If you’ve ever done any resistance exercise, often also called weight training or strength training, you’ll most likely have experienced a fair amount of soreness in the muscles you used. Maybe even soreness in muscles ‘you didn’t know you had’.

This is what exercise professionals call delayed onset muscle soreness (DOMS). DOMS usually hits you 24 to 48 hours after exercise and can last several days in the more extreme cases (1).

It tends to happen if you’re new to resistance exercise, had a prolonged layoff from the exercise, or made significant changes to the exercises and/or intensities you use from one session to the next.

This article is more about addressing the first two scenarios – you’re new to exercise or had a long layoff. 

Sufficiently bad DOMS can put you right off ever trying it again. Even if you decide to try it again, bad DOMS will probably make you wait a few extra days before coming back.

Exercise is extremely beneficial to health, but, as discussed in previous blogs, many of its health benefits only last hours to a few days after the exercise, so how frequently you exercise matters. 

You want time to recover and for your body to adapt, but you probably also want maximum benefit, so doing a little often is a good approach.

Reducing DOMS starts by knowing what kinds of activities cause it, specifically what types of muscle contractions.


Types of muscle contractions

We move when muscles shorten/contract to create movement around a joint. Muscles don’t push, they only pull. Of course the result of the right muscles pulling will cause us to push.

To understand how to minimise DOMS while still benefiting from exercise, we need to understand the main types of muscle contraction because one type is mostly responsible for DOMS.

Classic resistance training involves either moving an object through space or moving yourself through space repeatedly. This involves concentric and eccentric muscle contractions.

Concentric contractions are when the muscle shortens against a load, be that load in the form of something external or your bodyweight.

Eccentric contractions are when the muscle lengthens, but is still resisting the load.

Let’s look at some familiar exercises in terms of the concentric and eccentric part.

Squatting – when you lower yourself down, e.g. to sit in a chair, this is the eccentric contraction because muscles in your thighs and around your hips are lengthening against resistance. The resistance in this case is your bodyweight and anything else you might be carrying.

Chin/pull ups – pulling yourself up involves concentric contractions of the relevant muscles in your arms, shoulders, and back. Lowering yourself under control would require eccentric contractions of those same muscles.

You might wonder what happens when you just hold something, without shortening or lengthening muscles – this type on contraction in which the muscle neither lengthen nor shorten but do exert a force is called an isometric contraction.

An example of an isometric contraction most people use multiple times a day would be holding onto something. Taking the chin/pull up example, your forearm muscles involved in gripping would be undergoing an isometric contraction to help you hold on throughout the movement.

Hopefully that’s all starting to make sense, and you’re beginning to see possibilities for using different types of contractions.

Just one final example though. Have you ever noticed how a day or two after walking a long distance downhill, you get sore legs? That’s because the muscles of your thighs were constantly having to decelerate you, i.e. undergo lots of eccentric contractions.

Walking up hill feels harder, but if you didn’t walk back down, you’d have less DOMS afterwards.


Removing Eccentric Contractions

To reduce DOMS, reduce/remove the eccentric contractions.

Weightlifters do it by dropping the weight. But if you’re reading this, then you're probably not a seasoned weightlifter, so for safeties sake I’d recommend not dropping weights.

A safer option is to use a sled as shown in the videos below. You can easily make one by nailing an old car tire on a wooden board. With some straps and handles you can mimic many of the exercises you might associate with dumbbells or cable machines.

The nature of the sled is such that when you stop pulling/pushing, it stops moving. There is no eccentric contraction, it’s all concentric and isometric (the grip).

These videos are just a few examples. You’ll get more out of it if you have a longer path so you can do more repetitions without having to reset, but this is the space I had to work with. This is also something you may be able to do outside depending on your circumstances.

Another approach is to use cardio machines, especially stationary bikes and rowing machines. The circular motion of pedalling means that you are really just using concentric contractions.

On the rower, you only need strength for the pulling/concentric part, so again there is minimal eccentric work.

This article is about resistance training though, so you’ll need to change the resistance and pace to make these machines work for you.

On the stationary bike you’ll need to set the resistance so that you really have to push hard to grind out perhaps 30-40 revolutions per minute. Go for a minute or so at a time.

The rower is less versatile with the resistance, so you’ll need to pull with speed and treat it like weight training exercise – pull as hard as you can, pause for a second, then repeat for 15-20 repetitions.

Make sure you are well warmed up first, and that someone who knows what their doing has shown you good rowing technique.


Reducing Eccentric Contractions

Aside from trying to eliminate the eccentric contraction entirely, you can simply not emphasise them.

For example, some instructors and coaches suggest that you do the concentric contraction quickly and then slow the eccentric contraction right down. 

That has it’s place, but when you’re just (re)starting out, do both in a controlled manner, but only as slowly as you need to to feel in control of the exercise/weight.

If you are at a gym with the right equipment you can also use a reverse-band set-up. See the videos below for some example. 

The elastic bands will reduce the resistance during the eccentric part of the exercise. Just make sure to set them up as shown with the band inside any weight plates you are using and with collars to safely keep the plates and band on.


Isometric Holds

You could do away with movement all-together and just get into a position and hold it for 10–30 seconds.

Classic examples of isometric exercise would be planks and wall-sits. These examples aside, any time you hold a static position is an isometric exercise.

You can even load a bar with more weight than you can move and just push or pull on it as hard as you can. Just make sure whatever your pushing or pulling really isn’t going to move on you.


Final Word

Eccentric contractions are fairly potent for building strength and muscle, so I’m not suggesting you do away with these entirely. 

Instead, reduce the amount eccentric work you do when you’re just starting out. After a few sessions add in a little more eccentric work, and build up from their.

