J Phys Fitness Sports Med, 10 (3): 151-164 (2021) DOI: 10.7600/jpfsm.10.151
JPFSM: Review Article
Rationale and optimising of outcomes in high-intensity
interval training for health and disease
Shigenori Ito
Received: August 25, 2020 / Accepted: October 7, 2020
Abstract
High-intensity interval training (HIIT) research has drastically increased globally in the last decade. This might be owing to the application of HIIT in various fields such as among cardiovascular disease and cancer populations, and its increasing popularity in the me-dia and fitness industries. It is crucial to realize the substantial benefits of HIIT, keeping safety in mind, for these target groups. In this narrative review, HIIT is discussed from multifaceted perspectives. First, I describe the rationale behind the improvement in aerobic and metabolic capacity with HIIT requiring less time compared to moderate-intensity continuous training, as well as the enjoyable and affective factors and the broad applicability of HIIT due to the “rela-tive” high-intensity training. Second, I describe ways to maximize the effects of HIIT, which include optimising a potential genetic factor in HIIT responder, decreasing non-responders by attaining a targeted intensity, and adhering to the exercise intensity and unsupervised long-term participation. Recent development of HIIT/sprint interval training protocols and several unique clinical studies in the world have helped overcome the barriers against high adherence. Finally, safety and potential risks were only discussed briefly due to insufficient available data. In con-clusion, to utilise the benefits of HIIT effectively and safely for unfit subjects with lifestyle-related and chronic diseases, optimising HIIT protocols to include high adherence to exercise intensity and long-term participation should be considered.Keywords : HIIT, sprint interval training, cardiac, health, chronic disease, safety
Introduction
The number of published papers on research of “high-intensity interval training” (HIIT) has dramatically in-creased globally in the last decade according to PubMed search results (Fig. 1). Numbers have increased in Japan and globally, following a similar trend, with the increas-ing popularity of HIIT among physically active individu-als1). This positive trend might partly reflect the increase
in research conducted in fields that have adapted HIIT. Since 1990, HIIT has been adapted for individuals with obesity, sedentary, and cardio-metabolic risk factors1), as
well as those with cardiovascular diseases such as isch-aemic heart disease (angina pectoris and old myocardial infarction [MI]) and chronic heart failure (CHF)2-5). In
the last decade, HIIT has also been applied to patients with other chronic diseases such as cancer6,7), cerebral or
cognitive disorders8), and mental disorders (particularly
depression)9,10). This universal application11) might lead to
the increase in the volume of published literature on HIIT. The American College of Sports Medicine (ACSM)’s Health & Fitness Journal® presents the annual results of its
worldwide fitness trends survey, in its 14th year in 202012).
Over 3,000 health and fitness professionals ranked 38 pos-sible trends on a scale of 1 (least likely to be a trend) to 10 (most likely to be a trend). Number 1 (since 2016) was wearable technology and number 2 (in the top 5 between 2014 and 2020) was HIIT. Despite warnings by some fit-ness professionals of increased injury rates during HIIT, this form of exercise has been popular in gyms all over the world. As in the past, the results of this annual survey will help the health and fitness industry to make some impor-tant decisions regarding future growth and development. The media also present HIIT as an alternative means by which individuals achieve health benefits similar to those of moderate-intensity continuous training (MICT)1). The
popularity of HIIT in the community and industry might be related to its development in academic societies. HIIT was introduced in the first half of the 20th century
and often used during military training. In addition, it was credited for impressive performances at the Olympic Games by athletes. Paavo Nurmi, a Finnish middle- and long-distance runner, used fartlek training as a prototype of HIIT and won nine gold and three silver medals for his 12 events in the Olympic Games. One of the most renowned Olympic medallists is Emil Zatopek, a Czecho-slovak long-distance runner who adopted interval training and won 3 gold medals (5,000 m, 10,000 m, and mara-Correspondence: shigeito918@gmail.com
Division of Cardiology, Sankuro Hospital, 7-80 Kosaka-cho, Toyota-shi, Aichi 471-0035, Japan
thon) in the Helsinki Olympic Games of 1952. Since then, HIIT has been used in many events of athletic sports, fol-lowed by applications of HIIT in health development and disease treatment. It is notable that HIIT can be applied to universal individuals in two extreme polar fields: in top athletic performance and in advanced cardiac disease man-agement. In this narrative review, I give an overview of the current global trends and highlight issues regarding the rationale and optimisation of HIIT for health and disease. Latest adaptation of HIIT exercise to wider medical/ social fields
