Science AMA Series: My name is Ulrik Wisløff, and I am a professor of physiology at the Norwegian University of Science and Technology. My research focuses on the cardiovascular benefits of exercise both in healthy individuals and in individuals with lifestyle related diseases. AMA!


Hi Reddit! I'm Ulrik Wisløff. I head the Cardiac Exercise Research Group and the K.G. Jebsen Centre for Exercise in Medicine at the Norwegian University of Science and Technology. In 2005, my research provided the first causative evidence that low exercise capacity (in other words fitness) was sufficient to dramatically decrease the risk of heart disease. Subsequently, we investigated the effects of high-intensity interval training (HIIT) in both healthy individuals and in individuals with disease and showed that HIIT is not only safe (even for patients with heart disease), but also leads to superior improvements in aerobic capacity and cardiac function. Using clinical and epidemiological data, my group then developed a fitness calculator, which estimates one’s fitness and translates it into fitness age. In other words, your fitness determines your physiological age. The American Heart Association now recommends the fitness calculator. In fact, in a recent statement of the American Heart Association, which I co-authored, we demonstrated that having a good age-specific fitness is much more important for health than the number of hours spent being physical active. Or simply put, you may exercise many hours per week, but if you remain unfit you have very limited protection against developing, and dying prematurely from lifestyle related diseases. On the contrary, if you only exercise a few minutes per week, but that exercise is sufficient to make you fit, you are more likely to live healthier longer.

While we know that physical activity is good for us, the prescription regarding how much physical activity one needs to stay healthy is at best vague. That is why I invented a Personalized Activity Intelligence (PAI). PAI calculates how much weekly physical activity one needs, to live a longer and healthier life. PAI is now a wearable with a simple way to understand weekly score output, ranging from 0-100 PAI. The goal is to get a score of 100 PAI every week. PAI is personalized to the user and depends on the user’s fitness, meaning that 100 PAI for me will not be the same as the 100 PAI for a 70-year old heart failure patient. A score of 100 PAI can be earned using any activity of personal preference: running, walking, swimming, cycling etc. The higher the intensity of activity, the higher the PAI score. It is up to you to decide if you want to do hours of low intensity activity and earn your 100 PAI, or if you want to earn your 100 PAI as fast as possible by ramping up the intensity of your training. Our research has shown that obtaining a weekly score of 100 PAI secures a good age-specific fitness, and also cancels the negative effects of prolonged sitting-time.

I am currently working on a book with the working title "Welcome to the fitness world", where I concentrate complex science into an easily understandable message with practical tips and concrete, doable training programs for those with limited time looking to quickly improve their fitness and so their health. I will be happy to share some insights from that book and give tips on how to exercise in order to get maximal health benefits in a time-efficient way.

I will be back at 4 pm ET to answer your questions, AMA!

Hi Professor Wisløff:

  1. How many times a week is ideal for HIIT?
  2. How would you combine HIIT with other forms of exercise like low-intensity steady state cardio and weightlifting?
  3. Are there any major differences between the various HIIT protocols? (Tabata, Wingates, 4x4, etc.)

Thank you from r/HIIT for doing this AMA!


Hi! Good questions that is impossible to give one-answer that fits all. #1 depends upon your goal. I would say if you have time to exercise 4-5 times per week, at leat two of them should be HIIT. However, if you have time to exercise twice per week I would say that every second week both should be HIIT, and every second week at least one of them should be HIIT. The reason is that it is so time-efficient and health-efficient. Observations from our lab is that people that has on average (or below) fitness that undertake two HIIT's per week quickly improve their fitness level - and with the side effect that they start to be more physical active during their daily living - not a bad side effect of HIIT!

If you have fitness level above average two interval sessions will slightly increase your fitness level - then get stable - and when you get really fit you have to have two HIIT sessions just to keep your fitness level stable. For less-than-average and average fit people I have made a 7 week training program to quickly increase fitness and basic strength capacity. In that program that I guarantee improve your fitness level also combine basic strength training and other cardio workouts, that I guess answer your question #2. It has been used by more than one million people now so give it a try - completely free of cost - only some sweat needed.

For question #3 there is a long and short answer. The short is that there are huge differences between the HIIT protocols - so you should be aware of what you wants from your work outs. All of them have good effects but mainly on different organs/functions. A long version of why HIIT is very effective and should be performed for more than one minute is given below. You may also see a paper in which we compared [three popular training regimens](Obese as well as a recent review where we write about the essentials of HIIT30051-8/fulltext) for more information.

