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Exercise, weight loss and health: Is high intensity the key?

Australia’s Physical Activity and Sedentary Behaviour Guidelines state that:

  • Any physical activity is better than none. If you currently do no physical activity, start by doing some, and gradually build up to the recommended amount.
  • Be active on most, preferably all, days every week.
  • Accumulate 150 to 300 minutes (2.5 to 5 hours) of moderate intensity physical activity or 75 to 150 minutes (1.5 to 2.5 hours) of vigorous intensity physical activity, or an equivalent combination of both moderate and vigorous activities, each week.
  • Do muscle-strengthening activities on at least two days each week.

Although being physically active and limiting sedentary behaviour are essential for health and wellbeing, 54 per cent of women and 51 per cent of men aged 18–64 did not meet physical activity recommendations in 2014–15. Clearly, we need new approaches to encourage and enable Australians to maintain a healthy lifestyle.

High-intensity interval training (HIIT), which involves repeated short bursts of intense exercise, has received a great deal of attention in both the mainstream media and scientific literature over the past decade. One of the main reasons for this interest is that proponents claim HIIT offers a way to improve fitness – gain cardiovascular health, lose excess bodyweight – in much less time and with much less physical exertion than traditional approaches. Does the evidence stack up?

The concept of fitness

First, let’s consider what physical fitness means. Most people think of fitness as a combination of a lean body and muscular endurance – having the ability to run, swim or play sport for a reasonable length of time. Essentially, this concept describes aerobic (or cardiovascular) fitness – the ability of the cardiovascular system to supply oxygen and energy to your body over time.
A common measure of aerobic fitness is VO2max, which is the highest amount of oxygen taken up during intense exercise, measured in millilitres of oxygen used in one minute per kilogram of body weight (ml/min/kg). It is measured while exercising in a standardised way, usually on a treadmill or stationary bike. Most people have a VO2max of 30–60 ml/min/kg; highly trained athletes can record over 90 ml/min/kg.

Nonetheless, a high VO2max and a lean body are not essential for high-level athletic performance. An Olympic weightlifter would consider herself fit if she could lift her target weight, but might have a body mass index (BMI, see below) that signals clinical obesity and low VO2max. A sprinter needs huge peak cardiovascular performance to be able to convert oxygen and energy into power and velocity, but usually cannot maintain this output for as long as an endurance athlete. A high jumper might be clinically underweight on the basis of BMI and have low VO2max, but achieving his competitive goal requires a dozen or so bursts of acceleration lasting only a few seconds each.

Fitness clearly means different things to different people. Nevertheless, because overall and long-term health is so heavily dependent on cardiovascular fitness, in this article we concentrate on that aspect, referring frequently to VO2max.


BMI, calculated by dividing a person’s weight in kilograms by the square of their height in metres, is a common and convenient but flawed measure of overweight and obesity. It is generally interpreted as an index of fatness, but in fact is an unreliable indicator of body fat percentage. In addition, the standard BMI thresholds for obesity do not work well across ethnicities. For example, Asians tend to have slighter builds, and therefore lower BMI, but higher body fat percentages than Westerners; and muscular or solidly built people can be healthy, fit and lean but have a BMI that classifies them as obese.

HIIT versus traditional ideas about exercise and fitness

The dominant understanding has been that exercising for longer at a given intensity produces more benefits – a dose–response effect – as does exercising for a given period at a higher intensity. Ultimately, the standard view among physiologists and sports scientists has long been that more total exercise, in terms of energy output, is better.

In contrast, HIIT involves short periods (10–30 seconds) of all-out (‘maximal’) exercise, typically running or cycling, interspersed with low-intensity recovery. Some research suggests that a beneficial HIIT exercise session can be performed in just three sessions per week, each lasting only 10 minutes each. As noted earlier, if true, HIIT is a way for time-poor and exercise-averse people to meet the guidelines for physical exertion and improve their cardiovascular fitness, leading to greater longevity and wellbeing.