You can also use the exercises suggested as a means to increase the amount of work you do without having to increase your rest between sessions.

Frequency is a great ally for getting you results fast, and work with sleds or on a bike or rower will let you exercise more frequently and more intensely without some of the associate discomfort or fatigue.

Lastly, start any exercise programme gently, and after getting the ok from your doctor. Use less weight than you’re capable of and do fewer repetitions at first. 

This advice is even more true if you used to be athletic in the distant past, or even in the more recent past if your layoff was due to serious injury or illness. Over enthusiasm can be almost as detrimental as too little enthusiasm.

Build up over time so that you stay motivated and relatively pain free.

Next post will be the start a new series on effective exercise for health.

Want personalised guidance on starting exercise from a certified clinical exercise physiologist?

Check out our one-on-one training service here. 

References

1. Lewis, P. B., Ruby, D. & Bush-Joseph, C. A. Muscle soreness and delayed-onset muscle soreness. Clin Sports Med 31, 255–262 (2012).
2. Newham, D. J. The consequences of eccentric contractions and their relationship to delayed onset muscle pain. Eur J Appl Physiol Occup Physiol 57, 353–359 (1988).

This marks the last of this series on high-intensity intermittent training (HIIT) where we’ll look at practical ways of implementing a HIIT programme. It’s worth revisiting Part 1, in which we took a close look at intensity, what it means in the context of HIIT, and how you can measure it.

Part 2 and Part 3 discussed the evidence for the many benefits of HIIT, but if you’re reading this, I’m assuming you’re keen to give it a go.

HIIT, despite the name, has been used in people recovering from heart surgery all the way to elite athletes. That’s because it’s the number one way to build cardiorespiratory fitness (see Part 2).

That said, you should get the all-clear from your doctor before taking up an exercise programme. Ideally, that would involve an ECG/EKG monitored exercise stress test, which would also give you an accurate maximum heart rate to work with.

If you do have a known heart problem, it’s best to do intense exercise under professional supervision.

That out of the way, let’s address some key questions when thinking about designing an effective HIIT programme for health.


The Faces of FITT

The American College of Sports Medicine uses the acronym FITT to describe the key characteristics of exercise that are important for making progress.

F.I.T.T. stands for Frequency, Intensity, Time, and Type. These are explained in the first video of our Move 101 series here.

In this case, frequency refers to how often you do a HIIT session in a week. Intensity is already set (see below). Time refers to both the duration of the interval and how many intervals in total (discussed below). And Type is already determined as HIIT.


What kind of activities can I do as HIIT?

Although HIIT in commercial programmes is often done using various bodyweight movements, i.e. calisthenics, the studies done to date and discussed in Parts 2 and 3 never assessed calisthenics.

Most studies done to date, at least in people with medical conditions and even just none-athletes, have been done using stationary bikes, treadmills, and occasionally running on a track or swimming.

As the goal of HIIT is to drive up heart rate, it’s best to use activities that use a lot of muscle, e.g. walking/running, cycling, swimming, or rowing. These can also be done indoors using equipment such as treadmills, stepmills, steppers, cross-trainers/ellipticals, rowing machines etc.  If using your legs is out, permanently or temporarily, you can also use a hand bike. 

Of these the rower takes the most technical skill, so find someone with appropriate credentials to show you good technique that: 1) protects your lower back; and 2) uses your arms efficiently so they don’t become what limits you.

Because I work mainly with people who have some kind of health issue, I usually prefer low impact options that are very low injury risk, so in most cases nothing beats a stationary bike, either upright or if that’s a problem, recumbent. 

Outside of that, treadmills are handy, but instead of running just keep to a fast walk but increase the incline. The thing to avoid is holding onto the rails as this changes a natural activity of walking into a strange ineffective leg swinging exercise.

If you aren’t yet secure enough and need to hold on, use the side rails not the front. Using the side will make it more difficult to drag yourself along with your arms while essentially stumbling forwards – something I see a lot of people in commercial gyms do.

If you go the outdoor route, I’d also recommend picking a hill to walk up fast instead of running.

Most gyms I know don’t have stepmills, but if you have access to one, then I recommend them as long as the step height is reasonably comfortable for you. Stair climbing is a great exercise to build cardiovascular fitness and strength in the muscles of the legs and buttocks.

Basically, you want to choose one or more exercises that allow you to maintain some quality in your movement even as you push yourself hard. The more technical the movement, the more difficult that will be.


How long should an interval be?

In Part 1 I noted that HIIT comes in two broad categories, one that involves high but below maximum intensity for roughly 1-4 minutes, and all-out (maximum) efforts of 30 seconds or less often called sprint interval training (SIT).

SIT has been tested almost exclusively in healthy, usually athletic groups, so it’s general safety record in other groups is not well establish. Such all out efforts can put the heart under considerable strain and increase blood pressure rapidly.

The prudent approach is to keep intensity high, but not maximal and instead use intervals of 1-4 minutes. We’ll cover this a little more under the topic of progression below.

You can get special interval timers, use a smartphone app, or stick to simply using a watch or clock. 

How many intervals in one session?

The question is more flexible if phrased in terms of how much total time should be spent at high-intensity. The answer being 10–20 minutes.

The less fit someone is to begin with, the more inclined I’d be to start at the 10 minute end of that range. In practice, that might mean doing 10 one minute intervals, or 5 two minute intervals.

One option that is also available is to have the intervals be different lengths. As the intention of HIIT training is to keep the heart rate high, arranging your intervals so you spend maximum time in your target range is a smart thing to do.

