No doubt HIIT or High Intensity Interval Training is a very powerful method to inculcate in your training. But there are also certain misconceptions associated with its safety, primarily due to the level of intensity involved.
According to WHO reports, Coronary artery disease (CAD) is one of the most common causes of death worldwide, affecting 17.5 million people each year. Ischemic disease and chronic heart failure (HF) are lethal, causing 8.76 million deaths worldwide.
According to a 2013 study in the journal Current Heart Failure Reports, by researcher P. Meyer & team, aerobic exercise training is strongly recommended in patients with heart failure (HF) to improve symptoms and quality of life. Moderate-intensity aerobic continuous exercise (MICE) is the best-established training modality in HF patients. For about a decade, however, another training modality, high-intensity aerobic interval exercise (HIIE), has aroused considerable interest in cardiac rehabilitation. Originally used by athletes, HIIE consists of repeated bouts of high-intensity exercise interspersed with recovery periods. The rationale for its use is to increase exercise time spent in high-intensity zones, thereby increasing the training stimulus. Several studies have demonstrated that HIIE is more effective than MICE, notably for improving exercise capacity in patients with HF.
According to a 2013 study in the Journal of Clinical Exercise Physiology, Dr. S.J. Keteyian states that Interval training was systematically formalized and applied to elite athletes some 80 years ago by Drs. Woldemar Gershler and Herbert Reindel, a professor of physical education and a physician, respectively. Among athletes, the intermittent nature of HIIT or high intensity interval training allows for less fatigue because of the relief periods, which leads to the achievement of a higher intensity of effort during the work interval. As a result, the adenosine triphosphate phosphocreatine (ATP-PC) and glycolysis energy systems are used over and over, promoting an increase in the energy capacity of the skeletal muscles. Additionally, the multiple recoveries or relief bouts associated with HIIT allow stroke volume to reach its highest levels multiple times during a single bout of exercise rather than just one time with a continuous bout of exercise. These repeated bouts may provide a better stimulus for improving maximal stroke volume. In addition to HIIT being associated with less fatigue, the higher intensity training stimulus results in more total work being accomplished during a training session and greater improvements in exercise capacity (as measured by maximal or peak oxygen uptake (VO2).
Meyer & team found that, until the late 1980s, heart failure (HF) was widely regarded as a classical contraindication to exercise training in the belief that patients with a severely reduced left ventricular ejection fraction (LVEF) had an excessive risk for exercise-related morbidity and mortality. In the edition of Braunwald’s heart disease textbook published in 1988, one could read: “Reduced physical activity is critical in the care of patients with HF throughout their entire course”. This fear was supported by a study performed in the pre-angiotensin converting enzyme inhibitor and beta-blocker eras, which suggested adverse cardiac remodeling in patients with recent anterior myocardial infarction after a 12-week low-level exercise training program.
However, the chain of above myths was broken by the 1988 study in the journal Circulation, by M.J. Sullivan & team, and further by various studies, which demonstrated that regular exercise is safe and provides many benefits in the care of patients with chronic HF.
A major study in 2009 in the Journal of American Medical Association, by C.M. O’Connor & team, demonstrated modest but significant benefits related to aerobic training and did not reveal safety issues compared to the non-exercise group.
The stamp that exercise is a vital recommendation for heart patients came from the 2012 study in the European Heart Journal, by the European Society of Cardiology, where researcher J.J. McMurray & team, acknowledged, first time in 25years, aerobic exercise as a class I, level of evidence, a recommendation in patients with HF.
Meyer & team concluded that, looking at the current evidence,
- HIIE is more effective than MICE to improve VO2peak in patients with HF
- HIIE was shown to be more efficient, meaning that the same energy expenditure can be performed in a shorter time period, which may be an important practical aspect to consider during a rehabilitation program.
- HIIE acute and training studies performed in selected HF populations, including predominantly men with ischemic heart disease, did not reveal safety issues.
- No specific HIIE protocol has been proven superior, and therefore no single regimen can be recommended.
A 2014 study in the British Journal of Sports Medicine, by K.S. Weston & team, found that it was the first time in the 1952 study in the Journal of American Medical Association, when Levine & Lown, challenged the idea of complete immobilization by introducing the then-controversial ‘armchair treatment’ where patients were encouraged to sit in an armchair as much as possible during hospitalization post-myocardial infarction. Slowly the evidence poured in which recommended light-to-moderate physical activity for heart patients.
It was in 1972 study in the Journal of Chronic Diseases, by V.N. Smodlaka, when interval training was said to have been studied in cardiac patients when patients were asked to cycle at high workloads for 60 s with a 30 s rest between intervals. Using the intervals, the patients were able to exercise for at least twice as long as what they were able to do when cycling continuously.
