High-intensity interval training, or HIIT, is one of the most time-expedient and effective ways to work out. The beauty of HIIT is that you improve aerobic fitness and anaerobic fitness, when you work at near maximal effort (90% of your V02 max or higher). In terms of aerobic fitness, it improves oxygen delivery to muscle tissues similarly to moderate-intensity cardio, thereby increasing endurance.
In terms of anaerobic fitness, HIIT can enhance the ease with which you do short bursts of intense activity like sprinting and intense weight training. With steady-state endurance exercise, you almost exclusively work slow-twitch muscle fibers while with HIIT training you force slow-twitch AND fast-twitch muscle fibers to adapt, which leads to greater explosive power and strength along with endurance.
HiiT Training and Active Rest Intervals
Now a new study published in Medicine and Science in Sports and Exercise highlights another benefit of high-intensity interval training. During HIIT training, you work hard, often near maximal effort, for a short period of time and then recover before repeating the sequence. During the active intervals, when you work above your anaerobic threshold, you build up hydrogen ions, lactic acid and waste products that quickly force you to stop. During the recovery period, you flush out some of those waste products so you can max out again.
The reason you’re able to work so hard is because the intervals are short and each active interval is followed by recovery. Can you imagine working at such a high intensity continuously? For one, you’d have to dial back the intensity to sustain it longer, and if you reduce the intensity enough, you’re working mostly your aerobic system and slow-twitch fibers, rather than fast-twitch fibers. So much for building speed and power.
Secondly, trying to sustain a high level of intensity for longer than 30 to 60 seconds feels uncomfortable, maybe unpleasant enough to make you want to quit. That’s exactly what researchers in this new study found – shorter intervals, although intense, don’t feel as strenuous as longer intervals of a minute or 2 minutes. So, if you want to get the benefits of HIIT training with less pain, keep the active intervals short.
In the study, researchers asked a group of overweight, untrained adults to do a session of intense exercise in a continuous manner or in intervals, similar to HIIT training. Active intervals were 30 seconds, 60 seconds or 120 seconds in length with a 1 to 1 ratio of work to recovery. The goal was to see whether the participants perceived interval training to be less strenuous than continuous exercise and whether the length of the active interval had any effect on perceived exertion. It showed the rate of perceived exertion was lower for interval training than for continuous exercise and shorter intervals of 30 seconds had the lowest rate of perceived exertion. Their conclusion:
“Performing more intervals of shorter durations appears to produce lower post-exercise RPE (rate of perceived exertion) values than performing fewer intervals of longer duration and equal intensity.”
Short and intense seems to be the sweet spot for interval training. This isn’t surprising. If you know you only have to work intensely for 30 seconds, you can push yourself hard because the discomfort is brief. Through interval training, using short active intervals, you can work out at higher intensities than you would be able to sustain in a continuous manner. As a result, you build endurance AND explosive power. Plus, if you’re pressed for time, you can get the benefits of an aerobic and anaerobic workout in short time period, as little as 15 minutes of HIIT training.
The Benefits of HiiT Training
Why is HIIT so effective? When you work out at a near maximal pace, you recruit more fast-twitch muscle fibers and more total muscle fibers. Calling fast-twitch muscle fibers into play triggers greater metabolic stress and release of anabolic and fat-burning hormones like testosterone, catecholamines and growth hormone that help you get lean. Yet, you’re still working slow-twitch, endurance fibers with HIIT training, so you’re getting multiple benefits during a single workout.
You might think that because HIIT workouts are shorter, you’ll experience less improvement in endurance than with moderate-intensity cardio. Not so. When 36 untrained men were asked to do steady-state running for 150 minutes a week versus running in intervals for 40 minutes weekly, a significant time savings, the interval runners experienced GREATER improvements in aerobic capacity after 12 weeks than those who ran at a continuous pace. (14% versus 7% improvement) The conclusion was that exercise intensity is more important than time for improving aerobic capacity.
Many cardiologists now recommend high-intensity training, under supervision, for patients after a heart attack due to the superior cardiovascular benefits it offers. Most research shows HIIT training is safe in a supervised setting for people with stable cardiovascular disease.
HIIT training may be superior for improving body composition as well. Some studies show intense interval training is better for trimming belly fat, and you enjoy an after-burn where your body has to work harder to recover afterward, leading to greater post-workout calorie burn. Plus, long periods of moderate-intensity cardio elevate cortisol more and leads to muscle breakdown. Due to the intensity of the training, HIIT transiently increases cortisol but for shorter periods of time. When cortisol stays up, it can lead to fat redistribution with fat storage where you don’t want it – on your waist and belly. With HII training, you also get the release of anabolic hormones that counter the effects of cortisol.
Another benefit of high-intensity interval training is you can modify it to meet your fitness level. If you’re just starting out, you don’t have to “max out” on the active intervals. Take it slow and keep the recovery intervals longer until you build up a higher level of fitness.
The Bottom Line
As hard as it is, HIIT training has a lower rate of perceived exertion than continuous exercise, so you can push yourself harder without losing your motivation. Even if your primary goal is to boost aerobic fitness, HIIT training is an effective choice. Plus, you get the additional benefits, like a more sustained after-burn, which you won’t get with moderate-intensity cardio. To make an interval training workout feel easier, keep the intervals short, around 30 seconds.
Poloquin Group. “Is Aerobic or Anaerobic Training Best for Getting Rid of Belly Fat?”
Medicine & Science in Sports & Exercise: May 2015 – Volume 47 – Issue 5 – p 1038-1045.
Skoluda, N., Dettenborn, L., et al. Elevated Hair Cortisol Concentrations in Endurance Athletes. Psychoneuroendocrinology. September 2011.
Circulation. 2012 Sep 18;126(12):1436-40. doi: 10.1161/CIRCULATIONAHA.112.123117. Epub 2012 Aug 9.
“Effectiveness of high-intensity interval training in patients with coronary heart disease: a systematic review protocol” Adrian Elliott PhD1,2, David Bentley PhD2, Edoardo Aromataris PhD1.
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