HIIT in highly trained athletes

Research has shown that training at different intensities produces quite different responses in well-trained endurance athletes. In a study published in 2017, Thomas Stöggl and Glenn Björklund randomly assigned 33 male and three female runners, cyclists, triathletes and cross-country skiers to one of the following regimens over nine weeks:

  • HIIT – 27 high-intensity (HI) and 20 low-intensity (LI) sessions, averaging 66 hours in all
  • polarised training – 37 LI, 3 moderate-intensity (MI) and 14 HI sessions, averaging 104 hours
  • a high-volume low-intensity control group – 36 LI and 18 MI sessions, averaging 93 hours.

All participants underwent a maximal anaerobic running/cycling test before and after the training period to give baseline and follow-up measurements of neuromuscular status, anaerobic capacity/power and acute heart rate recovery (HRR). Only the HIIT group achieved improvements in peak power output (6.4 per cent). Acute HRR was improved in the HIIT (11.2 per cent) and polarised (7.9 per cent) groups, but did not change in the control group.

HIIT in non-athletes

Of course, highly trained athletes are physiologically different to the rest of us, so results obtained with them do not necessarily translate to the general population. However, considerable research has now been done on the effects of HIIT on ‘normal’ people.

In one of the earliest HIIT studies, 40 healthy, moderately trained male subjects were randomly assigned to one of four groups:

  • long slow distance (70 per cent maximal heart rate, or HRmax)
  • lactate threshold training (exercising at an intensity at which lactate begins to accumulate in the blood faster than it can be removed; 85 per cent HRmax)
  • 15/15 running – 15 seconds of running at 90–95 per cent HRmax followed by 15 seconds of active resting at 70 per cent HRmax
  • HIIT – 4 x 4 minutes of interval running (4 minutes of running at 90–95 per cent HRmax followed by 3 minutes of active resting at 70 per cent HRmax).

All training protocols resulted in similar total oxygen consumption and were performed three days per week for eight weeks. The HIIT group significantly increased its average VO2max compared with the long slow distance and lactate-threshold training groups (P<0.01). The 15/15 group increased average VO2max by 5.5 per cent, and the 4 x 4 group by 7.2 per cent. Heart stroke volume increased by approximately 10 per cent after both 15/15 and 4 x 4 interval training (P<0.05).

Given the increasing recognition of the interrelationships between brain and body health, it’s worth noting that very recent research has shown that HIIT can improve cognitive inhibition and attention capacity. Thirty-six overweight inactive men (aged 18–30 years) were randomly assigned to a HIIT group (4 x 4-min intervals at 85–95 per cent HRmax interspersed with four minutes of active recovery at 75–85 per cent HRmax), progressive resistance training group (PRT – 12–15 repetitions per set of six exercises targeting all major muscle groups), combined training group (PRT + HIIT) or control group (no exercise); three treatment groups achieved the same total energy expenditure. Numerous measures of cognitive inhibition and concentration improved significantly in the HIIT and combined PRT + HIIT groups relative to the PRT and control groups.

HIIT and ageing

A review published in 2015 concluded that lifelong physical training based on short, high-intensity exercise is at least as beneficial for healthy ageing as the traditional model of prolonged submaximal exercise focused on endurance. In particular, the authors noted that ‘the rate of decline in VO2max seems to be significantly smaller in [ageing] sprinters than in endurance runners and untrained individuals’.

In 2016, Heiskanen and colleagues wrote that:

exercise-induced hypertrophy [growth] of the left ventricle (LV), so-called athlete’s heart, is generally associated with excellent health outcomes … [In contrast] studies have reported that prolonged intense exercise leads to reduction of right ventricle (RV) function … studies on the RV adaptations to HIIT in healthy subjects are scarce.

The authors investigated RV adaptations to HIIT and whether HIIT and moderate-intensity continuous training (MICT) have similar or different effects, measuring RV glucose uptake (RVGU – lower levels indicate better heart function) and fat metabolism in 28 untrained, healthy 40–55-yr-old men. Participants performed six supervised cycle ergometer training sessions over 2 weeks (HIIT: 4–6 × 30-second all-out cycling, 4-minute recovery intervals; MICT: 40–60 minutes at 60 per cent VO2max). RVGU decreased after both HIIT and MICT, but fat metabolism, RV mass and stroke volume remained unaltered. They concluded that only 2 weeks of either HIIT or MICT decreases RVGU in previously sedentary middle-aged men.

