Updated 11/3/19. Interval training (also called “HIIT”) exercise programs are very popular these days. You can't pick up a magazine or surf to a website without seeing something on the health and weight loss benefits of interval training. As an exercise physiologist, I feel the public is not learning all they should do HIIT programs safely. I also feel the popularity of HIIT programs has been skewed by clickbait websites, magazines, and commercials touting exercise gizmos. So, for those who want to start an interval training program, let me offer 5 guidelines/opinions on how to do it safely. I also call attention to some controversies and myths.
What Is Interval Training?
Interval training (also called “HIIT“) is a way to exercise that involves periods of low-intensity activity followed by a burst of higher-intensity activity. The lower intensity intervals are often called the “rest periods,” and the higher intensity intervals are called The “work periods.”Each work period and recovery period equals one set.
The idea is to alternate between work periods and recovery periods at regular intervals. For example, someone might:
- run for 1 minute
- walk for 1 minute
- run for 1 minute
- walk for 1 minute
That person might perform a series of sets of running and walking until they performed the number of sets they wanted.
From an energy system standpoint, as you exercise at the higher levels, your body works more anaerobically (burns more carbs) and during the lower intensities, you work more aerobically (burns more fat). This often provides a more broad spectrum of benefits than if someone were to just exercise aerobically for example.
Interval training is often touted as being superior to steady-state exercise, where people exercise at the same intensity. However, I don't always think this is true, for reasons I'll outline below.
HIIT vs HIT: What's The Difference?
As mentioned above, a common name for interval training is HIIT – High-Intensity Interval Training. It's important that people do not confuse this with another training protocol called “HIT.” Here, HIT, stands for High-Intensity Training.
So, what's the difference between HIIT and HIT? Well, the big difference is that interval training (HIIT) traditionally refers to aerobic activity, while HIT refers to resistance training that is performed to muscle failure.
Also, if the exercise program involves doing 2 different, intense activities back-to-back, it's not HIIT.
While often confused, HIIT and HIT are not the same thing.
Interval Training Benefits
There are many excellent articles on the benefits of interval training so I won't try to reduce them here. Instead, let me just summarize some of the benefits and link to research for those who want to learn more
Interval training has been shown to:
- Improve VO2max (aerobic capacity)
- Improve anaerobic ability
- Improve how well the mitochondria work
- Improve fat burning ability
- Improve insulin sensitivity (good for diabetics)
- Improves antioxidant enzymes
The list goes on, but you get the point. There are definite health benefits to HIIT training programs.
I feel these benefits, though, have caused people to start interval training programs before they are ready. I also feel the research has made interval training programs the go-to answer for personal trainers who are tasked with the job of helping improve health.
That said, I feel there is a right way and a wrong way to incorporate HIIT programs to improve health. So, here are the four things I feel everyone needs to know about interval training programs to do it safely.
Interval Training Guidelines
Let me offer five guidelines about interval training that I feel will help you do it safely and bust a few myths along the way. These are not in any particular order of importance.
1. Don't Call It HIIT Training
I'll be honest, I do not like the acronym HIIT. I wish nobody would use this name. Here's why: HIIT stands for High-Intensity Interval Training. Look at those words; notice you only see “high” in the name.
Where is the low?
Where is the low-intensity period?
It's nowhere to be found in that acronym!
By calling interval training “HIIT,” it ignores the low-intensity portion of the program. This is a problem because if you only did the high-intensity portion, you take the interval out of interval training.
By definition, interval training programs MUST be a low-intensity period.
If you only focus on the high intensity, you might as well as do high-intensity, steady-state exercise (for example, run for 20 minutes at 8 mph).
I know what you're thinking; you are thinking, “Joe I'd never do that.” That's great. I can only say that over the years, I've seen many people make this mistake. I've also met personal trainers who have made this mistake too.
For me, a better name than HIIT is High /Low Training. If we call it this, we keep the low-intensity period front and center so it's not discriminated against. Yes, I said, discriminated. I'm using this word on purpose because I feel some people think the low-intensity period serves no purpose.
That's just not true.
The low-intensity portion of the program is when the cardiovascular system, etc., is recovering from the more intense activity you just did. But, because you are still moving – albeit at a lower intensity- you are forcing the body to adapt faster than it might if you just stopped.
2. Don't Start With Interval Training
While interval training gets a lot of attention and can be used by many people, I don't feel beginners should not start with HIIT programs. Why? I think the program is probably too intense for some people.
I feel a better choice for beginners is low-intensity, steady-state exercise. In some circles, this is called “LISS.” For example, someone might walk on the treadmill for 30 minutes at a speed of 2.5 mph.