Also, in the literature there is now agreement to term [4x4 intervals](: or long intervals OVER 1 min duration). Also be aware of that the scientific community no term the one minute interval and shorter is either called SIT or by their specific names (such as Tabata).

Here we go: A cardinal question when I design training programs is to find what limits the capacity I would like to influence. So for cardiorespiratory fitness (measured as Peak VO2) – what is the limitation? Is it the working muscles or is it the heart?

For most people working muscles have the capacity to handle 3-4 x more blood than the heart can pump at maximum. Therefore, for most people the pumping capacity of the heart limits PeakVO2.

The pumping capacity of the heart is made up from stroke volume (how much the heart pumps per beat) multiplied by maximal heart frequency. Maximal heart frequency is inborn and is reduced about a beat/min every second year. Thus, the only thing we can stimulate to increase the pumping capacity of the heart, is the stroke volume! Then an important question for choice of training intensity is at which intensity do we reach maximal stroke volume? Now we know that maximal stroke volume is reached somewhere between 85-95% of maximal heart frequency. Further, how quick do we reach maximal stroke volume? A critical question for the duration of the exercise! Maximal stroke volume is reached after 1-2 minutes or as fast as you reach 85-95% of maximal heart frequency. So from this, if we stop let’s say after a minute when we have reached maximal stroke volume we do not get many seconds of optimal training for the heart. The heart is as any other muscle, it must be loaded to get trained – and the only healthy way to challenge the hearts pumping capacity is to fill it with maximal amounts of blood for longer periods of times. It is obvious that one cannot exercise for very long periods of time at 85-95% of maximal heart rate and that is why we recommend to use interval training, with periods that get you up to the needed intensity, interspaced with active breaks to get rid of the lactic acid that builds up during the interval (because it is not pure aerobic metabolism going on during such intervals aiming for 85-95% of maximal heart rate). For how long are we able to exercise at maximal stroke volume? An important question for deciding the duration of the training! Our experience is that absolutely everyone can do 4 minutes, including very untrained individuals (if not they simply exercise at too high intensity - we have done this in individuals with PeakVO2 levels as low as 11 ml/kg/min and as high as 87 ml/kg/min). That is why we often recommend 4x4 minute interval training as described [CERG's webpage]( for individuals that would like to have time efficient and very effective training program. We have done this in a lot of patient groups as well as in highly trained individuals – and it works.

So what if the heart is not a limiting factor? In some very untrained persons factors inside the muscle may limit Peak VO2. Still doing the 4x4 is very effective in terms of improving the factors inside the muscle – and as a side effect one also train the heart very efficiently. That said it is evolving evidence that the sprint interval training protocols are more effective in terms of improving factors inside the muscle when compared to the 4 x4 interval training I often recommend – but then you do not get very good cardiac training. A combination is certainly a good compromise and I do that my self in my own training.

Can you go into how you determine fitness a bit more? What factors are you looking at to determine if someone is fit or not? I think for many of us, it is hard to imagine how someone who only exercises ten minutes a week is more fit than someone who runs a mile a day.


Thanks for your question - I like it! So what is cardiorespiratory fitness (CRF)? I recently co-authored A Scientific Statement From the American Heart Association about fitness and its importance for health. There is no doubt that CRF, measured as Peak Oxygen Uptake (PeakVO2) is the single best predictor of current and future health (better than if you have heart disease, hypertension, type 2 diabetes etc). The reason is that every single cell in the body requires oxygen to work properly over time. PeakVO2 is the sum of how good your lungs are to breath in oxygen (and CO2 out), how good the oxygen is transferred to the blood stream, how good the heart is to pump oxygen rich blood (stroke volume - se response to previous question), how good the blood vessel are to transport the blood to the different organs, and how good the organs are to receive and utilise the oxygen offered. I will elaborate more abut how we determine fitness below but to your last question about running a mile a day vs. only 10 min exercise with high intensity per week. There is really no large studies supporting that 10 min per week is better than running a mile per day in order to CRF. That said - I have seen individuals with low age-specific fitness level that exercise at low-to-moderate intensity every day - so that is absolutely possible as they do not tax the heart that in those people is the key limiting factor for improved fitness (see question above). In such cases 10 min with intensive exercise may lead to a higher PeakVO2 vs only low/moderate exercise intensity. In such cases the 10 min weekly high intensity work out may be time-efficient. But again when we compared 4x4 min interval with the 1-minute workout 3 times per week (i.e. 12 minutes in total + warm-up + cool down) and isocaloric moderate intensity training we could not find any differences between moderate intensity training and 12 minutes SIT over a few weeks). So in my view there exist no quick fix in my view regardless of what anyone claims - there is no proof of that in the long run!