People with lifestyle-related and chronic diseases and/ or older people are often physically unfit and may have cardiovascular risks13). Thus, some effective techniques
and attention to an individual’s condition are required when encouraging exercise training with adequate safety and high adherence, considering the existing limitations. Physical exercise offers preventive treatment effects, as well as numerous health-related benefits, for individuals with cancer14). Epidemiological research has primarily
been concerned with conventional exercise training that aligns with the recommendation of 150 min of moderate to vigorous physical activity per week15). This
recom-mendation is safe and effective at improving physical and psychosocial outcomes. Recently, researchers have begun to explore HIIT as a novel training regimen that may pro-vide additional health benefits and/or improved adherence even for cancer. Specifically, exercise at higher intensities may offer more or different benefits than conventional training approaches, with potentially profound effects on the tumour microenvironment15). The increasing body
of research can be divided into 3 groups: preoperative care16), during chemotherapy17), and primary and
second-ary prevention14). The main target cancers that have been
reported so far include lung (non-small cell)16), breast17),
prostate, and colorectal14) cancers. In preoperative care,
HIIT was used to improve cancer-related fatigue during a short period (as short as 3 weeks) of permitted time frame for preparation of operation. During chemotherapy, a randomised control trial (RCT) showed the beneficial ef-fects of HIIT in cancer-related fatigue and quality of life compared to control (usual care), but similar to the MICT group6). Therefore, future research should consider a
stan-dardised approach, implementation in cancer types across different delivery timings, and safety and feasibility in unsupervised settings14).
Terminology of HIIT: its wide utilisation
A wide range of terms have been used in fitness and health research fields to describe various interval train-ing protocols, which have led to many acronyms without general standardisation in the scientific literature to date. Interval training can be broadly categorised into two main
types: aerobic-based and resistance-based. Aerobic-based interval training generally involves exercises that engage a large mass of muscles (e.g. cycling, running, swim-ming), performed with variable intensity, which improve cardio-respiratory fitness (CRF) and cardiometabolic parameters18). This type of interval training includes the
following: (1) HIIT involving vigorous but submaximal effort, in which the relative intensity during the intervals usually elicits ≥ 80–90% of maximal heart rate (HRmax)
or > 70–80% of maximal oxygen uptake (V・
O2max)2,4,19,20);
(2) sprint interval training (SIT) involving “all out” or “supra-maximal” (at least > 100% V・
O2max) effort, in
which the absolute workload or speed exceeds that which would elicit V・
O2max21-25); and (3) less demanding types of
intermittent exercise, typically characterised by alternat-ing periods of light and moderate exercise (e.g. interval walking26,27)). Common examples of HIIT and SIT are
gym-based ergometers or treadmills and Wingate test22)
or Tabata-style intervals, respectively. During the acute and chronic phases of cardiac rehabilitation (CR), some patients with very low capacity need to split the low- to moderate-intensity continuous training to perform physical training according to their physical capacity. This is also called HIIT in patients with advanced heart failure28-30).
Rationale of HIIT as a training protocol
Recently, due to the effectiveness and “superiority” of HIIT, it has been purported as a time-saving alternative to “traditional” MICT in CR and other target fields de-scribed previously. However, the rationale for HIIT adop-tion is not fully substantiated in the scientific literature31).
In this section, potential protocols with explaination of the rationale for adopting HIIT are explored.
Peak oxygen uptake (VO2peak) and long-term prognosis. Accumulated evidence suggests that V・
O2peak is the
stron-gest predictor of future health, all-cause/cardiovascular mortality32-34), and cardiovascular risks35,36). Moreover,
previous studies35,36) have suggested that even people with
established cardiovascular disease (CVD) risk factors or chronic diseases can attenuate the risk of CVD and premature mortality if they obtain high V・
O2peak. Thus, it
has become a major goal in fitness and medical fields to improve V・
O2peak in patients with lifestyle-related diseases
with or without cardiac disorders, as a primary or second-ary prevention strategy18). The main benefits of HIIT that
have been reported are higher or at least similar levels of V・
O2peak improvement within a shorter period compared to
MICT, as shown by many studies that targeted a variety of participants including sedentary and older individuals37)
and those with obesity, metabolic syndrome38,39),
diabe-tes40), and cardiac diseases4,41,42). Therefore, HIIT could be
a first-line choice of exercise that is used as therapy and not merely as a training regimen.