Lower-intensity steady-state exercise produces many of the same benefits as HIIT programs but doesn't put as much stress on the muscles, ligaments, tendons and cardiovascular system.
The way I see it, you have to build the foundation first.
Many people forget that intervals often require the person to move faster than they are used to. The nervous system controls our muscles. Remember muscle memory? Muscle memory doesn't last forever.
Eventually, we forget how to move quickly.
This can result in injury from falls.
This is something few people talk about when it comes to HIIT programs but I feel it's very important to consider.
True story: I was once contacted by an attorney who wanted me to be an expert witness for his client who was suing a personal trainer because of an injury that occurred during a training session. The personal trainer had his client perform sprinting drills on the running track. She tripped, fell and broke her arm.
Who was this client? A woman in her 60s who had not run or worked out in decades! Her running muscle memory was atrophied from years of not doing anything.
So when the trainer had her run, she tripped over her own two feet.
So, what does it have to do with interval training? It's basically the same thing. Whether you are running or doing intervals, if you are moving faster than you are used to, injuries from falls can occur.
When designing an exercise program, the person's fitness level needs to be considered. Not just aerobic fitness, but neurological fitness too. That means muscle memory.
3. HIIT Doesn't Have To Be Hard
Another reason I don't particularly appreciate calling it HIIT is because it gives the impression that the workout has to exhaust you to provide benefits. While, I can understand how this is true if you are really fit, if you are just starting out or have health issues, this doesn't have to be the case.
When starting intervals, the idea is you only need to push the envelope a little bit and then back off. By pushing just a wee bit past your boundaries and then reducing the intensity, you eventually train the body to adapt to the greater overload.
For very fit people, that overload might mean sprinting for 30 seconds, but for others, it might mean only increasing the treadmill speed from 2 mph to 2.3 mph – and then back to 2 mph again.
It really all depends on what your fitness and health are.
4. Make The Recovery Time Longer
When beginning an interval training program, make the recovery period 2-3 times longer than the work period. For example, if you were running for 1 minute, walk for 2-3 minutes. Then run for 1 minute again… and so on.
Novices take longer to recover from exercise, so the recovery period should be longer. As fitness level improves, the person will bounce back faster, and so the recovery portion can be reduced.
5. Use RPE Instead Of Heart Rate
Most everybody determines how hard they exercise by measuring how many times a minute their heart beats. This is very common in spinning classes. Some may even use a heart rate monitor to determine the percentage of maximum heart rate they are at.
While noting your heart rate is great to know, for beginners or those who don't want to invest in a heart rate monitor, I prefer using the RPE Scale. The RPE Scale (also called Borg Scale) is just a series of numbers from zero to 10. The higher the number, the harder the exercise feels – for you. Here are the numbers of the RPE Scale and what they mean:
- Zero: You feel like you are doing nothing
- 1: Really easy
- 2: Easy
- 3: Moderate exertion
- 4: Somewhat difficult
- 5: Difficult
- 6: Pretty difficult
- 7: Very difficult
- 8: Very very difficult
- 9: Almost at your maximum
- 10: Hardest you can do. Any second, you'll have to stop
For most healthy people, staying somewhere between a 3 (mild exercise) to a 6 (pretty challenging) is fine. For those who are not so healthy, a rating of 2-3 may be more appropriate. Almost nobody needs to ever be at a 9 or 10 on the RPE Scale.
The beauty of the RPE Scale is that it works for everyone, even those who may be taking blood pressure drugs that slow down the heart rate. The only drawback to the Borg Scale is that you have to know what the numbers mean. So memorize them. The RPE Scale is less accurate if you don't know what the numbers mean.
For greater accuracy, you can combine RPE with the Talk Test. The Talk Test is the easiest way to determine how hard exercise is. When you can't carry on a conversation, the intensity is too great for you. The talk test works well with beginners and those with health problems.
How Many Times A Week Can You Do HIIT?
There is no formally accepted answer to this question, so I'll give my opinion. For those just beginning an exercise program, I would begin with only 1 HIIT workout per week. I would not do more than 2-3 sets (2-3 work periods and 2-3 recovery periods) in that first week.
The intensity of those work periods would be determined by the person's health and fitness level. The rest of the workout can be filled with steady-state exercise and/or resistance training.
Each week, as the person felt comfortable, slowly increase the number of sets performed until arriving at the desired amount (for most people, no more than 10 sets at most are needed). Again, I'd keep this to 1x per week. When the desired number of sets has been attained, then try to do 2 interval training workouts per week, not back to back but spread out during the week.