A little bit more about the how to determine fitness for most people. Despite that PeakVO2 is a strong predictor of future health and mortality, it is not routinely measured in clinical practice. The reason for this is simply due to the costly and time-consuming procedure of exercise testing that requires trained personnel and expensive equipment. Based upon all the evidence, Peak VO2 should in my view be regarded as a vital sign on line with blood pressure, cholesterol, obesity etc. Low fitness as a risk factor has the power of the risk of blood pressure, smoking and high blood pressure combined. I realized 10 years ago that it would take a long time before fitness can be measured objectively at a GP’s office.

Therefore, I decided (in 2006) that we should aim to develop the best algorithm to estimate fitness that exists. So we tested directly with expensive equipment and trained personnel. Peak VO2 in a diverse population of 4637 healthy participants (aged 20-90 years) to develop (The Fitness Calculator)[] that should be possible to undertake within minutes anywhere in the world by any person that has access to Internet. We were able to make a robust and reproducible algorithm that predicted Peak VO2 - and we had access to "everything" about the participants - even their genome. However, if the fitness number does not relate to hard endpoint such as disease or death it is not really worth much. So we assessed whether our algorithm that we baptised The World’s Fitness Calculator could predict long-term all-cause and cardiovascular disease mortality in a total of about 40 000 men and women that took part in one of the most complete health survey ever performed; [The HUNT studies in Norway]( We had data to calculate Peak VO2 for all these persons using our Fitness Calculator. So we did, and by connecting Peak VO2 to complete end-point (cause of death) registry in Norway we were able to see if the fitness numbers (Peak VO2) calculated by our Fitness Calculator actually predicted death. Indeed we found that the risk of dying unnecessary early (premature death) was much higher in those below the age-average Peak VO2 vs. those above. Also, we observed about 20% lower risk of premature death per MET increase in fitness number (1 MET = 3.5 ml/kg body mass/min) in both men and women.

CERG is now making the first catalogue of global fitness for "normal people" that could be used for a variety of purposes, especially to define large-scale public health policies. We have now fitness data from about 120 countries, more than 6 million users, and on average about 2000 new users worldwide register their fitness and health data every day. Global Fitness Data30020-8/fulltext) first the first 730 000 participants have been published.

In a recent study published in Mayo Clinic Proceedings30628-0/fulltext), we tested the value of estimated fitness in predicting the risk of premature death from either heart disease or any other cause, alone or in combination with other risk factors such as high blood pressure, smoking status, alcohol consumption, family history of heart disease, and diabetes. In other words, we investigated whether adding estimated fitness to traditional risk factors could improve the reliability of predicting premature death.

We found that estimating fitness was enough to predict future risk of premature death from all causes. There was no need to perform complicated risk score algorithms that traditionally are used to calculate risk. With the increase in lifestyle-related diseases around the world, estimated fitness is an easy, cost-effective method that could significantly help medical professionals identify people at high risk and improve patient management. And just as importantly, it is a test that individuals can easily use to assess his/her own Fitness Number and Fitness Age, and in cases of low fitness do something about it! The only thing needed is access to the Internet and/or a smartphone as we have made this tool freely available. Importantly, the Fitness Calculator is now officially recommended to be used in the Scientific Statement from American Heart Association about Fitness as a Vital Sign. To make it user-friendlier we introduced Fitness Age. In my view Fitness Age is the best measure we have of biological age, simply because it is the best predictor of future health there exist. So if a 60 years old person has the fitness level (Peak VO2) of 40 years old, we say that this person’s Fitness Age is 40. Very often we see older adults with Fitness Age of younger people and vice versa. So it is absolutely possible to turn back the clock! – But also speed up it’s ticking if one remains unfit.

What is the best activity for a person with a heart disease? What is your take on reversing heart disease with vegan diet? What diet would you recommend? What food supplements would you recommend?