According to Weston & team, in 1979 (Journal of Applied Physiology, D.H. Paterson & team), it was suggested that high-intensity exercise was required to provoke the necessary training adaptations needed to improve exercise capacity in patients with recent myocardial infarctions. One of the first studies (1981, Circulation, A.A. Ehsani & team) to investigate intense exercise in patients with cardiovascular disease found that if the exercise is intense and prolonged enough, then it can instigate a reduction in myocardial ischemia.
For their 2014 study, Weston & team carried out a systematic review & a meta-analysis to quantify the efficacy and safety of HIIT compared to MICT in individuals with chronic cardiometabolic lifestyle diseases. 10 studies with 273 patients were included in the meta-analysis. Participants had coronary artery disease, heart failure, hypertension, metabolic syndrome, and obesity. The researchers saw that there was a significantly higher increase in the VO2peak after HIIT compared to MICT, equivalent to 9.1%.
A 2012 study in the journal Circulation, by a Norwegian research team led by O. Rognmo, examined the risk of cardiovascular events during organized high-intensity interval exercise training and moderate-intensity training among 4846 patients with coronary heart disease in 3 Norwegian cardiac rehabilitation centers. The results of the study indicated that the risk of a cardiovascular event is low after both high-intensity exercise and moderate-intensity exercise in a cardiovascular rehabilitation setting. Considering the significant cardiovascular adaptations associated with high-intensity exercise, such exercise should be considered among patients with coronary heart disease.
A 2012 study in the Archives of Physical and Medical Rehab, by C. Freyssin & team, tested 26 patients with chronic heart failure who were enrolled in a cardiac rehabilitation program for 8 weeks. Patients were randomly assigned to 2 groups that performed either interval training (IT) or continuous training (CT). IT consisted of 3 sessions of 12 repetitions of 30 seconds of exercise at very high intensity, followed by 60 seconds of complete rest. The CT group performed CT exercises, which consisted of 45 minutes of aerobic exercise. The study demonstrates that a rehabilitation program of 8 weeks with very high-intensity IT in patients with chronic heart failure can result in considerable improvements in physical capacity. CT programs seem to have a lower impact on this capacity.
A 2015 study in the Journal of Cardiopulmonary Rehabilitation and Prevention, by a Spanish research team led by Dr. K.V. Jaureguizar, tested 72 patients with ischemic heart disease who were assigned to either HIIT or MCT for 8 weeks. The study saw that high intensity interval training resulted in a significantly greater increase in VO2 peak compared with MCT. The aerobic threshold increased by 21% in HIIT and 14% in MCT. Furthermore, there was a significant increase in the distance covered in the 6-minute walk distance test in the HIIT group when compared with the MCT group. Both training protocols improved quality of life. No adverse events were reported in either of the groups.
A 2015 study in the Journal of Applied Physiology, by a Canadian research team led by Siddhartha Angadi, found that heart failure with preserved ejection fraction is a major cause of morbidity and mortality. Exercise training is an established therapy in heart failure; however, the effects of high intensity interval training (HIIT) in HF are unknown. We compared the effects of HIIT vs. moderate-intensity aerobic continuous training on peak oxygen uptake (VO2peak), left ventricular diastolic dysfunction, and endothelial function in patients with HF. Nineteen patients with HF were randomized to either HIIT (4×4 min at 85-90% peak heart rate, with 3 min active recovery) or moderate-intensity aerobic continuous training (30 min at 70% peak heart rate. Patients trained 3 days/wk for 4 wk. The results show that significant improvements in VO2peak and diastolic function are possible after just 4wk of HIIT in HF. These improvements are consistent with previous findings on the effects of moderate-intensity exercise training programs of much longer duration in HF. It is also important to note that HIIT was extremely well-tolerated in this cohort consisting predominantly of older (age range 49–80 yr), overweight/obese individuals with significant contributory comorbidities (4 of 9 subjects had type 2 diabetes). No significant cardiovascular events or musculoskeletal injuries were reported during training.
A 2016 study in the journal Heart, Lung & Circulation, by an Australian research team led by K. Liou, performed a meta-analysis of published randomized controlled trials to compare high intensity interval training (HIIT) and moderate-intensity continuous training (MCT) in their ability to improve patients’ aerobic exercise capacity (VO2peak) and various cardiovascular risk factors. The researchers included patients with established coronary artery disease without or without impaired ejection fraction. Ten studies with 472 patients were included for analyses. Overall, HIIT was associated with a more pronounced incremental gain in participants’ mean VO2peak when compared with MCT. Moderate-intensity continuous training, however, was associated with a more marked decline in patients’ mean resting heart rate and body weight.
A 2016 study in the Annals of Physical and Rehab Medicine, by a Canadian research team led by P. Ribeiro, suggests that short interval HIIT(high intensity interval training) was found beneficial for CHD patients with lower aerobic fitness and would ideally be used in initiation and improvement stages. Medium and/or long interval HIIT protocols may be beneficial for CHD patients with higher aerobic fitness and would be ideally used in the improvement and maintenance stages because of their high physiological stimulus. They found that, for CHD patients, HIIT showed greater or equivalent benefits as compared with CAET (continuous aerobic exercise training) for most of the parameters reviewed. The use of HIIT does not seem to decrease exercise compliance or increase cardiovascular events (when properly prescribed) and is well tolerated and appreciated by the patients.