Heart rate doesn’t jump the instant you apply effort. It’s a lot like a car, even when you put the pedal to the metal, it takes time to reach top speed. A high performance body just like a high performance car will accelerate faster, but it’s never instant.

In my research with people who have non-alcoholic fatty liver (1), volunteers started with two minute intervals and it often took until late in the third or fourth interval for the heart rate to be in the desired range.

One way around this is to go from longer to shorter intervals, e.g. 4 minutes, then 3 minutes, then 2 minutes, and finally 1 minute. This gives 10 minutes in total.


How long should the recovery periods be?

The whole point of HIIT is that it allows people who couldn’t do more than a few minutes of high-intensity activity at a time to accumulate 10-20 minutes at high intensity.

The way to do this is to break up those 10-20 minutes into shorter intervals with a recovery period in-between. Those recovery periods can involve either complete rest, or some lower-intensity activity.

If you opt for the latter, which we’ll call active recovery, you have the option of staying with the exercise you’re using for the intervals, e.g. cycling, or doing something else.

That something else is extremely versatile, you could for example do some stretching or mobility work, visit our Move Better video library for some ideas.

Whatever you choose, the recovery period should allow you to catch your breath, have a sip of water, and let your heart rate come back down somewhat.

That still begs the question of how long? As long as it takes for you to repeat your effort in the preceding interval and no longer. That’s in theory anyway. In practice you have a few options:

1. Recover for a set time. Most often that involves a recovery period roughly as long as the interval. That might mean 1 minute intervals separated by 1 minute recovery periods. You may prefer to start with longer recovery periods than intervals at first.
2. Use the Perceived Recovery Status Scale (download here) to help guide you (2). When you would rate your recovery around 5 or 6, do the next interval.
3. Use a heart rate monitor and when you’re heart rate drops below a certain target, do your next interval. That target can be somewhere between 60% and 70% of your maximum heart rate.

We’ll look at the most important ins and outs of heart rate in a future blog.


How intense should intervals be?

We discussed this in Part 1, but to recap – contrary to how HIIT programmes/protocols are often described, heart rate targets are not very practical. Go with Ratings of Perceived Exertion (download here) (2).

Using the scale, and interval should feel in 16-17 (meaning very hard). 

The saying It never gets easier, you just get better applies here, so even though you might find you have to slow down during an interval, the whole interval should be a very hard effort.

Remember, you have a recovery period just around the corner.


Warmup

Always start your exercise session with a warmup where you go from an intensity of about 9 (meaning very light) in the first minute or so, up to a 13 (meaning somewhat hard) in the last minute of the warmup.

Typical warmups are for five minutes, but there is nothing magic about that number, so feel free to go up to 10 minutes if it makes you feel more ready.

Going much above 10 minutes may take energy away from your intervals, so the idea is to warmup just enough to give a good effort when you launch straight into your first interval.


Progression

Exercise like medication has a dose, and that dose needs to be appropriate to your needs. That topic is covered in more detail in the second video of our Move 101 series here.

Exercise dose can’t be measured as conveniently as the dose of a drug as it’s really a product of FITT – frequency, intensity, time, and type. 

Nonetheless, if one or more of these parameters increases, e.g. you do more sessions per week, more intervals in a session, longer intervals, or even decrease your recovery periods, then you’ve increased the dose of exercise.

That’s assuming you don’t cut back elsewhere, e.g. more intervals but shorter times. To be an increase in dose you have to be either doing more work in total, or the same amount of work in less time.

If we accept that exercise has a dose, then it should make sense that if we want to keep improving over time, we have to adjust that dose accordingly.

Another word for it would be progression. You need to make progress in your exercise to keep making progress in your health. Although there is a point where physical performance and health no longer travel in parallel, most of us aren’t anywhere near that point, so greater fitness equals better health prospects as long as we give our bodies enough time to adapt.

We exercise to make out bodies change, but our bodies want to conserve resources, so they only change as much as they feel is necessary given what we expose them to. That means if we don’t increase exercise dose as we get fitter, we’ll stop getting fitter.

So how can you gradually increase the dose? I gave some examples above, but lets be systematic about it.

Some progress is actually built in when we set an intensity. Recall from Part 1 that intensity is relative to your personal capacity. That means that as you get fitter, the amount of work you do, e.g. how far you cycle, swim, walk etc. during a given interval will increase if you are working at a given capacity, e.g. 85% of your maximum.

That’s why I recommend giving yourself at least three consecutive sessions and up to six, so 1-2 weeks, in which you make no intentional changes to your HIIT sessions. Keep all aspects of FITT the same.

Over time though, aim to progress from 10 minutes worth of intervals to 20. That might mean adding an extra 10 seconds per interval per week or fortnight. 

Alternatively, add intervals. E.g. go from 10 intervals of one minute to 11 intervals, and then 12, and so on until you’re doing 20 one minute intervals.

Once you’ve reached 20 minutes, work on decreasing the rest time. That said, keep each interval to four minutes or less, and keep the recovery period long enough 

Our bodies do get fairly efficient at activities we do often, and when it comes to exercise for health and or fat reduction, it’s actually good to do things we aren’t super efficient at because the greater efficiency equals less energy used for a given amount of work.

That means when you’ve reached your 20 minutes of accumulated high intensity activity per HIIT session, and your total recovery time is in the 10-15 minute range, it’s time to switch activity, e.g. from cycling to incline walking, or rowing.


How frequently should you do HIIT?

Preferably no more than two days in a row as immune cells start to decrease with multiple consecutive sessions (3). High-intensity exercise by its nature takes longer to recover from than low-to-moderate intensity physical activity.