Hi! I am not an expert on dietary advise at all so I cannot give any advise in that matter. In regard to which type of activity a person with heart disease should undertake it is not a one-size fit all answer. Persons with heart disease should anyhow consult their doctors before undertaking exercise. That said the probably most risky thing to do is to do no exercise! A few years ago we published a study showing that it is in general safe to undertak exercises both with moderate and high intensity for patients with heart disease We have over the last 20 years studied the effect of mainly moderate intensity exercise training, which is the most used in clinical practise, with High Intensity Interval Training (HIIT). Some examples where HIIT is more effective in improving CRF, cardiac function and/or quality of life in patients with heart disease are in patients with post-infarction heart failure, coronary artery disease, patient that have suffered a myocardial infarction. In patients with COPD we found similar effect of HIIT and moderate intensity training. In patients with atrial fibrillation HIIT was also found very effective. In individuals with metabolic syndrome, type 2 diabetes, obese adolescents, hypertension HIIT has been found to be a very effective medicine.

atrial fibrillation

In your expert opinion, does the scientific community have a strong enough grasp of exercise science to make claims to the public yet, or are there still a lot of basic questions to answer first?

I ask because we see lots of advertisements and PSAs encouraging things like, 'be active for 60min/day,' or, 'walk 10k steps/day,' but the recommended quantities--and even the recommended activities--are constantly changing.

So is most of what makes its way to the public just extrapolated from relatively preliminary scientific data, or do you think the advice we get is good even though it's not as consistent as it could be? Obviously we know that exercise in general is important, but I'm more concerned about specifics.


The questions you brings up are highly relevant in my view. First of all, studies saying that exercise and physical activity increase longevity are purely observational/epidemiological studies. That means that those that report to be physical active live longer - but that may imply that they are physical activity are physical active because they are healthy, or that those being physical inactive are that because they are sick or have an unknown underlying disease. To really answer the question whether physical activity prolong life one need to undertak longitudinal randomised clinical trials with long follow-up on hard endpoint (deaths), and such studies are extremely costly. However, we have been crazy enough to start one such study called the Generation 100 study in which more than 1500 elderly (aged 70-77 years old) are exercise trained for 5 years aiming to improve fitness level in order to study whether this actually gives more and healthier years. First baseline data from the Generation 100 study have been published and we have the final answers on the "big question" towards the end of 2018.

That said - I honestly physical activity is very important for all individuals health. However, as you state the guidelines changes frequently - and who shall the layman listen to? I think people are tired of all types of advises out there - we need advices with a robust evidence base. Here is my view: Although physical activity levels are associated with mortality, this association disappears after adjustment for fitness (PeakVO2). PeakVO2 is a major determinant of future cardiovascular health and longevity and our most recent findings show that high age-relative PeakVO2 mitigates the increased odds of having CVD-risk factor clustering associated with sedentary time. A robust body of evidence suggests that recommendations for PA SHOULD emphasize PA-modalities that effectively result in improved or maintained high age-relative levels of PeakVO2.

So select physical activity that ensure an average or above CRF level and forget counting steps, kcals, minutes of this and that activity. As stated above PAI is a super practical way to ensure that you obtain above average fitness level. It is much more personalised than any metric I know of and that is what we need - advise based upon how each individuals body responds to exercise training/physical activity. It require the use of Mio Slice today the plan is that it soon will be available on most major brands. For people that do not want to use a heart rate monitor visit our 7 week training program to get examples how to efficiently improve your fitness level.

As a 50 year old, I am working to stay in and get back in shape. My question is: what should be my expectations for how good my fitness levels could be? I ask because I get injured more easily and heal more slowly than in my youth. When I lift heavier weights, I end up hurting my shoulders. When I run, I injure my knee or ankle. Should I just accept I am old and stick to walking my dog?


I would definitely look into our 7 week training program that has been used by over a million people with success. When that is done you will find a new program - the next 7. I am 100% sure you will feel improvement after 3 weeks. I would not accept that you stick to just walking your dog - however, you may bring your dog if walking is you preferred exercise. In my vies you should consider using relatively steep uphills during walking to reduce the chance of injuries on your knee - and also consider cycling and/or swimming or similar activities for period to get started. Also, if you do not have a heart rate monitor or do not want to use it please check out the program I mentioned as there is no need for a heart rate monitor if you really do not want to use that. Somre more information is also given under our 4x4 FAQ. If you like to use a heart rate monitor you should consider using the Mio Slice for now to make sure you keep your PAI-level about 100 per week. For more details about PAI see response above or on PAI Health.

Thanks for answering questions here. My question is how does the cardiac adaptation caused by HIIT differ from the adaptation of steady state cardio- such as a long run at a moderate pace?