A 2016 study in the journal Physiotherapy Research International, by I.L. Aamot & team, found that exercise adherence, in general, is reported to be problematic after cardiac rehabilitation. Additionally, vigorous exercise is associated with impaired exercise adherence. Over seventy-five participants were put under a 12-week HIT cardiac rehab program. The study saw that both home-based and hospital-based HIT in cardiac rehabilitation induce promising long-term exercise adherence, with the maintenance of peak oxygen uptake significantly above baseline values at a one-year follow-up.
A 2017 study in the Annals of Physical and Rehabilitation Medicine, by a Canadian research team led by Paula Ribeiro, found that, for CHD patients, HIIT showed greater or equivalent benefits as compared with CAET (continuous aerobic exercise training) for most of the parameters reviewed. The use of HIIT does not seem to decrease exercise compliance or increase cardiovascular events (when properly prescribed) and is well tolerated and appreciated by the patients.
Ribeiro & team, states that “Different HIIT protocols (intensity, stage duration, nature of recovery, number of intervals) have been tested and used for CHD patients. Three different categories of HIIT have been described for CHD patients:
- long intervals: 3 to 15 min at 85% to 90% VO2peak
- Medium intervals: 1 to 3 min at 95% to 100% VO2peak
- Short intervals: 10sec to 1 min at 100% to 120% VO2peak
Furthermore, HIIT can be performed with different exercise modes such as cycling, running, walking with inclination, rowing, swimming or other activities. Exercise intensity is generally determined with % VO2peak, %HRmax, percentage maximal aerobic power, percentage maximal short exercise capacity.”
In a 2017 study in the European Journal of Preventive Cardiology, a Brazilian research team led by M.G. Neto performed a meta-analysis to investigate the effects of high intensity interval training versus moderate-intensity continuous training of coronary artery disease patients. Twelve studies met the study criteria, including 609 patients. High intensity interval training resulted in an improvement in peak oxygen uptake compared with moderate-intensity continuous training. No significant difference in the physical, emotional, and social domain of quality of life was found for participants in the high-intensity interval training group compared with the moderate-intensity continuous training group.
A 2017 study in the journal Circulation, a team of almost thirty researchers led by O. Ellingsen, compared 12 weeks of supervised interventions of HIIT, MCT, or a recommendation of regular exercise (RRE). Two hundred sixty-one patients were randomly assigned to HIIT at 90% to 95% of maximal heart rate, MCT at 60% to 70% of maximal heart rate, or RRE. The findings of these extensive studies were kind of controversial to the previous studies. Though there were no safety issues with HIIT, HIIT was not seen to be superior to MCT in changing left ventricular remodeling or aerobic capacity, and its feasibility remains unresolved in patients with heart failure.
Another 2017 study in the journal Biomedical Research International, by a Chinese research team led by Xie Bin, carried out a meta-analysis to compare the effects of high intensity interval training (INTERVAL) and moderate-intensity continuous training (CONTINUOUS) on aerobic capacity in cardiac patients. Twenty-one studies involving 736 participants with cardiac diseases were included. This study showed that INTERVAL improves aerobic capacity more effectively than does CONTINUOUS in cardiac patients.
A 2018 study in the Spanish journal Revista Española de Cardiología, by a team led by I.B. Garcia, identified the most effective doses of HIT to optimize peak VO2 in coronary artery disease (CAD) and heart failure (HF) patients. The researchers analyzed multiple studies and reported significant improvements in peak VO2 after HIT in both diseases with a higher increase in HF patients. Nevertheless, in HF patients, there were no improvements when the intensity recovery was <40% of peak VO2 and the frequency of training was <2 d/wk. There were significant differences regarding duration in CAD patients, with greater improvements in peak VO2 when the duration was < 12 weeks. In HF, programs lasting < 12 weeks did not significantly improve peak. Hence, the recovery intervals should be active and be between 40% and 60% of peak VO2 in HF patients. Training frequency should be >2 d/wk for CAD patients and >3 d/wk for HF patients.
Another 2018 meta-analysis by an Australian research team led by A.L. Hannan, in the Journal of Sports Medicine, within the cardiac population that investigated cardiorespiratory fitness changes resulting from HIIT versus MICT and to collate adverse events. Seventeen studies, involving 953 participants were included in the analysis. HIIT was significantly superior to MICT in improving cardiorespiratory fitness overall. There were no deaths or cardiac events requiring hospitalization reported in any study during training. Overall, there were more adverse events reported as a result of the MICT intervention than the HIIT intervention. Improvements in cardiorespiratory fitness are significant for CR (cardiac rehab) programs of >6-week duration. Programs of 7–12 weeks’ duration resulted in the largest improvements in cardiorespiratory fitness for patients with coronary artery disease.