On the other hand, as we found out in Part 2, some of the positive effects of HIIT, notably improved glucose control, tends to wear off after 48 hours.

If you are mostly or exclusively doing HIIT as exercise, then optimal is every other day, or if it’s more practical go for three times a week, but preferably with a 48 hour break in-between sessions.

If you do other forms of exercise, e.g. resistance training, you have the choice of combining the two training styles into a single session or have them on separate days.

There are no hard and fast rules here. But if I had to make a recommendation, I’d go with two HIIT sessions per week and 2 resistance exercise sessions. Or, three sessions combining both with the resistance exercise first, and with a days break in-between.

If you have the time and inclination, it’s also worth doing a weekly session of longer duration moderate-intensity exercise for 30 minutes to a a few hours. A nice hike would be a good example, but you may need to work up to that.


What if I have to take a break?

If injury, illness, or other circumstances stop you doing your HIIT sessions for more than a few days, don’t start where you left off. Instead, take a step or more back to give your body the chance to build up again gradually. 


Take homes

If you want to get benefits like greater cardiorespiratory fitness, body fat reduction, improved glucose control and blood pressure from HIIT, then:

Do 2-3 evenly spaced sessions per week.

Base your intensity on the Rating of Perceived Exertion Scale.

Start with a warmup and then 10 minutes worth of high-intensity intervals, lasting 1-2 minutes. E.g. 10 x 1 minute intervals or 5 x 2 minute intervals.

Your recovery period can be based on a specific time, score, or heart rate range, but the goal is always to allow you to do a similar amount of work in each interval without a major drop off from first to last.

Ensure you keep making progress by building up the total interval time from 10 to 20 minutes, and cutting back on recovery time. Do this slowly, only making adjustments every 3-6 sessions, and then keeping those adjustments small, e.g. adding 10 seconds to each interval, or taking 5 seconds off recovery.

Finally, check with your doctor before starting a HIIT programme.

Want personalised guidance on starting or modifying a HIIT programme from a certified clinical exercise physiologist?

Check out our one-on-one training service here. 

References

1. Hallsworth, K. et al. Modified high-intensity interval training reduces liver fat and improves cardiac function in non-alcoholic fatty liver disease: A randomised controlled trial. Clin Sci (2015). doi:10.1042/CS20150308
2. Laurent, C. M. et al. A Practical Approach to Monitoring Recovery: Development of a Perceived Recovery Status Scale. J Strength Cond Res (2010). doi:10.1519/JSC.0b013e3181c69ec6
3. Borg, G. A. Psychophysical bases of perceived exertion. Med Sci Sports Exerc 14, 377–381 (1982).
4. Navalta, J. W., Tibana, R. A., Fedor, E. A., Vieira, A. & Prestes, J. Three consecutive days of interval runs to exhaustion affects lymphocyte subset apoptosis and migration. Biomed Res Int 2014, 694801 (2014).

Disclaimer: The tables you’ll see links to in the text are taken directly from my PhD thesis and are not user friendly. If you want the summary, just read the text, but if you want the fine detail, it’s all laid out for you in the tables. Also, I’m coming from the perspective of a clinical exercise physiologist specialising in healthy ageing and helping people with type 2 diabetes, metabolic syndrome, fatty liver, and similar conditions. These articles aren’t aimed at competitive athletes.

Part 1 described what high-intensity intermittent training (HIIT) actually is, and looked at the safety of this exercise approach. In Part 2 we looked at the benefits of HIIT in terms of improved cardiorespiratory fitness and what that means for health, as well as effects on the heart, nervous system, and blood pressure. Please review those parts if you aren’t familiar with them.

Now we’ll look at the effects of HIIT on body composition, blood glucose control and insulin sensitivity, and blood lipids.


HIIT for a leaner you?

Advertising portrays exercise as a great way to slim down, extolling the calorie burning effects of exercise with perhaps a nod to the vague concepts of shaping and toning. The research and most people’s experience is usually different.

That’s really the focus of another article, but worth recapping here. Exercise boosts many areas of health and is certainly part of healthy weight reduction (1). Exercise, especially the right kind, accelerates reduction of liver and visceral fat with or without weight reduction (1-3).

If you’re unclear as to just how beneficial that is, consider reading Is type 2 diabetes all about the fat?

The graphic above is from an excellent article by Peter Janiszewski and Robert Ross that highlights the health benefits under different circumstances (1). PA stands for physical activity and CMR cardiometabolic risk – in other words the risk of heart and other vascular problems as well as risk of things like type 2 diabetes.

What the graphic summarises is that physical activity, the umbrella under which exercise sits, has health promoting effects whether or not it results in weight reduction.

Another reason for pairing diet with exercise is to preserve lean mass – everything that isn’t fat, including muscle, bone, and organs (4-7). Loosing lean mass, especially in the form of muscle is bad news for physical function and your body’s capacity to burn calories, so not something you really want.

Most exercise only studies at best report a modest reduction in weight. That said, no other well studied approach to exercise has shown the body fat reduction effects of HIIT.

Note: most of the programmes published in books, magazines, and used by many trainers have not been assessed in studies. That doesn’t make them bad, but it does make it harder to comment on. 

You can see the individual programme and study designs as well as results in Table 3, which summarises 27 studies of which 13 reported weight, body mass index (BMI), and/or fat reductions of 1.1 – 8.4% 8-21, and the rest no change (22-39).

The 8.4% reduction was what we call an outlier because all the other studies showed 0–2.5% reductions.

Of 14 studies that specifically estimated fat mass, 8 reported a reduction (8,11,16,17,19,21-23), and six observed no change (24,32,33,37-39).

