As high intensity interval training enjoys newfound popularity in mainstream fitness, Wendy Sweet explores the research – and highlights the cautions – relating to HIIT training for fitness professionals.

Intermittent work:rest ratio of exercise performed to exhaustion has been a training modality of elite athletes for years. More recent research, however, suggests that the packaging of high intensity interval training (HIIT) strategies into exercise programs for clinical and overweight populations is not only an effective way to improve aerobic metabolism in more sedentary individuals, but may even provide a more time-efficient and enjoyable way to work out than steady-state endurance exercise.

As well as being employed in the training of elite athletes, sprint interval-type training has been evident in the fitness industry for some time: it is not a new concept in how to get the ‘best’ results. Since my time teaching ‘aerobics’ throughout the early 1980s and 1990s, we knew that both the intensity of the cardio component, as well as how we structured the class (‘harder’ alternated with ‘easier’ music tracks), offered the secret to success, not only for fitness improvement but also for weight management. We could see that those members who regularly attended harder 50-minute classes that were structured in a hard:easy, ‘multi-peak’ way seemed to derive greater physiological and psychological benefits from each workout compared to members who worked out in what were, back then, marketed as ‘fat burner’ classes – up to 90 minutes of continuous, low intensity exercise. Those of us teaching in the days of ‘fat burning’ classes also knew what happened after about 45 minutes – those new to exercise, and also those with perhaps the most ‘fat to burn’, left! Not only did they have other things to do with their time, but after a while they became disillusioned as the fat failed to burn off!

Fast-track nearly three decades and we now know a lot more about the physiology behind what we were seeing (but not fully understanding) in those early days of exercise-to-music classes. Firstly, thanks to the work that went into the Wingate Protocol of anaerobic power testing for cyclists in the 1970s, we know more about quick, intense (maximal) workouts and the effect on anaerobic fitness and muscular power.

Secondly, based on the exercise physiology research that has emerged over the past two decades, we have greater understanding of energy systems and intensity of training, particularly as it relates to lipolysis (fat burning) and the muscular and mitochondrial changes that accrue from the effect of the post-exercise ‘after-burn’ (Excess Post-exercise Oxygen Consumption [EPOC Response]).`

Based on this knowledge, the last decade has seen harder exercise-to-music classes being restructured accordingly, and personal trainers who keep up-to-date with the latest research including ‘aerobic interval training’ strategies in their weight loss clients’ weekly programming regimes.

Today, thanks to more recent physiological research specific to low volume (time-efficient) HIIT training, we have even more knowledge about the physiological mechanisms behind this type of exercise strategy and, more importantly, why it is emerging as a ‘hit’ with trainers, clients and medical practitioners alike.

What is HIIT?

HIIT describes a regime of training that is based on the development of the Wingate test, i.e. a sprint, interval training workout consisting of 30 seconds of ‘all-out’ cycling effort against a supra-maximal load, immediately followed by up to four minutes of ‘recovery time’. Training bouts of between four and six work efforts followed by subsequent rest intervals mean that intense, anaerobic workouts can be completed in around 20 minutes.

The physiology behind HIIT

Studies on these types of high intensity interval workouts have demonstrated substantial increases in the oxidative capacity of skeletal muscles, as reflected by the activity of mitochondrial enzymes. Increasing the oxidative capacity of muscles is critical for a host of physiological adaptations in the human body, such as improved fat burning, improved cardiovascular fitness and more efficient functioning and use of stored glucose (glycogen) in muscles and the liver. According to researchers (Gibala, Little, MacDonald & Hawley, 2012), other adaptations have also been seen. These include increased resting glycogen content (good for restoring energy quickly); reduced rate of lactate production (this means new exercisers won’t ‘burn out’ as quickly while working at a harder intensity); increased rates of whole-body as well as skeletal muscle lipid oxidation (fat utilisation); and, finally, improved exercise performance as measured by time-to-exhaustion.