For most people, 2-3 HIIT workouts per week should be enough to provoke health benefits. Even in most highly fit people, I would not do more than 4 per week.
Rest is essential between HIIT workouts. When just starting out, I would wait at least 48-72 hours before doing it again. As fitness improves, this can be shortened to 24-48 hours.
Who Should Not Do Interval Training
Everybody likes to talk about the benefits of interval training. Few like to discuss who it may not be right for. There is no doubt that interval training has proven beneficial for several health issues such as diabetes. Because HIIT programs usually involve moving faster or more intensely than normal, caution should be exercised in the following conditions. This list is not complete:
|Atrial fibrillation||Recent heart attack||Recent stroke / history of aneurysms|
|Peripheral artery disease||Chest pains at rest/during exercise||Arthritis / joint problems|
|Fibromyalgia||Balance problems||Breathing problems / COPD|
|Blind/visual impairment||Osteoporosis||Low back problems|
|Chronic fatigue syndrome||Multiple Sclerosis||Those with overtraining syndrome|
|Recent rhabdomyolysis||The very old or very frail|
I am painting with a very broad brush here. While I'm sure there are those who will take exception to some of the conditions I listed, my desire is to give people something to think about. I'm not trying to sell you an exercise gizmo. I want you to be safe.
If you are on the fence about whether this is right for you, talk to your doctor first and then consult an exercise physiologist or qualified trainer who can design a safe interval training program for you.
What Is Intermittent Exercise?
Intermittent exercise can be thought of as a real low-intensity version of interval training. It's a program that combines periods of activity with no activity. For example, someone might walk for a few minutes and then sit down for a few minutes to rest. This is repeated until the person has completed the desired amount of exercise.
Intermittent exercise is usually reserved for those who have a very low level of fitness, an injury or who have a lot of atrophy. For example, I remember having my grandmother do intermittent exercise when she was 103 years of age (she lived until she was 104).
The idea with intermittent activity is that, hopefully, fitness level improves so that the person can undertake continuous exercise (like steady-state activity or maybe even intervals) one day.
Does HIIT Raise Metabolism?
One of the reasons interval training is so popular is because of the promise that you will burn lots of calories for a long time after you stop. This is often called EPOC – Excessive Post Exercise Oxygen Consumption. This is fancy talk for the elevation in metabolic rate that occurs after you stop exercising.
The idea is that HIIT will raise metabolism for a long time afterward, leading to more calories burned when watching tv, sleeping, etc. In this way, HIIT workouts are touted as best for weight loss.
It all sounds great. The problem is, not everybody agrees this really happens. Humans are complicated creatures. There are a lot of things that factor into the metabolic rate.
Here's what I think. We work harder (and use more calories) when we do things we are not used to performing (like HIIT workouts). This might lead to an elevation in metabolic rate (EPOC) for some period of time afterward (studies are all over the place when it comes to how long metabolism stays elevated).
As the body adapts to exercise, it becomes more efficient (burns fewer calories), leading to a lower EPOC?
The bottom line to this is, if you like doing HIIT programs, great. But, whether or not they are best for weight loss or elevating metabolism long term, I think needs more study.
What About Muscle Confusion?
If you read the above section, you may have started to wonder if the body adapting to exercise (and using fewer calories) was a reason to confuse the body to keep the metabolism engine purring all the time? In other words, is this evidence for “muscle confusion?” I don't think it is and here's why.
The idea of muscle confusion is a myth that was used to help sell exercise DVDs. There is no scientific principle of muscle confusion, and I'm not aware of the clinical benefits of muscle confusion promoting greater weight loss or fitness or improved metabolism.
If anything, muscle confusion – the idea of doing different activities on different days – is just a sexy name for cross-training. Cross-training is an effective way to reduce overuse injuries and improve exercise adherence.
I'm straying from the main point of this post, but I thought it was wise to mention this because I know many people who believe muscle confusion is real. It's not.
Is HIIT Training Best?
So, by now, most people have probably come to the conclusion that I'm totally against interval training. I'm really not. I know it has benefits, some of which I showed above. That said, I think the research (and clickbait articles) have hypnotized people into believing HIIT is the best overall program for everybody. I don't feel it is.
For example, in this study of 55 untrained people, 8 weeks of interval training produced no greater improvements in aerobic capacity than steady-state exercise. Both exercise programs improved aerobic ability by about the same amount.
At the end of the day, everybody is different. There is no doubt interval training programs have many health benefits. That said, I don't feel they are right for everybody. If you are healthy enough for interval programs, great. If you have some health problems that preclude you from doing HIIT training, I want you to know that this is ok. Any activity you do is better than no activity. Keep moving by any means you can. That is what is important.