High-intensity training has been a hot media topic over the last couple of years, from magazine and newspaper articles to prime time television shows on the BBC. It seems that the idea of achieving as much benefit within less time is a solution that appeals to many people in a world where available workout time is at a premium. However, the idea of using higher intensity training to boost physical benefits is certainly not a new one. Early research by scientists like Bahr, Tremblay and Tabata, all of which have become renowned in this field, date back to the early 1990s. Take note that not everything published in the media or blogosphere on high-intensity training is justified by the scientific literature. Fitness professionals and enthusiasts often use the basic concept of high-intensity interval training and embellish the truth a little, perhaps unknowingly, to suit their own desired outcome. The intent in this post is to glean the facts around this popular training method from the scientific literature so that you are correctly informed going forward in your utilisation of HIIT as a training modality.

• Tremblay showed in 1994 that a 15 week HIIT programme reduced total skinfolds 14 mm subcutaneous body fat compared to a 20-week endurance training (ET) programme that only reduced by 4mm total skinfolds. The HIIT protocol being 3.5 times more effective.  The ET programme was steady-state exercise beginning at 30 minutes at 60% and progressing to 45 minutes at 85% HR max as the test subjects were able. The HIIT protocol was 30 minutes of short bursts, beginning at 10 x 15-second bursts progressing to 15 x 30-second bursts as the test subject was able.
• In 1996 Tabata published a study demonstrating that 8 bouts of 20 seconds at 170% VO₂max with 10 seconds rest in between each set had the same benefits to the aerobic system as 60 minutes of steady-state training at 70% VO₂max. However, the HIIT protocol also caused a 28% improvement in anaerobic capacity that was not observed in the low-intensity protocol.
• Borsheim and Bahr are renowned for their work on increased metabolism following exercise, also known as excess post-exercise oxygen consumption (EPOC). In 2003 they carried out a substantial review of the scientific literature and showed that exercise intensity has a curvilinear relationship with EPOC whereas exercise duration is linear. Increased EPOC for 8-12 hours after intense training periods was common.  In simple terms, you get more bang for your EPOC buck by driving CV intensity up rather than spending longer at moderate intensities.
• Talanian in 2006 showed that a 2 week (7 sessions) aerobic HIIT training protocol (10 x 4 min, 90% bursts with 2 min rest periods) significantly (up 25%) increased muscle fatty acid oxidation.
• Trapp in 2008 showed that HIIT training is effective even in overweight deconditioned women as well. A 20-minute cycling HIIT protocol was compared to a 40-minute steady-state cycling regime of the same frequency for 15 weeks. Both groups had similar CV improvements but the HIIT group had significantly greater body fat loss on legs and trunk and improvements in insulin resistance.
• Boucher in 2010 reviewed the available scientific evidence surrounding HIIT and concluded that while there was valid evidence to show it is more effective at decreasing both subcutaneous and abdominal body fat compared to steady-state training, there is also clear evidence of individual variation in response – not all participants appear to receive the same level of fat loss benefit.
• Resistance training has also been shown in scientific studies (Melby 1993, Laforgia 1997) to influence EPOC and fat burning when lower volume, higher intensity weight training is utilised in preference to higher volume, moderate resistance work, but the research is still ongoing in this field.

While this is just a brief look at some of the science on high-intensity training, it does illustrate that in comparison to steady-state exercise, the benefits of HIIT training are:

• it may be as good at providing aerobic training benefits
• it is better a stimulating anaerobic training benefits
• it significantly improves body fat reduction
• it increases EPOC for up to 12 hours’ post-training
• it improves insulin sensitivity to working muscles

It is important to note that the majority of these studies have been performed in a highly controlled environment and most often using a cycle ergometer (bike) or a treadmill. Whilst it is reasonable to assume these benefits may also carry over to other training modalities, such as circuits, group training or resistance work there is much less current evidence to suggest this is true. Perhaps in time the science will more fully support and provide confirmation that high-intensity training has a broader application across a range of training modalities with the same beneficial results.

In the meantime, while we wait for science to catch up, it is clear that the many different methods of HIIT training can be great fun, they definitely save on time in the gym and it can deliver a real motivational boost to your training. If it does provide an increased fat burning boost as well, then all the better!

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