Carla Coetsee and Elmarie Terblanche investigated the effects of resistance training, HIIT and moderate continuous aerobic training on the cognitive and physical functioning of healthy older adults. They randomly assigned 67 inactive people, aged 55 to 75 years, to a resistance training group, HIIT group, moderate continuous aerobic training (MCT) group and a control group for 16 weeks. The HIIT group showed the greatest practical (and statistically significant) improvements in reaction time, walking endurance and functional mobility. MCT participants had very large practical and significant improvements in reaction time on executive cognitive tasks.

HIIT and weight loss

With overweight and obesity among the most significant health concerns for Australians (and as shown in a previous In Brief, especially among people in Victoria’s western region), any relatively easy or efficient method of reducing excess body weight is obviously important.

Stefano Lanz and colleagues compared 2-week training modalities – continuous intensity for maximal fat oxidation (Fatmax) and HIIT – in 19 obese men (BMIs ≥ 35). Both groups performed eight cycling sessions comprising equivalent total work. Aerobic fitness and fat oxidation rates during exercise were significantly higher in both groups after training, but insulin resistance was significantly reduced only for the Fatmax group.

In August 2018, Tomas Tong and colleagues published a randomised controlled trial of the ability of two forms of HIIT, ‘traditional’ HIIT and sprint interval training (SIT), to reduce abdominal visceral fat in 46 obese young women. SIT involved 80 × 6-second all-out cycle sprints interspersed with 9-second passive recovery; HIIT comprised repeated 4-minute bouts of cycling at 90 per cent VO2max separated by three minutes of rest; both achieved 400KJ of work. Participants performed 3–4 sessions per week for 12 weeks. Abdominal visceral fat area and abdominal subcutaneous fat area were measured using computed tomography scans. SIT and HIIT reduced abdominal visceral and subcutaneous fat significantly in comparison to the control subjects (who did no exercise). HIIT reduced abdominal subcutaneous fat area more than SIT, but VO2max increased significantly more in the SIT group than the HIIT group. The authors concluded that the lower training load and time commitment of SIT represented a considerable advantage over traditional HIIT, and had similar fat reduction benefits in obese young women.

Real-world studies

The research described above was undertaken under close supervision in laboratory conditions, producing effects that might not be replicable in the real world. A recently published article describes one of few studies of the effectiveness of HIIT in a real-world setting. The research involved an unsupervised HIIT program in 250 overweight and obese adults over 12 months. Participants could choose HIIT or follow current exercise guidelines of 30 minutes per day of moderate-intensity exercise. HIIT participants (n=104; 42 per cent) received a single training session and were asked to independently perform HIIT three times a week (activities reported included running; running up hills and stairs; using bikes, elliptical trainers and rowing machines; burpees and star jumps; gym workouts; playing futsal and squash). HIIT participants reported significantly greater enjoyment of physical activity, but at 12 months there were no significant differences over time or between the groups in weight and visceral fat volume. This was considered to be due to poor adherence to training; only 23 per cent of HIIT participants adhered fully over 12 months. However, adherent participants achieved significantly greater average reductions in weight (–2.7 kg) and visceral fat (–292 cm3) than non-adherent participants. These adherent participants were mostly male and leaner than non-adherent participants, but did not differ in age, BMI, physical activity or aerobic fitness at baseline.

Risks of HIIT

Any movement that involves ‘sprints’ – bursts of high-energy activity – means the limbs must be accelerated rapidly, and this generates much higher forces on the body than more moderate forms of exercise. Such activities can overload tendons and ligaments, which is particularly problematic for the less-resilient bodies of older adults. However, according to Krzysztof Kusy and Jacek Zieliński, research shows no significant association between Achilles and patellar tendon injuries and sprint training in master track-and-field athletes. These authors also noted that:

Competitive master athletes participating in long-term intensive endurance training regimens may have a somewhat higher risk of deleterious cardiovascular structural and functional changes than the general population. An analogous risk in aging sprint-trained athletes is not known.


The research to date suggests that various forms of HIIT have benefits for aerobic capacity, heart health and weight loss, regardless of age or baseline physical condition. Unsurprisingly, research also shows that these benefits are only obtainable if subjects adhere to the HIIT regimen. Nonetheless, the reduced time involved in HIIT makes it an undeniably attractive option for people needing to incorporate exercise into their lives.