For those individuals new to exercise or trying to improve their health and reduce their weight, this is exciting information. As it has been traditionally known that these adaptations mainly occurred as a result of low intensity (65 per cent effort or less) continuous, endurance exercise as advocated by current public health guidelines, i.e. 60 to 90 minutes of continuous exercise, HIIT is now seen as offering a time-efficient strategy of training. With obesity, insulin-resistance, type 2 diabetes and cardiovascular disease on the increase in both Australia and New Zealand (International Obesity Taskforce, 2010; Ministry of Health, 2012) and with ‘lack of time’ known to be the greatest disincentive to exercise for many people, HIIT offers fitness professionals and health promoters another ‘tool’ for motivating, supporting and engaging a wide range of the population, including higher-risk individuals, into structured exercise.

Structuring a HIIT session

As with interval training for elite athletes, a number of variations of the HIIT training methods are emerging in the wider fitness industry. Most studies support the notion that it takes at least six weeks of low-volume HIIT for physiological effects to be seen. A protocol of HIIT suggested through research undertaken at McMaster University’s Centre for Active Living in Canada, (Gillen, August 2012), is to have clients or participants warm up on a stationary cycle for three to four minutes, before increasing the resistance on the bike until they are at about a Level 9 out of 10 on their rate of perceived exertion (RPE). At this level of intensity they perform one minute of hard effort, cycling as fast as they can. They follow this by reducing the workload until their effort drops down to an easy Level 4 RPE. This work:rest ratio is undertaken about 10 times.

Other HIIT protocols have also emerged using either different modes of exercise (e.g. swimming, running, stepping and rowing) or, depending on the starting fitness level of new exercisers, a longer rest component is allocated, e.g. one minute hard effort, followed by up to four minutes of light activity or rest (this lengthened rest period allows for some lactate clearance).

Cautions with HIIT training

Any exercise has cautions associated with it, and fitness professionals must recognise that it’s still a case of ‘one size doesn’t fit all’ when it comes to newly promoted training regimes.

With this in mind, here are a few cautions to remember:

  • New exercisers presenting with high health risk need medical clearance in accordance with best practise as promoted by ACSM (American College of Sports Medicine) Guidelines
  • At present the research has only been conducted on healthy male graduate students, older males and some obese women. What is not known is the effect of HIIT for women experiencing hormonal fluctuations and associated visceral weight gain of menopause, younger females (under 25), post-natal women and youth (12-18 years).
  • Exercise practitioners need to look at weekly structures of exercise which allow for recovery from HIIT. As in usual interval training regimes, two sessions a week is enough.
  • As per usual exercise safety, a warm up and cool down period are crucial. In higher risk populations, such as those with cardiovascular risk, the warm up and cool down periods should be lengthened to allow for less risk associated with the adrenaline response in exercise.
  • As in any intense exercise, effective monitoring of the client as they work at higher levels of intensity is crucial. Look for visual signs of stress during work intervals, monitor their heart rate and adjust the rest interval accordingly.
  • Keep your clients motivated and supported. Metabolic and mitochondrial changes accrue after around six to 12 weeks.
  • For clients undertaking HIIT for weight loss, it is vital that they understand nutrition and hydration options post-workout. It is known that high intensity training leads to increased exhaustion and a subsequent post-workout craving for sugar. Many people then resort to ‘quick-fix’ high sugar foods as well as caffeine. Support and educate your weight loss and diabetic clients accordingly!

Gibala, M., Little, J., MacDonald, M. & Hawley, J. (2012). Physiological adaptations to low-volume, high-intensity interval training in health and disease. The Journal of Physiology, 590 (5), 1077-1084

Gillen, J. (August 2012). Low-volume, high-intensity interval training: A practical fitness strategy. WellSpring Newsletter, 23,(4). University of Alberta: Alberta Centre for Active Living.

International Obesity Taskforce (2010). Obesity prevalence worldwide. Retrieved from: www.iaso.org/iotf/obesity

Ministry of Health [MoH] (2012). The health of New Zealand adults 2011/2012. Key findings of the New Zealand health survey. Wellington: Ministry of Health. ISBN: 978-0-478-40219-3


Wendy Sweet, MSpLS, BPhEd, RGN
Wendy has a masters degree in sport and leisure studies from Waikato University in New Zealand where she lectures in a number of sports science and exercise prescription papers as well as undertaking PhD studies. She is an award-winning REPS exercise professional and 30-year veteran of the New Zealand fitness industry. In 2010 she was recognised by Fitness NZ with the award for Most Outstanding Contribution to the Fitness Industry following her involvement in establishing the Les Mills Personal Training program as well as education programs for personal training throughout New Zealand.