Also, you mention how HIIT improves both cardiac function and aerobic fitness. To what extent are those the same thing, and what other non- cardiac adaptations are involved in aerobic fitness?


Please see response to my first question today. Both your questions have been answered there. Also visit [CERG's]( webpage for more details.

I see your algorithm for determining PAI is proprietary, and I don’t suspect you’d release it here, but I wonder if you could clarify some things.

  1. Does the criteria for reaching 100 in PAI dynamically change?

  2. Does your algorithm differentiate between activity induced heart rate elevation and stress or medication induced heart rate change?

  3. Does your algorithm utilize predicted cardiac output or just heart rate?


Good questions. First of all it will be meaningless to publish the algorithm here as it is dynamic and it will be extremely un-practical for people to calculate their own PAI based upon that as it depend of how many PAI's you have at a certain time point. Here are some more information about PAI that should answer your questions:

You can earn your 100 PAI using any form of physical activity. The type of activity you chose to do depends on your own personal preference and could include anything from walking to work, cleaning, climbing stairs, dancing, exercising at high or moderate intensity, to playing with your children or grandchildren. You can even swim, as the Mio Slice is waterproof. You can combine activities of different intensities and durations to earn PAI. The higher the intensity - the more PAI points you earn at any given time. In other words, you can accumulate 100 PAI by doing low intensity activity, but this will take you longer. If you are pressed for time, high intensity activity will get you to the 100 PAI faster. You could walk 10,000 steps in one day without getting a single PAI if you are in good shape, as walking might not increase your heart rate. If you are untrained, walking 10,000 steps per day could get you your PAI, if it results in increased heart rate. This explains why some people, even when they believe that they exercise a lot, fail to see improvements. That said, please note that we applaud all physical activity, as any physical activity is better than none!

The PAI algorithm is designed to make it easier for you to reach the first 50 PAI compared to the next 50 PAI. This means that if you repeat a workout two days in a row, you will get less PAI the second time around. This is because our research shows that the risk reduction towards developing lifestyle related disease is greatest when progressing from a couch potato state to beginning to be physically active. Mio Slice App enables you to see at all times if your heart rate is in the PAI point earning zone.

You need an activity level that gives you 100 PAI or more per week to achieve optimal protection from lifestyle-related diseases. To achieve this, you do not necessarily need to be physically active every day. For example, if you know that you are going to be busy at the end of the week with no time to exercise, you can "bank your PAI " today. This will allow you to take it easy for the next few days with good conscience. However, you cannot allow yourself to do this often, as taking it easy for too long costs you in interest and deductions. For example, the PAI points you have earned on Tuesday one week will be lost Tuesday the following week. It is only possible to earn 75 PAI points per day.

Research has shown that achieving 50 PAI points per week also yields significant health benefit, but achieving 100 PAI is optimal. We found no additional benefits with a score higher than 100 PAI per week.

Research has shown the same beneficial effects of 100 weekly PAI even if you have a few extra kilos, had high blood pressure or type 2 diabetes.

It also shows that obtaining a 100 PAI per week abolish the negative health effects of prolonged sitting time.

Why is PAI a more accurate metric? Goals such as "10000 steps" or "30 minutes" of physical activity per day do not give a precise picture of whether you are active enough. PAI is based on your heart rate, which is a more accurate reflection of the intensity of physical activity you do. It does not matter what activity you are doing as long as it elevates your heart rate.

To your question #2: Yes - there is an inbuilt accelerometer in the Mio Slice that recognise whether you are active or not - so higher heart rate due to stress will not count. That said - heart rates due to stress is normally below the threshold of earning PAI anyway - if not you would probably have noticed and you should seek advice from your GP. We have had a few examples of earning PAI while sitting more or less inactive - one was a person that on a daily basis recorded heart rates above 130 beats per minute using the Mio Slice and earned PAI - she questioned this and showed data to her GP - that immediately noticed that she had atrial fibrillation - the other person had similar, but higher heart rates, and after seeking medical advise that person got an ICD implanted - in both cases they had used PAI over several weeks with normal values so that they had a long history of heart rate data that formed the basis for diagnosis - both now successfully treated. However, for 99% of stressful situations you will not earn PAI as the heart rate will be below a certain threshold of earring PAI.