Thirteen studies compared HIIT with continuous moderate-intensity exercise, i.e. classic aerobic exercise, for weight change (8-11,15,16,19,21,22,26-31,33,40,41). Seven reported no weight change in either group (22,23,26-31,41), five reported largely comparable modest reductions of 1-3.9% (9-11,14,15,19,33), and one reported greater weight reductions of 8.4% vs. 5.9% in the HIIT relative to the group continuous moderate-intensity exercise groups (8). 

This larger reduction followed 12 weeks of track running five times per week 8. This is in contrast to the majority of studies, which assessed three sessions per week programmes done on stationary bikes or treadmills.

Seven studies compared HIIT with continuous moderate-intensity exercise for change in body composition (8,11,16,19,22,31,33). Six reported body fat reductions in the HIIT and/or continuous moderate-intensity exercise groups relative to before exercise with either no obvious difference between groups or slightly greater reductions with HIIT (8,11,16,19,22,33).

The studies varied in a lot of ways, including length of time, the specific exercise programmes, and the health and fitness status of the volunteers. Studies reporting weight and/or fat mass reduction tended to be longer and/or have volunteers with a higher starting cardiorespiratory fitness.

This makes sense – you’d expect to gain more benefit from something you did for several weeks or months than something you only did for a few days. You’d probably also expect to burn more calories when you do more work, e.g. go faster and/or further. 

All taken together, doing HIIT gives you a good chance of reducing your body fat and maybe even your weight if you have a lot to shed, but paying attention to your nutrition is still a important


Glucose control and insulin resistance

Table 4 shows you the results volunteers got after a single session of HIIT; this is what we call the acute effect. 

Most studies in healthy people showed little effect of HIIT on fasting blood glucose, insulin sensitivity (how well the body responds to signals from the hormone insulin), or insulin and blood glucose response to a meal after single session (42-45). 

One study did show less insulin needed in response to a meal after a session of HIIT (46). Interestingly, this study had the largest number of volunteers and the fittest, it also required volunteers to expend 500 kcal during their HIIT session.

In contrast, volunteers with type 2 diabetes experienced reductions in 24-hour blood glucose both immediately following meals and between meal periods (47).

Volunteers with metabolic syndrome (i.e. pre-diabetes) also experienced reductions in fasting glucose following HIIT (that study only measured fasting glucose) (40).

Comparison of HIIT with continuous moderate-intensity exercise, showed that fasting blood glucose stayed reduced for 72 hours after HIIT, but was only improved immediately after continuous moderate-intensity exercise (40).

Although it’s nice to know that HIIT has benefits after your very first session, you’re hopefully planning something more long-term, so let’s look at what happens after several sessions over a few weeks.

Things do get a bit cloudy here, so if you want the finer detail have a look at Table 5 (9,11,17,20,21,23,24,26,28,33,42,48-50). 

Some studies did report reductions in fasting glucose and/or fasting insulin (9,11,33,48,50), but the majority didn’t.

You’re hopefully wondering why. Although it’s impossible to be sure there are some likely explanations for why a study may not find a change even when such a change is likely for the majority of people doing HIIT:

1) When exercise works to improve glucose control, one reason is that the exercising muscles take up more glucose from the blood. This process seems to be driven largely by the muscles contracting forcefully and also their own glucose stores getting low (51-53). In some volunteers, e.g. with serious heart disease, one or both of these requirements may not have been met (26,28).

2) Most studies didn’t report how long after the last HIIT session they did their tests, so it’s possible that the effects may have worn off by the time they took their measurements, e.g. one study found improvements 24 but not 72 hours after the last HIIT session (20).

3) Fasting measurements aren’t really that informative unless they are in people for whom these are usually outside the normal range, and many, though not all studies, relied exclusively on these.

For me the most interesting study was done in volunteers with type 2 diabetes, the group with the largest room for improvement, and in this group HIIT improved 24-hour blood glucose, both immediately following meals and in-between (54).

Also worth noting is that even if the study shows an average improvement, this may not have been the case for everyone taking part, e.g. in one study of 12 volunteers, the insulin sensitivity of 10 improved, 1 person experienced no difference, and one got worse after HIIT (42).

Six studies compared HIIT with continuous moderate intensity exercise (9,11,26,28,33,50), and two with resistance training (23,33). 

Overall, the reports suggest HIIT is similar or slightly superior to continuous moderate intensity exercise or conventional resistance exercise for improving glucose control (9,11,33,50).

So on balance, it’s worth trying HIIT if one of your main goals is to improve glucose control and insulin sensitivity. Assuming you aren’t doing any other form of exercise between sessions of HIIT, doing a session every other day, or three times a week is likely to maintain any benefit you do experience, but more on that in part 4.


Blood Lipids

Three studies tested the effect of a single session of HIIT on post-meal blood lipids and all found a reduction in post-meal triglycerides (also called triacylglycerols) (43,45,46). The response was longer lasting after HIIT than after a session of continuous moderate intensity exercise (45).

Nine studies reported fasting blood triglycerides after several sessions of HIIT (9,17,20,21,23,26,27,29,38,50), five reported total cholesterol (21,31-33,50), nine high-density lipoprotein (HDL) cholesterol (20,23,26-33,50), and five and low-density lipoprotein (LDL) cholesterol (21,26,31,33,50). Yet only two studies that didn’t also involve changing diet showed any change – an increase in HDL-cholesterol (generally considered a good thing) (27,29,32). One other study did show a change in very low-density lipoprotein cholesterol (38). 

Once again, those studies comparing HIIT with continuous moderate intensity exercise or conventional resistance exercise showed the latter two were no more effective.