Furthermore, HIIT improves cardiac performance, met-・
4 min HIIT and 45 min MICT. The participants reported a higher rating of perceived exertion (RPE) in the HIIT session compared to the continuous endurance session, yet also reported equal enjoyment51). A systematic review
showed that although HIIT exercise may be recommended for obtaining positive psychological responses in the acute phase, chronic studies should clarify the applicability of HIIT for exercise adherence52). SIT, an all-out and more
strenuous exercise than HIIT, might be inappropriate for a largely sedentary population due to the generation of aver-sive feelings53).
Optimising outcomes in HIIT
There are large inter-individual differences among exer-cise training responses54). One method for optimising the
exercise benefit is to decrease the ratio of non-responders in the corresponding exercise programs. Several factors that affect the response to exercise could be considered.
Responder/non-responder to the exercise: genetic matter.
Genetic factors have been proposed from various points of basic and clinical research55-57) regarding the response
to exercise. In the HERITAGE (HEalth, RIsk factors, ex-ercise Training And GEnetics) Family Study revealed that the trainability of V・
O2max is highly familial and includes a
significant genetic component58). By contrast, Bonafiglia
et al. revealed in a randomised crossover study design that the individual response to exercise training is highly variable following different training protocols and that the incidence of non-response to exercise training may be reduced by changing the training stimulus for non-responders to three weeks of endurance training59). This
study suggested that the existence of true non-responders to exercise training is unlikely and that different training protocols should be considered when optimising individ-ual exercise prescriptions. Treating genetic disadvantages is not realistic; therefore, the most efficient exercise mode should be explored for each participant.
Importance of high intensity in the HIIT protocol to decrease non-responders. One in five adults,
follow-ing physical activity (PA) guidelines, are reported to not demonstrate any improvement in CRF60). Seventy-eight
healthy adults were divided into five groups comprising one, two, three, four, and five 60-min exercise sessions per week, respectively, but otherwise following an identi-cal 6-week endurance training programme. The preva-lence of CRF non-response gradually declined exercising by 60, 120, 180, 240, or 300 min (1–5 times of 60-min workouts a week) for 6 weeks. In participants exercising for 240 or 300 min, there were no non-responders. How-ever, with 60, 120, or 180 min, there were non-respond-ers. Following a successive identical 6-week training period but comprising 120 min of additional exercise per week in these groups, CRF non-response was universally abolic and vascular function such as insulin resistance,
and endothelial function in clinical conditions5). SIT has
also been reported to have effects similar to those of MICT43).
Promising way to maintain high VO2peak in less time, but with comparable health benefits. Considering the
prog-nostic value of changes in V・
O2peak, it is important to know
how to effectively improve V・
O2peak44). Traditionally,
en-durance exercise training (ET) that improves health-relat-ed outcomes has consisthealth-relat-ed of low- to moderate-intensity training. MICT remains the gold standard recommenda-tion in several guidelines45-47). However, a growing body
of evidence suggests that HIIT has been recognised as an alternative, which is more time-efficient and a more en-joyable protocol than MICT48). The ACSM also assessed
HIIT for cardio-metabolic disease prevention1).
High intensity in HIIT is “relatively” high intensity. HIIT should not be easily excluded in individuals who
are sedentary or older or have a chronic disease or com-plications with cardiovascular diseases due to reasons that they are unfit and have risks. Although HIIT intrinsically includes “high”-intensity intervals, this “high” level is a relative one calculated based on the individual baseline fitness level (estimated by V・
O2peak and HRpeak) in each
participant (Fig. 2). That is, HIIT should not be operated as a one size fits all protocol but as a finely tailored one. As an extreme case, critically ill cardiac patients with a left ventricular assistance device could undergo HIIT in CR with inotropic agents29). Thus, two subjects exercising
next to each other may look very different, one running very fast and the other walking, even when they both are exercising at the same relative intensity44) (Fig. 2).
The differences stem from different CRF levels, but they would be experiencing the same relative exercise stress. From a practical point of view, it has been pointed out that in severely deconditioned patients, as in those with heart failure, the normal PA of daily living actually corre-sponds to high-intensity ET. Hence, using the same inten-sity during structured ET may, in a short period of time, increase these patients’ CRF level, making daily tasks less strenuous44). Thus, HIIT can be adapted to a wide range of
clinical conditions after appropriate clearance.
Enjoyable protocol. An increase in enjoyment during
exercise has also been suggested as a rationale of adopt-ing HIIT as a trainadopt-ing protocol48). By contrast, MICT has
been rendered as a monotonous and boring protocol48).