Question #3: we do not predict cardiac output today but could do. We have also developed a new algorithm that can predict cardiorespiratory fitness in a good way and similar to the accuracy of the Fitness Calclulator that is more than good enough in terms of accuracy to tell whether a person have healthy lifestyle or not. Importantly, it is better than all other estimations you can find in any wearable worldwide as it is developed on several thousands individuals from over 2-3 decades.

This and more information about PAI can be found on [CERG's homepage]( and at PAI Health

Does the PAI look at anything besides heart rate to measure activity level?


Here is the background for developing Personalised Activity Intelligence - PAI for those that do not know what it is yet. Thereafter, a short answer to your question.

Individualised lifestyle advice to ensure improvement in CRF. A major challenge in health promotion of PA is to provide clear feedback to individuals with personalized and meaningful information that motivates individuals to increase or sustain PA levels. Goals such as ‘10,000 steps per day’ or ’30-minutes of moderate activity per day’ do not operationalise PA intensity and do not reflect the body’s response to each activity. It is well accepted that the most personalized, accurate way to track and measure the body’s response to PA is by monitoring a person’s heart rate. Unlike all other PA metrics, such as distance walked, number of steps, and frequency or duration of activity, heart rate changes reflect the body’s response to PA regardless of the type, context or intensity of activity performed (walking, running, swimming, cycling etc.). Heart rate in it self is also meaningless – it must be related to a meaningful endpoint. By studying 45,000 people, in the [renowned HUNT-studies]( over more than two decades, we have defined a weekly beneficial heart rate response during PA that translates into reduced long-term risk of premature CVD- and all-cause mortality. This algorithm is the Personal Activity Intelligence or PAI. PAI analyses a continuous stream of heart rate data acquired from the user over a week and provides a single measure (arbitrary units from 0 to 100 PAI/week) for whether current PA level is sufficient to obtain or sustain a good health profile. The main data behind the PAI31069-5/fulltext) is as follows: After >1 million person-years of observations during a mean follow-up time of 26 years, there were 10,062 deaths. Healthy men and women with a PAI-level ≥100/week had about 20% reduced risk of CVD mortality compared with the inactive groups (0 PAI), respectively. Obtaining ≥100/week PAI was associated with significantly lower risk for CVD mortality in all age-groups (20-70 years) and in participants with known CVD risk factors such as smoking, hypertension or overweight/obesity and type 2 diabetes. In terms of risk, no further reductions in CVD or all-cause mortality were observed for scores progressively higher than 100 weekly PAI. Participants with <100 PAI had a on average 5 years fewer years of life than those with ≥100 weekly PAI, while the difference was more pronounced in middle age participants. Importantly, obtaining 50-99 PAI per week also gave substantial health benefit (about 70% of that obtained from >100 PAI per week), whereas no significant benefit in terms of mortality risk was observed whatsoever in those with <50 PAI per week. This means that an intermediate goal could be to reach at least 50 PAI per week in people with extreme lack of motivation to being physical active. IMPORTANTLY, participants who did not obtain ≥100/week PAI had increased risk of dying regardless of whether or not they met PA recommendations.

Rather than taking a “one-size-fits-all” approach to fitness assessment and monitoring via step count, number of minutes of PA or distance tracking, PAI represents a personalized reflection of the body's response to PA based on heart rate. OF PARTICULAR PRACTICAL INTEREST is the fact that obtaining 100 PAI/week is a practical way for patients and primary care workers to know when they have done enough PA in order to increase their CRF levels.

The data behind the PAI-algorithm shows that 100 PAI can be achieved at different intensities, frequencies and duration over the course of a week, according to an individual’s preferred physical activity type (gardening, swimming, cycling, walking, running etc.) and pattern (e.g. moderate intensity and long duration vs. higher intensities and shorter durations). The key is to accumulate adequate active time above a certain heart rate threshold in order to earn PAI and the higher the intensity the shorter the time needed to obtain 100 PAI. PAI can be achieved by performing exercise/PA at various doses of quantity and intensity.

When we developed PAI we had access to an enormous amount of personal and medical information from the [HUNT studies]( ended up with an algorithm that includes gender, age, resting heart rate and maximal heart rate. That was all that was needed.

For more information about how to earn PAI and other related questions about PAI - take a look at PAI FAQ questions

When should a daily exercise starts? A young age? (E.g 10 yrs old)

What advice do you have for people who doesn't want to get fit but wants to be healthy?


