High – intensity interval training or HIIT is the most time expedient way to train. Who doesn’t like a short, sweaty but effective workout? To perform a high-intensity interval workout, you alternate periods of high-intensity exercise with recovery intervals where you rest or do an active recovery. The rest periods are long enough to allow your body to partially recover. Then, it’s on to the next interval! You cycle back and forth in this manner until you’ve completed your workout. Phew!
Why is HIIT training so popular? Studies show that high-intensity interval training boosts aerobic and anaerobic capacity without long sessions of repetitive endurance exercise, like jogging, cycling, or running. That’s good news if you like short and sweet workouts. Short doesn’t mean easy though. With high-intensity interval training, you work harder during the active intervals than you do with endurance exercise. but your workouts can be as brief as 10 minutes.
Why the focus on HIIT? Studies show that high-intensity intervals can boost aerobic capacity in the same way that moderate-intensity exercise can. But what interval structure is best? With high-intensity training, the active intervals can be as long as a minute or as short as 10 seconds. During an ultra-short interval, you’ll be able to work harder than with a 60 second interval. It’s tough to maintain a very high intensity for a full minute!
What HIIT interval structure is best for enhancing aerobic capacity? A study at Liverpool John Moore’s University in the United Kingdom looked at this issue. They asked 26 healthy, untrained men and women to do high-intensity interval workouts three times per week for six weeks. One group did 4-8 rounds of 30-second intervals with 4 minutes of rest in-between. Another group did 6-10 cycles of 60-second intervals and rested for a minute between each active interval.
Before the study and at six weeks they measured the subjects’ aerobic capacity, the stiffness of their arteries, and their body composition. They found that subjects who did HIIT training using 60-second active intervals improved their aerobic capacity the most. In contrast, those who worked out using 30-second intervals followed by 4 minutes of rest experienced no significant change in aerobic capacity. This doesn’t mean they got no benefits from the workout, but the interval structure didn’t boost their aerobic capacity. Also, keep in mind that this was only one study too.
Why would the 30-second active interval structure fall short? Researchers believe the rest period of 4 minutes between active intervals was too long. During a 4-minute rest interval, the heart rate drops, and this may interfere with gains in aerobic capacity. In contrast, the heart rate would increase more during 60-second active intervals and the 60-second rest was short enough to keep the heart rate from falling.
High Intensity Interval Ratios
One way to describe a high-intensity interval is the ratio of the time you spent in the active phase versus how much you spend in the rest or recovery phase in between. For example, a high-intensity interval structure of 60-second activity followed by 60 seconds of rest or recovery is a 1:1 ratio. Research shows that an interval ratio of around 1:1 is optimal for enhancing aerobic fitness capacity. So, such a workout would be structured so the active interval is around the same length as the rest interval.
As mentioned, high-intensity interval training improves anaerobic capacity too, the ability to perform at high capacity during brief but vigorous exercise. If you have better anaerobic capacity, you’ll be a better sprinter and will excel at activities that require a short-term burst of energy. If the goal is to boost anaerobic capacity, you would structure your intervals differently.
To target the glycolytic energy system, that supplies the most fuel during bursts of exercise that last under one minute, an active-to-rest interval of 1: 3-5 would be more effective. For example, if you do 30 seconds of vigorous exercise during the active intervals, your rest interval would be 90 seconds to 150 seconds. If you’re trying to enhance the very short-term energy system called the creatine-phosphagen system to become efficient at doing explosive moments, an active-to-recovery interval of 1: 10-15 is ideal.
As you can see, the energy system you’re targeting should impact how you structure your intervals. For boosting aerobic capacity, around 1 to 1 works best. As the study mentioned shows, long rest intervals aren’t as effective for boosting aerobic capacity.
How Long Should a HIIT Workout Be Total?
Another question people often have is how long should a total high-intensity interval workout take? Most fitness experts believe 30 minutes should be the limit. If you’re working at a high intensity during the active intervals, you’ll fatigue quickly. Studies show that if the intensity is high enough, you can get benefits from an HIIT workout as short as 10 or 20 minutes. If you try to lengthen it to 30 minutes or longer, a few things can happen. Your performance will suffer because you’re tired. Also, the stress of working so hard for so long can cause a sharp rise in the stress hormone cortisol. The rise in cortisol suppresses your immune system and increases the risk of catching a viral infection such as influenza or a cold. Plus, elevations in cortisol also create a catabolic state that boosts muscle loss.
Also, don’t do high-intensity interval training every day. You need not do them more than twice a week to get the benefits. After an exhausting workout, your body need at least 48 hours to recover. Be sure to include a warm-up and a cooldown at the beginning and end an HIIT workout respectively.
The Bottom Line
High-intensity interval training has multiple benefits and it can even be a time-expedient way to enhance your body’s aerobic fitness. Now you know how to structure your intervals to best do that!
- com. “Here’s How Long Your HIIT Intervals Should Be, According to New Research”
- American Council on Exercise. “8 Things to Know About Aerobic Capacity (And How to Improve It)”
- J Physiol. 2013 Feb 1;591(Pt 3):641-56.
- IDEA Fitness. “Interval Training: The New and Better Way to Train Your Clients?”
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