However, there are controversies regarding this matter. The level of “enjoyment” of all of the 3 protocols (MICT, HIIT and SIT) declined over the duration of the study49).
The insufficiently active group displayed lower affective responses over time (sessions 4 to 10) than the active group to the low-dose HIIT protocol20,50). Sagelv et al.
ex-amined exercise enjoyment following one session of 4 × ・
abolished. The potential for CRF improvement may be present and unveiled with appropriate exercise training stimuli in healthy individuals without exception60). This
study showed that total exercise amount per week might be related to increased response in endurance training. On the contrary, a high intensity was important in the following studies. In a study by Ross et al, abdominally obese participants were randomly assigned to (1) low-amount, low-intensity exercise (LALI), (2) high-low-amount, low-intensity exercise (HALI), or (3) amount, high-intensity exercise (HAHI)61). At 24 weeks, 38.5%, 17.6%,
and 0% of the participants within the LALI, HALI, and HAHI groups, respectively, were CRF non-responders. At a fixed exercise intensity, increasing exercise amount re-duced the rate of non-response by 50%. At a fixed amount of exercise, increasing the exercise intensity eliminated non-response. Low-intensity exercise may not be suffi-cient to improve CRF for a substantial proportion of sed-entary obese adults. In this context, HIIT could be more beneficial than MICT to optimise the outcome. However, even in HIIT, the actual intensity during intervals should be higher to obtain more benefits. Moholdt et al. assessed the impact of exercise intensity during HIIT on the in-crease in V・
O2peak among patients with coronary artery
disease (CAD) for 12 weeks62). The increase in V・
O2peak
for the three intensity categories were 3.1, 3.6, and 5.2 mL/kg/min for the <88%, the 88–92%, and the >92% of HRmax categories, respectively, Thus, even within the
high-intensity training zone, exercise intensity was an im-portant determinant for improving V・
O2peak.
Learning from multicentre RCTs comparing HIIT and MICT with negative results in CR. Less workout time
with identical total energy expenditure for more gain has been the main rationale of HIIT protocol compared to MICT even in patients with cardiac disease and cardio-metabolic risk factors. This has been proven to be true in multiple single-centre RCTs2,3,41,63-67) and
meta-analy-ses68,69). However, two multicentre RCT studies5,70,71) that
compared an increase in V・
O2peak between HIIT and MICT
in patients with CAD (multicentre study on aerobic in-terval exercise training in CAD: SANITEX-CAD)70) and
CHF (study of myocardial recovery after exercise training in heart failure: SMARTEX-HF study)71) showed negative
results in the superiority of HIIT in V・
O2peak improvement.
These studies have highlighted the challenges of the prac-tical feasibility of HIIT in CR44). In particular, adherence
to ET intensity and continuously increasing ET workload throughout the ET period may seem challenging in mul-ticentre studies44). Analyses of these results clarified the
issues of adherence to exercise intensity in both HIIT and MICT groups. In these studies70,71), it is unknown if there
were peripheral limitations to ET, such as a high degree of muscle wasting, and limitations in patients’ ability to exercise at a high intensity; however, a large overlap in ET intensity between HIIT and MICT groups was seen in
both studies and could be a key factor explaining no dif-ference between the groups44). That is, the intensity was
lower than expected in the HIIT group, and by contrast, higher in the MICT group. There might have been a vari-ance in practical techniques (tips) to yield optimal gain by pursuing intensity among the participating institutions. The low feasibility to ET intensity prescription in HIIT could also be due to the patients’ ability and/or motivation to exercise at a high intensity or the challenges of imple-menting the ET program in a multicentre setting.
Another indispensable factor for exercise response to occur over time is the principle of increasing exercise workload as training adaptation progresses44,61). The
in-creased workload seems to be greater in singe-centre studies than in RCTs. The increased workload per session was 0.83 and 2.6 W in MICT and HIIT, respectively, in a single-centre study4). Similarly, in another single-centre
study, it was 1.2 and 1.7 W in MICT and HIIT, respec-tively72). Meanwhile, it was 0.28 and 0.42 W in MICT and
HIIT, respectively, in the SMARTEX-HF study71) and 0.97
and 1.05 W in MICT and HIIT, respectively, in the SAIN-TEX-CAD study70). These might have affected the extent
of increase in V・
O2peak and the difference in V・O2peak increase
between HIIT and MICT protocols. The practical implica-tions of the findings may be to keep a combined focus on both target ET intensity62) and continuous progress in ET
workload to secure sufficient relative exercise intensity as V・
O2peak improves throughout the exercise period44).
