Despite a reduction in cardiovascular mortality in recent decades in industrialized countries, the prevalence of cardiovascular diseases is still increasing because of lack of control of cardiovascular risk factors, including physical inactivity . Indeed, a large number of epidemiological and clinical studies have shown that regular physical activity (PA) reduces the main modifiable cardiometabolic risks, which leads to premature death due to cardiovascular disease . The direct health benefits of regular exercise are now irrefutable, and exercise appears as efficient as pharmacologic strategies to control chronic diseases . Therefore, it seems that “exercise is medicine” .

Thus, a dose of global exercise could be prescribed as much as a dose of any drug. This exercise dose could be defined by frequency, intensity, and duration, which is frequently summarized by energy expenditure. Studies have reported different amounts of PA to induce health benefits, and recommendations have evolved over the past 2 decades. Specific PA guidelines have emerged in an attempt to combat various chronic conditions such as hypertension, diabetes, dyslipidemia, and obesity that contribute to cardiometabolic risk. As a consequence, we have conflicting messages that are also potentially confusing to the public, arising as advice from clinicians or through the mainstream media or publicity. The recommended dose of PA reported by research studies depends on the chronic disease in question and on judgment criteria (e.g., mortality, physical fitness, symptoms, or quality of life), but they share a similar objective of prescribing adequate levels of PA to prevent cardiovascular disease.

During the 1970s and 1980s, prospective epidemiological studies estimated the amount of PA necessary to move from a sedentary lifestyle to a physically active one in the general population . These studies found that the difference between the two lifestyles was equivalent to an active energy expenditure ranging from 1000 to 1500 kcal per week. The public health recommendation in most developed countries is to walk for an equivalent of at least 30 min per day, most days of the week (i.e., 150 min per week) , which approximately corresponds to the extra 1000 kcal energy expenditure leading to health benefits. This has likely led to the simple and effective message “30 minutes of walking per day” heard in national communication campaigns.

In a recent paper in The Lancet , Wen et al. reported the results from a very large observational study ( n = 416,175) that 15 min/day of moderate-intensity PA, or 90 min/week of moderate-intensity PA, reduced all-cause mortality by 14%, cancer mortality by 10%, and mortality from cardiovascular disease by 20% . We agree that this advice is simple and probably easily achievable. The knowledge that as little as 15 min/day of exercise on most days of the week can substantially reduce a person’s risk of dying could encourage many more to include a small amount of PA in their busy lives . However, in the Wen et al. study, the benefits of PA followed a dose–response curve, which clearly indicated that “although a small amount of PA is good, more is better”.

Regarding the worldwide problem of obesity, the dose of exercise required to lose weight and to lower cardiovascular risk is not well determined, even if it can be summarized as “even a little is good, but more may be better!” for patients and clinicians .

Finally, all of these studies do not deal with the logic of the development of socio-ecological dimensions that recommend including doses of PA during each day as soon as possible, with no specific exercise plan. This lifestyle could be fostered by environmental and economic measures that favor active means of transport. This plan is supported by the study of Fan et al. (2013), who showed that even with slightly lower total energy expenditure, the repetition of short but high-intensity doses of energy expenditure in everyday life was more effective in helping men and women lose weight than was lower-intensity exercise, whatever the duration . This study supports the public health promotion message that “every minute counts.”

Moreover, Fitzgerald et al. (2015) found that daily sedentary time is positively associated with predicted 10-year coronary heart disease risk among mobility-limited older adults . Therefore, sedentary periods during the day (sitting position) should be limited, because the duration, not the intensity, of the activity appears inversely associated with the occurrence of coronary events in older adults. This recommendation can be summarized simply as “avoid the seat.”

In addition, a recent study underlined that running even at relatively low doses (5 to 10 min/day), below the current minimum guidelines of vigorous-intensity aerobic activity, was sufficient for substantial benefits that represented a 30% and 45% decrease in total and cardiovascular mortality, respectively (Lee et al.) . This finding agreed with The Copenhagen City Heart Study that monitored 1098 healthy joggers and 3950 non-joggers, also healthy, from 2001 to 2013 : the lowest mortality was associated with light jogging, at low intensity, 2 to 3 times/week. The mortality in intensive joggers and non-joggers was not statistically different. These studies support the message “train cool, live long”.

In specific populations with high cardiovascular risk, the question of the intensity of the exercise sessions, as well as their duration and frequency, is becoming a key issue . Although intensity can improve exercise capacity, it can also increase the risk of adverse events occurring during exercise. However, intensity ranges for the prescription of aerobic training are included in most guidelines and publications regarding secondary prevention and cardiac rehabilitation, even if they remain very wide, with no specification for a proper individualization .