So consistent with most exercise studies, the effects of HIIT on fasting blood lipids are minimal, but the post-meal effects are desirable as these suggest HIIT makes people better at dealing with fat in meals.


Take home messages

When it comes to fat reduction, exercise should be part of a broader lifestyle change. That said, HIIT is a good choice of exercise if getting leaner is one of your goals. The better your cardiorespiratory fitness, the better your chances of shedding unwanted pounds; luckily HIIT is great for getting people fitter.

The effects of HIIT on glucose control and insulin sensitivity are greatest in those with poor glucose control, and are time limited, wearing off after 48–72 hours, yet still longer lasting and usually greater than the effects of more traditional continuous moderate intensity exercise.

HIIT has a little or no effect on fasting blood lipids, but is beneficial in terms of helping the body to handle the fat in meals better.

Finally, just because a study shows something works on average, or just because you have a friend that had great or no success with an approach, doesn't mean things will work out the same for you. A degree of trial and error may be needed.

In Part 4 we conclude this series by looking at ways you can incorporate HIIT into your exercise practice, even if you currently don’t have one.

Want personalised guidance on starting or modifying a HIIT programme from a certified clinical exercise physiologist?

Check out our one-on-one training service here. 

References: click here for a full list of references cited.

Disclaimer: The tables you’ll see links to in the text are taken directly from my PhD thesis and are not user friendly. If you want the summary, just read the text, but if you want the fine detail, it’s all laid out for you in the tables. Also, I’m coming from the perspective of a clinical exercise physiologist specialising in healthy ageing and helping people with type 2 diabetes, metabolic syndrome, fatty liver, and similar conditions. These articles aren’t aimed at competitive athletes.

In Part 1 we laid the foundations of understanding what high-intensity intermittent training (HIIT) is, and addressed the safety record of this type of exercise. So please start there if you haven’t read it.

This week we'll look at the many health benefits of HIIT supported by research in non-athletic healthy people and in those with various chronic health conditions.

Cardiorespiratory Fitness

There is a strong link between cardiorespiratory, or what is sometimes called aerobic fitness and health. For example, one of the key differences between the 'metabolically healthy' obese and those with less healthy metabolism, e.g. metabolic syndrome, is cardiorespiratory fitness (1).

Staying fit throughout the lifespan also helps maintain glucose control, in fact this become more important later in life (2). Higher cardiorespiratory fitness also seems to help ward off or lessen the effects of non-alcoholic fatty liver disease (3,4), type 2 diabetes (5-8), metabolic syndrome (9,10), cancer (11), cardiovascular disease (12-17), as well as reducing the risk of falls (18).

On the broadest level, being fitter reduces something researchers call all-cause mortality (8,12), which is a catch all for saying at any given age fitter people are less likely to die irrespective of the cause of death.

Even if you’re less interested in putting years on your life and more interested in putting life in your years, good cardiorespiratory fitness is your ally. That’s because greater cardiorespiratory fitness is associated with greater health-related quality of life (19), and less burden from illness (20).

The list of references alone should convince you that being fitter is better in virtually every way, at least within reasonably achievable realms. Luckily, HIIT is the most time efficient way to substantially improve cardiorespiratory fitness.

Numerous studies totalling over 700 volunteers with a diverse range of health conditions have shown substantial improvements in cardiovascular fitness (21-70). 

Fitness has been measured in terms of VO2peak (see Part 1 for an explanation), peak work rate – basically how many watts you can generate per minute – and time to exhaustion at a given work rate. Changes ranged from 0-35% improvements in VO2peak; 6.5-33% improvements in peak work rate, and 23-56% improvements in time to exhaustion (See Table 2 for details).

Time to exhaustion is particularly relevant to everyday life as another name for it is work capacity – the higher your work capacity the less likely you are to find everyday tasks and activities challenging or fatiguing.

Although more session of HIIT predictably produced greater improvements, even six sessions spread over 2 or 4 weeks produced obvious improvements. That also means that HIIT can fit nicely around any other types of exercise you are doing. 

Three sessions per week seem about optimal, and we’ll discuss the nuances in Part 4.

Particularly pleasing to anyone with limited time to exercise is that comparisons between HIIT and ‘traditional’ moderate-intensity continuous, or what many have come to all endurance or aerobic exercise, showed similar or superior improvements from HIIT (26,29-32,36,38,40-44,49,51,52,54,56,58,60,66).

Also worth noting is that in the few studies that assessed men and women separately, both benefited to a similar degree (34,47,50,65).

Specific Effects on the Heart and Nervous System

HIIT improved cardiac output and left ventricular ejection fraction – both indicators of how well the heart is pumping blood – in volunteers with chronic heart failure and coronary heart disease (21,24,25), as well as in sedentary middle-aged volunteers (56,58), sedentary elderly (57), and overweight young women (60). 

The changes were greater with HIIT than moderate-intensity continuous exercise, the latter often showing no change (21,24,56,58,60). 

The only study to look at the effect of multiple sessions of HIIT on heart rate variability – an indicator of how well the sympathetic or ‘fight or flight’ and parasympathetic or ‘rest and digest’ parts of the nervous system are balanced – found an increase predominantly in parasympathetic activity, in elderly men after 14 weeks of HIIT (35). Meaning that these men tended to be more relaxed than stressed.

Blood Pressure

Research into the blood pressure lowering effects of HIIT has been a mixed bag with some studies showing modest reductions and others reporting no change. You can see detailed results in Table 2 here.

In women with a family history of hypertension (71), and men and women with hypertension, HIIT reduced ambulatory blood pressure with greater reductions reported overnight (56). 

