High-intensity interval training for health benefits and care of cardiac diseases – The key to an efficient exercise protocol
Aerobic capacity, which is expressed as peak oxygen consumption (VO2peak), is well-known to be an independent predictor of all-cause mortality and cardiovascular prognosis. This is true even for people with various coronary risk factors and cardiovascular diseases. Although exercise training is the best method to improve VO2peak, the guidelines of most academic societies recommend 150 or 75 min of moderate- or vigorous- intensity physical activities, respectively, every week to gain health benefits. For general health and primary and secondary cardiovascular prevention, high-intensity interval training (HIIT) has been recognized as an efficient exercise protocol with short exercise sessions. Given the availability of the numerous HIIT protocols, which can be classified into aerobic HIIT and anaerobic HIIT [usually called sprint interval training (SIT)], professionals in health-related fields, including primary physicians and cardiologists, may find it confusing when trying to select an appropriate protocol for their patients. This review describes the classifications of aerobic HIIT and SIT, and their differences in terms of effects, target subjects, adaptability, working mechanisms, and safety. Understanding the HIIT protocols and adopting the correct type for each subject would lead to better improvements in VO2peak with higher adherence and less risk.
Core tip: There are numerous of high-intensity interval training (HIIT) protocols, which can be classified into aerobic HIIT and anaerobic HIIT [usually called sprint interval training (SIT)]. Professionals in health-related fields, including primary physicians and cardiologists, may find it confusing when selecting an appropriate protocol for their patients. This review describes the classifications of aerobic HIIT and SIT, and their differences in terms of effects, target subjects, adaptability, working mechanisms, and safety. Understanding the HIIT protocols and adopting the correct type for each patient would lead to better improvements in VO2peak with higher adherence and less risk.
Accumulated evidence suggests that aerobic capacity (VO2peak) is the strongest predictor of future health, all-cause mortality[1–3], and cardiovascular risks[4,5]. Moreover, several studies have suggested that people with established coronary vascular disease (CVD) risk factors (such as high body mass index, hypertension, or diabetes) and high cardiorespiratory fitness have a highly attenuated risk of CVD and premature mortality[4,5]. Thus, it has become a major goal in the medical field to improve VO2peak in patients with lifestyle-related diseases with (as a secondary prevention strategy) or without (as a primary prevention strategy) cardiac disorders. For improvement in public health, performing regular physical exercise is indispensable together with a nutritional approach. Healthy young and middle-aged people can select from the many choices of exercise training methods, including recreational sports, in daily life. In contrast, people with lifestyle-related disease and/or elderly people are often sedentary and physically unfit. Thus, some useful techniques and limitations exist when encouraging exercise training with adequate safety and high adherence in these people. High-intensity interval training (HIIT) has been recognized as an alternative and more efficient protocol than moderate-intensity continuous training (MCT), which is the gold standard recommended in several guidelines[6–8]. HIIT and sprint interval training (SIT) for 6-8 wk increase VO2peak more than or at least comparable to MCT. In this comprehensive review, many protocols of HIIT and SIT for improving aerobic and metabolic capacity were evaluated for their effects in patients with sedentary lifestyle-related diseases with or without cardiac disease to determine appropriate protocol recommendations for different patient populations. General practitioners and cardiologists should pay more attention to exercise and physical activity rather than to the prescription of drugs.
EXERCISE IS MEDICINEⓇ (EIM) ENCOURAGES PEOPLE TO FORM EXERCISE HABITS
To improve primary and secondary prevention methods in cardiovascular medicine, physical activity should be promoted as a first-line strategy despite new drug developments in the medical treatment field.
Although the value of exercise for improving health is well recognized world-wide, widespread adoption of exercise habits has not been adequately achieved, especially in highly developed countries where the use of automobiles is highly prevalent. In a recent study from the World Health Organization, about 27.5% of the population in 2016 was recognized as sedentary (i.e., with insufficient physical activity). In this context, EIM is a global health initiative promoted worldwide by the American College of Sports Medicine. EIM encourages primary care physicians and other health-care providers to include physical activity when designing treatment plans, and to offer evidenced-based exercise programs to their patients or refer their patients to qualified exercise professionals. EIM is committed to the belief that physical activity promotes optimal health, is integral in the prevention and treatment of many medical conditions, and should be regularly assessed and included as part of health care. Irrespective of disease severity, exercise can bring improvements in aerobic and metabolic capacity as well as cardiac function if performed with an optimal dose, frequency, and intensity. Despite the continuous recommendations by the American College of Sports Medicine and related pro-fessional societies worldwide, the effects of such recommendations on public awareness have been very limited. Many kinds of wearable heart rate monitors and accelerometers are commercially available. Although these state-of-the art products could motivate sedentary people and increase their frequency of exercise training or participation in sports events, more efficient and effective exercise training strategies are still required.
For the success of EIM, professionals who can encourage target people to exercise in a planned way according to detailed exercise protocols, functioning as an intermediary between physicians and patients, would be very important.
