Monday June 3 2013
HIIT involves short bursts of intense activity
'Just 12 minutes of intensive exercise per week is enough to improve your health if you are overweight,' according to The Daily Telegraph. The paper reports on the findings of a study into the phenomenon of high-intensity interval training (HIIT). It's claimed that HIIT can deliver many of the benefits of conventional exercise in a much shorter time.
Two groups of overweight and inactive, but otherwise healthy, middle-aged men either followed a programme of:
- four bursts of intense exercise for four minutes, each separated by three minutes of lower intensity exercise, three times a week (plus a 10-minute warm-up and a five-minute cool-down), or
- a single burst of intense exercise for four minutes, three times a week (plus the warm-up and cool-down)
After 10 weeks, both groups had improved their bodies' maximal oxygen uptake (a measure of fitness) and lost weight.
This suggests that in healthy overweight men, a regular programme based on a single burst of intense exercise may bring similar fitness benefits to a programme involving repeated bursts of intense exercise.
However, this was a very small and relatively short-term study. Ideally, larger studies are needed to explore the longer term effects of this type of exercise programme in more diverse groups of people.
HIIT may not be safe for everyone, especially if you have health problems or are not currently doing any exercise. Check with your GP first.
Where did the story come from?
The study was carried out by researchers from the K.G. Jebsen Center for Exercise in Medicine in Norway, and other research centres in Norway, Canada and the USA. It was funded by the K.G. Jebsen Foundation, the Norwegian Council of Cardiovascular Disease, the Norwegian Research Council, St Olav's University Hospital, Norway, and the Eckbos Foundation.
It was published in the peer-reviewed open access medical journal PLoS ONE.
The Telegraph and Mail Online website reported this study quite accurately, but both suggested that the single burst of exercise group did just 12 minutes of exercise per week. In fact, including the warm-up and cool-down sessions, they actually did 57 minutes of exercise per week.
What kind of research was this?
This was a randomised controlled trial (RCT) looking at the effects of regular short bursts of intensive endurance training on fitness.
Current official guidelines in the UK recommend that healthy adults should do:
- at least 150 minutes per week of moderate intensity aerobic exercise a week, or
- 75 minutes of vigorous exercise a week, with an equivalent mix of moderate- and vigorous-intensity aerobic activity every week (for example, two 30-minute runs plus 30 minutes of fast walking)
Doing muscle-strengthening activities on two or more days is also recommended. Read more about the physical activity guidelines for adults.
Many people find that even though they want to get fitter through exercise, it can be difficult to find the time. In the current study, researchers wanted to assess the effects of two shorter, high-intensity exercise programmes on fitness levels.
This type of study is the best way of comparing two different treatments or interventions, as the groups should be well balanced at the start of the trial. This means any differences at the end of the trial can be attributed to the intervention received.
What did the research involve?
The researchers enrolled 26 overweight men (BMI between 25 and 30) who were healthy but inactive. They were aged 35-45 and had not exercised regularly for at least two years prior to the study. Men with heart problems, lung disease, high blood pressure, kidney failure, or orthopaedic or neurological problems were not eligible to participate.
The men were randomly assigned to do one of two exercise programmes on an inclined treadmill three times a week for 10 weeks. One programme involved a single four-minute intense period of exercise (single burst group), and the other consisted of four bouts of four minutes' intense exercise separated by three minutes of "active recovery" (multiple burst group). The researchers say that each burst was the equivalent of a four-minute quick walk uphill at an 8-10% gradient, or rapidly walking up six to 10 flights of stairs.
Before both types of sessions, the men warmed up for 10 minutes by walking, jogging and running on an inclined treadmill with the aim of reaching 70% of their maximum heart rate. During the four-minute intense parts of their exercise, they aimed to reach 90% of their maximum heart rate.
The men whose programme included active recovery periods aimed to reach 70% of their maximum heart rate in these periods. Both groups of men also had a five-minute cool-down session at the end of their workout. Overall, the groups did 19 minutes (single burst group) and 40 minutes (multiple burst group) of exercise in total.
The researchers took various measures of fitness and cardiovascular risk. The main outcome they were interested in was the men's maximal oxygen uptake (VO2max), which is the maximum amount of oxygen they could consume each minute per kg of bodyweight while doing exercise. A higher VO2max indicates a higher level of fitness, and previous research has linked a higher VO2max measurement with a reduced risk of death. They also measured:
- blood pressure
- levels of fats, sugars and other substances in the blood
- body composition
What were the basic results?
The group doing the single burst of intense exercise had slightly higher BMIs at the start of the study (average BMI 27.8 compared with 27.0 in the multiple burst group). Two men in this group dropped out of the study – one because of back pain and one because he moved out of the area – and were not included in the analysis.
After the 10-week study period, the groups did not show statistically significant differences in change in any outcome. Both groups showed an increase in maximal oxygen consumption.
The single burst exercise group showed a 10% improvement, and the multiple burst group showed a 13% improvement. The difference between groups in this outcome did not quite reach statistical significance.
Both groups showed a reduction in weight: 1.8kg in the single burst group and 2.1kg in the multiple burst group. However, the researchers point out that short bursts of intense exercise are unlikely to achieve the kind of sustained weight loss needed to tackle obesity.
Fasting blood sugar level was also reduced in both groups. Blood pressure showed a statistically significant reduction in the single bout group, but not in the repeated bout group. Levels of one form of cholesterol (oxidised LDL cholesterol) showed a statistically significant reduction in the repeated bout group, but not in the single bout group.
How did the researchers interpret the results?
The researchers concluded that a single short bout of intense exercise performed three times a week "may be a time-efficient strategy to improve VO2max and reduce blood pressure and fasting glucose in previously inactive but otherwise healthy middle-aged individuals".
This small study has suggested that short periods of intense exercise can increase fitness to a similar level as sustained exercise, and show similar effects on outcomes such as weight. But this study has some limitations, including:
- The study was very small, involving just 26 men. This may mean that the results may not be as representative of the general population of similar men as a larger sample would be. It also means that it is less able to detect differences between the groups in their statistical tests, even if they exist.
- The study only included overweight but healthy men. The results may not apply to other groups of people.
- There was no group that didn't exercise at all or do less intensive exercise, so we can't tell what would have happened with these types of programmes.
- The study was only relatively short term and therefore could not look at long-term outcomes, such as whether weight loss was maintained or if there was an increased risk of heart attack.
Overall, the study suggests that fitness may be improved by regular exercise sessions that include a short, intensive bout of exercise. This may be encouraging for people who feel they don't have enough time to exercise. And for most people, some exercise is better than none.
However, it's important to note that the men in this study were healthy, with no heart disease, high blood pressure, or orthopaedic or lung problems. Short bouts of very intensive exercise may not be suitable for people who have these conditions.
It is always good to check with your GP if you are committed to getting fit but have been inactive for some time. Read more advice about starting a new exercise plan.
Analysis by Bazian. Edited by NHS Choices. Follow Behind the Headlines on Twitter.