Ambulatory means that blood pressure was measured several times during the day and night to give a much more detailed insight into blood pressure than one or two tests in a clinic or laboratory.

Results in resting blood pressure measurements (one-off tests) have been less consistent. 

Despite using similar HIIT programmes in terms of type, time, and intensity, two studies in people with metabolic syndrome showed diverging results with one showing a decrease (36), and the other no change (37).

Some key differences between the studies were that the one showing improvements was 4 weeks longer (16 weeks instead of 12), improved fitness more, and increased flow mediated dilation more – the opening response of blood vessels when more blood flows through them.

HIIT reduced resting systolic and/or diastolic blood pressure in two sets of sedentary volunteers after 12 weeks (40,57). However, no such effect was observed in overweight men after two weeks (39), eight weeks (41), 12 weeks (42), or in elderly men after 14 weeks (35).

One study reported blood pressure reductions 24 but not 72 hours after the last HIIT session (45). 

The studies mentioned above usually didn’t specify when blood pressure was measured following the last HIIT session, so it may be that some measured after the effects had worn off. 

Given such mixed results it’s worth knowing that in comparisons of HIIT with continuous moderate-intensity exercise, HIIT resulted in similar or better blood pressure reductions (21,36,37,40-42,56,71).


Flow-Mediated Dilation

Although the effects on blood pressure measured many hours or a few days after exercise were inconsistent, HIIT did have a fairly consistent effect on the dilation (opening) of major blood vessels in response to increased blood flow (28,29,32,36,37,56,67).

As a side note, I can tell you from doing dozens of blood pressure monitored exercise stress tests that intense physical activity raises blood pressure substantially, but that the majority of people experience a drop in blood pressure relative to that before exercise.


Take Home Messages

In summary, HIIT has an excellent track record for improving cardiorespiratory fitness, and its positive effects on heart function are also well established. 

The effects of HIIT on blood pressure aren’t as uniform, but are on par with those you might get from continuous moderate-intensity exercise. 

HIIT may also help restore balance to the nervous system by promoting slight dominance of the parasympathetic or ‘rest and digest’ branch.

As with other forms of exercise, the effects of HIIT are temporary, and improve over time as physical changes in the body take hold.

In Part 3 we look at the benefits of HIIT in terms of changing body composition, blood glucose control, and blood lipids, so stay tuned.

Want personalised guidance on starting or modifying a HIIT programme from a certified clinical exercise physiologist?

Check out our one-on-one training service here. 

References: click here for a full list of references cited.

Disclaimer: The tables you’ll see links to in the text are taken directly from my PhD thesis and are not user friendly. If you want the summary, just read the text, but if you want the fine detail, it’s all laid out for you in the tables. Also, I’m coming from the perspective of a clinical exercise physiologist specialising in healthy ageing and helping people with type 2 diabetes, metabolic syndrome, fatty liver, and similar conditions. These articles aren’t aimed at competitive athletes.

Interval training or high-intensity intermittent training (HIIT), as exercise scientists often call it, seems all the rage now. You might even say that HIIT is the intermittent fasting of the exercise world – at first glance a little extreme, but on closer examination effective and widely accessible to virually everyone with appropriate modification.

Done correctly for your circumstances, HIIT can help reduce body fat, boost metabolism, rapidly and dramatically improve cardiorespiratory fitness, lower blood pressure in those with hypertension, and improve blood glucose control in those with metabolic syndrome and/or type 2 diabetes.

HIIT is particularly practical as the time commitment is less than that needed for moderate-intensity continuous exercise, or what many just call ‘aerobic exercise’. So if you ‘don’t have time to exercise’ HIIT has your back.

In this series we’re going to explore the science, and on the way learn how best to apply HIIT for health. First we have to get clear on what we're talking about when it comes to HIIT for health.


What is high-intensity intermittent training?

There is no one uniform definition for HIIT. Based on the published research we’ll be looking at in this series, I’m going to divide it into two sub-categories: 

1. I’ll use HIIT to describe any physical activity of high aerobic intensity done continuously for 1-4 minutes, multiple times with either complete rest or a lower intensity activity between intervals.
2. I’ll use the term sprint interval training (SIT) for any physical activity done continuously with an all-out effort for 10-30 seconds multiple times with either complete rest or a lower intensity activity between intervals.

The differences between the two are the duration and intensity of the intervals. That said, some cardiac rehabilitation programmes use intervals of less than a minute, but these aren’t all out efforts, so I’d class them as a form of HIIT. Like I said, there are no absolute definitions.

The next hurdle to effectively using HIIT is that people have their own ideas about the meaning of intensity, whereas exercise professionals have some fairly specific definitions.

When following a programme or even trying to make sense of a study involving HIIT or SIT, you’ll need to know the professional definition.


What is intensity?

The first thing to understand when it comes to the meaning of intensity as applied to exercise is that – although high-intensity exercise is hard work, just because something feels hard doesn’t make it high-intensity.

For example, if you ran or walked for two hours and found the last 15 minutes really tough, it still wasn’t high-intensity. Think about it, almost anything that takes effort becomes hard when you do it long enough.

The rule for intensity is simple – the higher the intensity, the shorter the length of time you can keep it up. 

A classic example is a sprint; at their fastest, someone’s average speed over 100 m is going to be higher than their average speed over 400 m. The 400 m will almost certainly feel harder, but the intensity was lower.

The other quality of intensity as used when describing exercise formally, is that intensity is relative. Specifically, the intensity of an activity is relative to the person doing it. 

One person may take less than a minute to cover 400 m, another would need to rest a few times before going that kind of distance and do so at a slow walk rather than a fast run, but the person that needs a rest on the way may actually be working at a higher intensity.

