intensity portion of the workout should be performed at an intensity of 80 per cent to 95 per cent of an individual's maximum heart rate. This can be measured through various methods such as a heart rate monitor or by using the rate of perceived exertion scale.

Duration: the high-intensity portion of the workout should last between 15 and 60 seconds, with the rest, or low-intensity portion, lasting twice as long. For example, a 30-second high-intensity interval followed by a 60-second rest interval.

Volume: the total volume of the workout should be between eight to 12 intervals, with a total workout time of 20 to 30 minutes.

Progression: the intensity, duration and volume of the workout should be gradually increased over time to avoid injury and to continue making progress.

Variety: to prevent boredom and plateaus, it is important to vary the exercises used during HIIT. As an example, one could sprint-swim or run in one session and lift weights or do burpees in the next.

What is a good example or method of HIIT?

One popular method is the Tabata protocol, which consists of 20 seconds of maximum effort followed by ten seconds of rest, repeated for eight rounds. Total workout is four minutes and can be repeated.

Is HIIT good for me?

It is important to note that most of the studies have been conducted on healthy young adults, and more research is needed to understand the effects of HIIT on specific populations such as older adults and people with chronic disease. Additionally, HIIT is a highly demanding form of exercise and should be approached with caution. A warm-out and a cool-down at the start and end of the session are recommended. Individuals new to exercise or with certain health conditions should speak with a healthcare professional before starting a HIIT programme.

Joseph Yammine is a consultant cardiologist at the Bermuda Hospitals Board. He is also the director of the outpatient multi-speciality clinics and director of the Cardiac Computed Tomography programme. Nisrine Atieh is a Boston-based paediatrician and an American-board certified obesity medicine specialist.

The information herein is not intended as medical advice nor as a substitute for professional medical opinion. Always seek the advice of your physician.