Almost 20 years after the first reported use of interval training in cardiac patients, the growing interest in high-intensity interval training (HIIT) in high-risk patients led to a number of studies in this area . The benefits of HIIT in patients referred for cardiac rehabilitation programs have now been clearly demonstrated with a low risk of cardiovascular events . Following the promising studies of Rognmo et al. and Wisloff et al. in cardiac patients, Tjonna et al. (2008) reported the same benefits of HIIT in patients with metabolic syndrome and high cardiovascular risk . HIIT improved peak maximal oxygen consumption (VO ₂ ), reduced the number of cardiometabolic risk factors, reduced the prevalence of metabolic syndrome and improved endothelial function and glucose metabolism to a greater degree than did continuous, long and low-intensity exercise . More recently, in a systematic review and meta-analysis, Weston et al. (2013) showed that HIIT significantly increased cardiorespiratory fitness, by a factor of almost 2 as compared with moderate-intensity continuous training in patients with lifestyle-induced chronic cardiometabolic diseases . Likewise, Hollekim-Strand et al. (2014) recently showed that diastolic and systolic function, flow-mediated dilation, haemoglobin A1c level, high-sensivity C-reactive protein level and peak VO ₂ were significantly improved in patients with type 2 diabetes and diastolic dysfunction after 12 weeks of HIIT as compared with moderate-intensity exercise .

These improvements reported after sessions of HIIT have led certain authors, such as Moholdt et al. (2013), to convey another message in the management of coronary heart disease, namely “the higher the intensity, the better” . This message seems to be supported by the Ismail et al. (2013) systematic review and meta-analysis of patients with heart failure . This study showed a positive relationship between increased exercise intensity and cardiorespiratory fitness and an association with increased adherence and safety. Moreover, Moholdt et al. (2012) showed that HIIT led to quantifiable improvements in peak oxygen uptake in a large cohort of patients ( n = 89) with myocardial infarction. In these rather frail and weak patients, the message to clinicians could be that HIIT is the standard mode of exercise as recommended in recent guidelines .

Generally, the 4 meta-analyses published on this subject since 2013 corroborate this message . However, their conclusions were based on randomized controlled studies with small numbers of participants and varying HIIT protocols. Therefore, if public health messages arising from this scientific literature advocate that the intensity of effort is the determining factor in benefits gained, pleasure derived and risks incurred during physical activity, we must remain cautious because a number of other studies, with larger cohorts, have had contradictory findings . However, even these studies have their drawbacks in that they used HIIT training protocols that were arbitrarily based on heart rate, peak power or other less clinically functional criteria .

In May 2013, a study was published in PLOS One by researchers from the Norwegian University of Science and Technology who investigated the minimum amount of exercise needed to acquire acceptable health benefits in apparently healthy overweight men . One group followed the most widely used HIIT protocol consisting of 4 × 4 min at 90% of maximal heart rate (HR _max ) interspersed with 3 min active recovery at 70% HR _max (4-AIT), and the other group performed only a single-dose protocol of 4 min at 90% HR _max (1-AIT). The authors found that a single short dose of high-intensity physical activity performed 3 times/week may be a time-efficient strategy to improve peak VO ₂ and reduce blood pressure and fasting glucose in previously inactive but otherwise healthy, middle-aged, overweight patients. If the results are the same whether a person exercises 4 or 16 min, perhaps the next message could be “the shorter, the more intense, the better”. If the main limitation in PA adherence is lack of time, the 1-AIT could be implemented in new mass campaigns that aim to improve public health. In addition, Coquart et al. (2008) showed that HIIT was perceived to be less arduous than moderate continuous PA in obese women with and without diabetes . Thus, the message could be “the more intense and shorter, the more enjoyable”.

The concepts cited above corroborate the results of 3 studies of obese patients from the Montreal Heart Institute . Patients participated in high-intensity interval training (repeated 15- to 30-sec doses at high intensity over 20 min) 2 to 3 times/week combined with counselling for a Mediterranean diet. Waist circumference was significantly reduced, ventricular repolarization parameters were improved, systolic blood pressure was reduced and aerobic fitness was improved over the first 9 months of the study, and blood sugar levels were improved in patients with diabetes. The authors developed an optimized and time-efficient model of HIIT. The originality of the study lies in the fact that training represented 10 min of real exercise . In other words, the message could be “low volume, high intensity, and individualized training”.

In conclusion, even though the key messages of these studies appear to be conflicting, one way to summarize the prescription of global PA might be efficacy, safety and cost-effectiveness. Therefore, public health policies could encourage behaviours that are both healthy and appropriate to prevent cardiovascular disease. This move requires thought and consensus on specific messages to be given to the public, the training of teams who supervise PA and the creation of flowcharts to help physicians and healthcare professionals choose and implement optimal exercise protocols. This protocol should begin with radical politically motivated prevention by increasing the amount of time spent participating in sports at school. In addition, medical training should include appropriate education components, so that future clinicians can properly appreciate the “epidemiological swing” induced by the aging of the population and chronic disease burden. Indeed, medical students spend years learning how to prescribe drugs, but they are rarely instructed on the value of exercise prescription for various medical conditions or how to prescribe it. As well, later, when chronic disease is established, therapeutic patient education is essential to optimize the therapeutic alliance around PA, regardless of the key messages. Given that different exercise parameters (i.e., large amount or high intensity) have different levels of efficacy on health indicators, the message must be adapted to the patient’s specific needs. This message has to be even more tailored to patients with comorbidities. Indeed, regardless of the strategy used with each population and the message delivered, advice should be centred around the efficacy, security and pleasure that can be derived from PA. Exercise prescribers should be adequately trained to adapt the parameters of PA to each clinical population.