GUIDELINE RECOMMENDATION: MCT AS A CLASSIC AND SIMPLE PROTOCOL
The current guidelines on physical activity for health recommend that adults should engage in at least 150 min of moderate-intensity activity or 75 min of vigorous-intensity activity per week, or any combination of activities that amount to the same total energy expenditure[6,12]. Similarly, in the field of cardiac rehabilitation, MCT has been the gold standard for many years for patients with cardiac diseases. The current guidelines on cardiac rehabilitation/exercise training recommend endurance exercises with a moderate intensity at 50%-85% (mostly 70%-85%) of the peak heart rate or anaerobic threshold level for patients with CVD or chronic heart failure (CHF)[7,8,14]. The latest guidelines suggest HIIT as an alternative protocol to improve aerobic capacity and cardiac function. However, the adoption of HIIT in the cardiac rehabilitation setting is still controversial among researchers. In Japan, only a few studies describing the effects of HIIT have been published[15–17]. On the other hand, MCT has been used as a control strategy in randomized controlled trials (RCTs) that evaluated HIIT or SIT. Thus, evidence for the same amount of MCT has been accumulated. In representative MCTs such as walking or jogging, each workout is time consuming and usually monotonous and boring. Therefore, although MCT has become a classic protocol based on evidence from RCTs, it remains difficult for most people, with lack of time being cited as a common hindrance.
HIGH INTENSITY IS THE KEY ELEMENT OF EFFICIENT EXERCISE PROTOCOLS: HIIT AND SIT
The inclusion of “adapted” high intensity (relative to a subject’s current physical ability) in the exercise protocol is a key component for exercise to be more efficient as a “medicine.” The clinical and physiological benefits of HIIT compared with those of MCT are shown in Table Table1.1. In multiple RCTs, a wide range of targets, including skeletal muscles[19–22], risk factors, vasculature[19–22], respiration[22,23], autonomic function, cardiac function[20,22,25–27], exercise capacity, inflammation, quality of life, physiological markers such as VO2peak, and endothelial function, showed better improvements with HIIT than with MCT.
|Skeletal muscle biopsy|
|Mitochondrial function in lateral vastus||O2 consumption|
|Fatty acid transporter in the vastus lateralis and FAS (a key lipogenic enzyme)|
|IR β subunit in skeletal muscle (peripheral insulin sensitivity)||Metabolic|
|Re-uptake of Ca2+ into the salcoplasmic reticulum|
|Improvement of ventilatory efficiency (increased value of PETCO2)||Respiratory function|
|Oxygen consumption at the first ventilator threshold||Cardiac function|
|Oxygen pulse||Cardiac function|
|Parasympathetic activity (HR recovery)||Autonomic function|
|Duration of exercise time||Autonomic function|
|Distance walked during the 6-min walk||Work capacity|
|Reversed LV re-modelling (LV end diastolic and systolic volumes)||Cardiac function|
|Diastolic function (e′, E, E/ e′, E/A ratio, higher proportion of e′ > 8 cm/s, E improvement during exercise),|
|Systolic function after 12 wk at rest and during exercise)|
|Deceleration time increase|
|Left atrial volume|
|Endothelial dysfunction (FMD)||Vascular function|
|Coronary plaque necrotic core reduction in defined coronary segments||Vascular function|
|High sensitivity CRP||Inflammation|
|insulin sensitivity (HOMA index)||Metabolic|
|Increased Short Form-36 physical/mental component scores and decreased Minnesota Living with Heart Failure questionnaire score||Quality of life|
|Frequency of metabolic syndrome||Risk factor|
HOMA: Homoestasis model assessment; IR: Insulin receptor; PGC: Peroxisome-proliferator activated receptor γcoactivator; FMD: Flow mediated dilation; FAS: Fatty acid synthase; PETCO2: End-tidal carbon dioxide; HR: Heart rate; LV: Left ventricular; BNP: Brain natriuretic peptide.
High-intensity exercise consists of aerobic HIIT and anaerobic SIT.
Figure Figure11 illustrates the representative protocols of aerobic HIIT and 2 anaerobic SITs, as well as a comparison of their intensities, duration, and frequencies. These exercise protocols require a shorter exercise duration to obtain the same benefit as that provided by moderate-intensity exercises. Although maintaining a high intensity exercise workout for a longer duration could be preferred, high-intensity exercise can be realistically tolerated by people with sedentary lifestyle, obesity, old age, or cardiac disease only in the form of interval training. In this regard, HIIT consists of brief, intermittent bursts of vigorous activity (less than VO2peak but usually involves < 100% [70%-90%] of VO2peak or 85%-95% of the peak heart rate) interspersed with active rest periods[22,28,29], whereas SIT is classically a Wingate-type protocol (all-out, vigorous-intensity exercise involving approximately 350% of VO2peak) interspersed with longer complete rest periods. These high-intensity protocols are demanding for the subjects even though the intensity is adapted to the individual’s aerobic capacity and the rest period. Although the most popular and evidence-rich protocols are the Wingate test for SIT, and the 4 × 4 min[28,32] or 10 × 1 min protocol for HIIT, many other protocols can be applied by modifying the workout duration, rest interval (work/rest ratio), workout intensity, and workout frequency. The difference between HIIT and SIT is that SIT refers to anaerobic supramaximal VO2max (all-out) intensity and HIIT refers to aerobic submaximal VO2max intensity. The peak power output (PPO) of SIT is about 350% of the power output at VO2max. Meanwhile, the common elements between the two protocols are the high work intensity adapted to the current aerobic capacity of the individual, and the aim of improving both aerobic capacity (VO2peak) and metabolic capacity. However, the risk of these protocols has also been a concern, and more studies are warranted before these protocols are adopted to more common use. A supervised workout is mandatory to maintain high-intensity adherence until the participants become accustomed to the intensity and to heart rate measurements during physical activity by using a wearable heart rate monitoring device. Home-based HIIT is also possible if experienced management programs are provided by renowned centers.