We are born to exercise so one should start as early as possible. Young children often naturally perform high intensity interval training without knowing it. Systematic training is important to start at school age in my view because todays school system to day make sure kids get inactive! We have a few studies showing huge beneficial effect of high intensity interval training to revers the impaired cardiac function often observed in obese adolescents.

About fit vs healthy see responses to questions above. The short answer is that you should aim to at least become so fit that you have a fitness level which is on average or above for your age group. We have shown that if your fitness level is less than 85% of what is expected for your age group the risk of disease and unnecessary early death is dramatically increased. Test your [Fitness Number]

Could you give a layperson explanation of what "fitness"means? I think most have a broad understanding, but could not give you an exact definition. Is it a set of certain physical characteristics?


See responses above but to make it simple, cardiorespiratory fitness is measured as peak oxygen uptake (PeakVO2), it determines how much work your body can perform over time. For instance for a world class cross-country skier PeakVO2 is a key determinant for the race result, whereas for a heart failure patient it is a matter of being able to perform daily activities. Take a look at our Fitness Calculator where we also present more about fitness levels and risk of disease and premature death. Other relevant information can also be found at different links on [CERGs homepage] (

Since many workouts consist of different exercises to work different parts of the body, how do you determine which is most important for the participant? If cardio has a higher PAI than an activity such as weightlifting or leg presses, is it still necessary to do those activities with lower PAI ratings to be considered fit? A better way to put it would be: Can I get all the exercise I need to fulfill my 100 PAI just doing a certain exercise? Or are many different kinds important even if they all have low individual PAI ratings?


Good and relevant questions. See replies above for some more details. It is clear that most studies focusing on fitness and health have determined the association between cardiorespiratory fitness and health outcomes (such as disease, death, quality of life etc.). That is the main reason why "strength capacity" often is overlooked. However, we recently demonstrated that a handgrip strength and chair-rise test performance30140-4/fulltext) predicted the risk of all-cause and cardiovascular mortality independent of physical activity levels. So strength capacity is also important - for some patients and inactive older adults one often first need to build up a certain strength capacity to be able to undertake effective endurance training regimens.

There is no doubt that the PAI scoring system award higher heart rate more than low-to-moderate heart rates. So a strength training regimen that hardly raises your heart rate is not good enough to get your cardiorespiratory fitness to an average, or above, age specific level. So just performing such strength training is not enough to protect your health in the long run. We have good experience that functional strength training such as cross-fits effectively increases the heart rate to earn plenty of PAI's so that is absolutely a good alternative to just cardio training. All training is good but make sure to keep you PAI level above 100 that is the cheapest health insurance you can get.

How can we encourage more people to exercise? Are you optimisitic about the future of fitness culture?


I think that the worldwide "trial" to get people to exercise 5-7 days per week has failed completely, as at best 20-30 % the worlds population fulfils todays advice for physical activity given by health authorities worldwide. In my experience inactive people can follow the advices for a month or three and then fail - and they really feel they fail because they cannot fulfilled the health authorities recommendations - and for that reason many stop exercise completely and get inactive most of their life. There is now enough evidence to say that do whatever you want as long as you make sure your Fitness Number is ok! That is the take-home-message we should tell the world about. That is also the reason American Heart Association now has a statement about the importance of fitness. We have to find practical ways to give scientifically proved advice that ensure a good age specific fitness level. PAI is probably the single best way to record this today - and as long as you obtain a weekly PAI score above 100 you get average or above Fitness Number. It is also super important to give the message that one do not need to exercise every day to obtain huge health benefits - the science say that you can do less than recommended from health authorities today as long as some of that is done with high intensity. Again, the PAI-scoring system take this into account and seems to be very motivating for people as they in busy periods can "bank" PAI in order to have no-bad-concius of not performing physical activity the next following days. Alternatively, or in combination with PAI, our 7 week training program guarantee for improved fitness levels - but you have to do the work!

I am relatively optimistic about the future fitness culture, and I really think wearable gadget is part of that future - also to improve interactions between health carers and patients. However, my concerns in use of wearables is that "everyone" wants to have their own physical activity metric and that the present to the user, just because it is technical possible, tons of information that is not proven to be important for health at all. This will attract the "first movers" that are active enough anyway. We need to simplify our advises - PAI can become a new world standard for activity training and motivation that ultimately leads to improved health world-wide. And that can become a reality now that PAI will be possible to use also with other gadgets, beside the Mio Slice, such as the Apple Watch very soon.

What is your opinion on calisthenics and how effective it is for building muscle?