This means that anyone can do high-intensity activity. It’s just that one person's high intensity is another may be another person's low-intensity.

This may become clearer when you understand how we measure intensity.


How do we measure intensity?

There are really two measures of intensity, which one we use depends on the activity we are doing. We’ll only look at the one relevant to HIIT here, and cover the one relevant to resistance exercise in another series.

Intensity, as mentioned above, is relative, and it’s specifically relative to aerobic/cardiorespiratory fitness (some call it cardiovascular fitness). 

In this context we measure intensity in terms of the volume of oxygen your body is capable of using when you’re working as hard as you can. This is often called VO2peak or VO2max , which stand for volume of oxygen at peak/maximum effort. The two terms mean slightly different things, but the distinction isn’t important here.

You would need the kind of equipment we mostly find in exercise laboratories  to measure VO2 (see below for a demonstration by some former colleagues). A more accessible alternative you can use is heart rate maximum (HRmax), which refers to your maximum heart rate. All you need is a heart rate monitor, which will also come in handy during exercise.

The laboratory I used to work in had all our volunteers undergo exercise testing to determine VO2peak and HRmax. We also monitored people with an ECG/EKG, which you will have had if you’ve ever been suspected of having had a heart attack as well as for various other reasons.

Tip: get an ECG monitored exercise stress test if you can afford it. This will give you an objective measure of your fitness, but more importantly it will give you some peace-of-mind when the ECG shows no problems with your heart, or provide a valuable warning if it does.

Intensity is therefore often reported as a percentage of VO2peak or HRmax. The closer you are to 100% of either, the higher the intensity. So in theory you would aim for say 85-90% of your HRmax for your intervals when doing HIIT. 

In practice this doesn’t work very well for HIIT. Even when you work your hardest, your heart doesn’t instantly go to 100%, nor if you work at 85% of your maximum does your heart go up to 85% HRmax and just stay there. Life would be easier if it did, but we have to make do with what is.

Studies often report that volunteers walked/ran/cycled etc. at some percentage of their HRmax or VO2peak, but what they mean is that they did an exercise test and then had people walk/run/cycle at a speed/incline/resistance consistent with that VO2peak or HRmax.

Let's say I do an exercise test on you and you get to 85% of your VO2peak pedalling against a resistance of 125 W (watts), then all I need to do is set the bike to 125 W during your intervals. The problem arises when you get fitter, or if you have an off day and find the 125 W too hard.

Bare with me, the solution is easier than the explanation, but I want you to understand the problem so that you don't get yourself into trouble if you use a heart rate monitor.

Below you see an example of what might actually happen to someone's heart rate while doing HIIT. 

The specifics will vary in terms of the maximum heart rate, and the degree of recovery during intervals. Someone with better cardiorespiratory fitness may achieve their target in an earlier interval and earlier in each interval, as well as returning to the resting heart rate more quickly.

You can see in the graph that heart rate gradually increases during the warmup and even more during intervals, and then goes back down somewhat during the recovery periods between intervals. 

This is what tends to happen if the recovery periods are for a predetermined time. If you have a heart rate monitor you have the option of varying the length of the recovery periods so you always start your intervals at the same heart rate, in which case things look a bit different. We'll discuss this more in the last article of this series.

So how do you determine the intensity of an interval is right given that basing it on heart rate doesn't work? Use rating of perceived exertion (RPE) scales (1). Here is one I’ve used in studies and use with clients.

You’ll see the target for both the warmup and the intervals clearly highlighted. The scale being subjective is actually an advantage in that it accommodates to how you’re feeling. 

I do recommend also using a heart rate monitor so you have an objective measure as well.

NB: to avoid any confusion it’s important to know that maximum heart rate doesn’t go up as you improve cardiorespiratory fitness. Maximum heart rate is a function of genetics and age – it goes down by roughly 1 beat per minute per year of life. Other things like stroke volume – the amount of blood your heart can pump per beat – do go up with fitness. That means that over time you’ll be able to do more at a given heart rate.

OK, that may be more than you wanted to know, but unless you have a coach or trainer, it pays to know these things so that you get the results you deserve.

Given that HIIT and SIT do make your heart race, an obvious questions is...


Is it safe?

SIT really hasn’t been assessed for safety, so my advice is to leave it to healthy, already very fit individuals. That’s because these are the people it has been tested in.

In truth, being alive isn’t safe, so no form of exercise is entirely safe either. Not doing exercise is however fairly detrimental to health too.

That acknowledged, HIIT has a good track record even in those with heart conditions. It’s been employed in cardiac rehabilitation by the Norwegians for several years. They collected data from over 46000 exercise hours and show the risks of their HIIT programme is similar to moderate intensity exercise (2).

Studies in much smaller groups with chronic heart failure (3-11), recent stroke survivors with hemiparesis (12), and patients with coronary heart disease have reported no serious adverse events (13). 

The health/fitness professionals among you might want to look at the pdf of Table 1 here to see the acute effects of HIIT on cardiovascular function.

For everyone else, it’s probably enough to say that everything either stayed within normal ranges or recovered quickly either during the recovery intervals or several minutes after the HIIT session ended, even in people with stable cardiovascular disease (13-20).  

That said, you should always check with your doctor before increasing the intensity of your exercise. Better yet, get them to order you an ECG/EKG monitored exercise stress test as recommended above.

In Part 2 we look at why HIIT should probably form part of your exercise programme by exploring the many health benefits that can be gained efficiently.

Want personalised guidance on starting or modifying a HIIT programme from a certified clinical exercise physiologist?

Check out our one-on-one training service here. 

References: click here for a full list of references cited.