That depends upon how fit you are. It will be effective to build muscle size if you are unfit. However, what is more relevant to me is that it is enough to keep your functional muscles fit. I have seen several obviously "strong people" in terms of how many kilos they can lift in bench press, squats etc but that have a poor functional strength as we every person need in daily life to avoid unnecessary injuries. Examples are given in our 7week training program.

Hi Dr. Wisløff! Thanks for doing this. My question: Although some types (like in your research, HIIT), maybe be more efficient in reaping benefits, is there any physiologic difference in the type of cardiovascular benefit (i.e. stroke volume, vascular resistance, venous return, etc.) a person gains from different exercise types, like cardio, weights, HIIT, and so on?


Absolutely. If you see my previous responses today there is clear differences!

Is there a way to track PAI with an Apple Watch?


Not yet! However it will be available for Apple Watch early in 2018. It will also be available for other brands next year. Updated information will be available on PAI Health's web page

What "extra" benefits would I get, if I put the effort in to exercise on a daily basis. I say extra because currently, I walk ~2-3 miles everyday at my University, and am currently 130 lbs with a height of 5'9", because of this, I don't really see any more of a benefit of physical activity. Another question, what can long term stress, or depression affect the body?


There is no doubt that you cover enough distance per day. However, what is not clear is whether you exercise/being physical active so that you get out of breath now and then - because that is, unfortunately, necessary to improve your fitness level as is probably the most important number we should care about for our health. I am not saying you should exercise at high intensity every day, but all people should do that at least once or twice per week. In my view it is not important to exercise every day either - the important thing is to increase fitness, like it or not! As mentioned above I have developed a 7+7 week training program that improve fitness level of regular people. In addition we have developed a Fitness calculator so you can calculate your fitness number. In addition to that PAI is a easy way to ensure that/when you have done enough physical activity per week in order to stay healthier for longer.

I literally just sat down after defending my masters thesis which was a literature review on how exercise effects HTN, Hyperlipidemia and Diabetes! What are the odds? My most interesting findings were on augmentation of nitric oxide and ox-HDL. Have you done any research on how exercise relates to prevention of heart disease on a chemical level?


There are several papers out there "feeding" patients with beet root juice and similar NO-donors with promising effects on HTN and other diseases. We have not done any such study in patients but we know that HIIT increase NO-production and we have shown that in a few of our studies as well. There is also some literature out there showing improved fitness level due to NO supplement in athletes. However, in top-endurance-athltetes we found no effect of NO-supplement whatsoever. However, as you have noticed this may be a promising path in some patients groups. I would note that NO is naturally produced in the body, for instance in the endothelial cells that covers the inner layer of blood vessels) as a response to for instance exercise/physical activity. In patients with angina pectoris, the ability to produce NO is reduced, and they must receive this artificially in terms of for instance Nitroglycerine to have their vessels work properly and avoid chest pain during physical activity. I am sceptical to that "regular people" starts to use NO supplement on a regular basis as that MAY reduce the bodys capacity to produce NO in the long run - so rather perform HIIT to learn the vessels to produce NO it self

Hi Professor, do you think think routine is ok, or should I reduce the amount of workouts I do in a week? Is HIIT better right after power lifting, or should I do them on separate times in a day?

My routine:

  • MWF - Powerlifting (currently at 40lbs squats/bench/overhead press, 80lbs deadlifts), 20s/2m x 5 reps HIIT on an Airdyne, done right after the lifts
  • TTH - 3 miles 1min walk/1min light jog
  • Sat - 15 mins Abs/core training + 10 reps/3 sets sandbag training (currently at 25lbs)
  • Mon-Sat - Jump rope 5-10 mins
  • Sun - rest day

Also, I noticed that when doing HIIT my fitbit shows that my heartrate doesn't really get up to more than 140-150 (max hr 190) during the high interval, but I do feel like I'm dying after. Am I on the right track, or am I not utilizing HIIT properly since I can't seem to hit 80% hr?


Hi! Impossible to respond accurately to this program as I need to know much more about your health status, time to rest, your exercise goal etc. and that will be beyond the time-frame we have of todays QA. But see previous responses - sure several of them will be relevant for you as well. Keep up the good work!

do you like Snoop Dogg? Why, or why not?


Sure I like Snoop Dogg! He even has a song called Exercise! I would be happy to record his PAI during his singing, and perhaps he needs some 4x4 intervals as well? At least some of his music is perfect as motivation for HIIT!

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