In patients with CAD, coaching in combination with use of heart rate (HR) monitors is necessary to achieve exercise within a target HIIT intensity zone. Without ac-tive coaching, patients tend to exercise below the lower limit of target HR (85% of HRpeak), despite the use of HR
monitors and a knowledge of target intensity. This may indicate that the implementation of HIIT might be more difficult than previously predicted. In addition, several of the previous single-centre studies have been performed using treadmill exercise2,3,41,63,67), while a majority of the
patients in multicentre studies70,71,73) used cycling as the
mode of ET. The effect of cycling versus walking or run-ning during HIIT thus needs to be further addressed, with potential differences in societies with a population more familiar with cycling, as opposed to those less trained with cycling exercises44).
Although there was no significant difference in im-provement in V・
O2max and left ventricular remodelling,
Karlsen et al. evaluated the predictors of good response in the SMARTEX-HF study74). Their analyses suggested that
age, left ventricular ejection fraction, New York Heart As-sociation classification, and the ability to improve V・
O2peak
might be considered when giving exercise training advice and evaluating exercise response in heart failure with reduced ejection fraction (HFrEF) as a data point to a gradient toward a poor exercise response in the oldest and most symptomatic HFrEF patients74).
Practical issues in improvement of adherence to HIIT Karlsen et al. proposed the key practical considerations for successful clinical and home-based HIIT. Based on their clinical experience, they emphasised the importance of appropriate warm-up, adjustment of exercise workload, and use of both the Borg scale and correct measurement of HRpeak44). Furthermore, guidelines for the delivery and
monitoring of HIIT in a clinical population would be ben-eficial for health personnel who are inexperienced in HIIT prescription75).
Protocol development to overcome barriers to exercise.
A 4 × 4-min HIIT is a globally well-known protocol that has been utilised for heath and disease treatment and is adaptable to older people and to people with cardiovascu-lar diseases2,4,19,76). This protocol was adopted in two
ran-domised multicentre studies in Europe71,73,77) to evaluate
the efficacy of HIIT compared to MICT for CHF in CR based on accumulated evidence2,4,5,19,78). However, a 4 ×
4-min HIIT takes at least 35 min including warm-up and cool-down and might need a pre-training period before adopting a 4 × 4-min protocol in sedentary participants. The most common external barrier among the middle-aged and elderly respondents was “not enough time” (46.7% and 48.4%, respectively); meanwhile, the most common internal barrier among middle-aged and elderly respondents was “too tired” (48.3% and 51.7%, respec-tively in a cross-sectional study79). Thus, the reasons for
nonparticipation in exercise could be multi-factorial79).
More sophisticated exercise protocols would be ex-pected to spread HIIT and SIT in wide target populations. In this regard, somewhat complicated, but participant-friendly, HIIT training regimens have been developed for targets with specific clinical/social subsets. To overcome the feeling of difficulty experienced during the 4 × 4-min protocol by sedentary or older individuals, a finely-tuned HIIT protocol, in which the frequency, workload, and work duration are initially set at low levels and altered during the training course, has been reported80-83). The
Japanese version of the High-intensity Interval Aerobic Training (J-HIAT) program is composed of 3 sets of 2–3-min cycling at vigorous intensity (first and second sets: 3 min at 85%–90% V・
O2peak; third set: 3 min at 80%–85%
V・
O2peak) with 2-min active rest at 50% V・O2peak between
each set (healthy, but sedentary young 20–30-year-old adults)82). This protocol was originally developed to
control energy expenditure for astronauts participating in long-term space missions and showed more V・
O2max
increase than MICT when applied for 8 weeks, 3 days a week, as a supervised exercise intervention with even lower energy expenditure81). The J-HIAT protocol has
been successfully modified for older adults (aged 60–69 years) by the same group as EJ-HIAT (The elderly Japa-nese male version of High-intensity Interval Aerobic Training) with 3 sets of a little lower intensity (0–10%
V・
O2max) and shorter duration (0–1 min) depending on the
1st–3rd bout, than those of J-HIAT83). A significant aerobic
and metabolic response was attained by the shorter proto-col than by the 4 × 4-min protoproto-col, with a completion rate of 100%83).
Growing evidence suggests that low-volume HIIT stim-ulates physiological remodelling comparable to MICT de-spite a substantially lower time commitment and reduced total exercise volume20,84-86), suggesting its importance
from a public health perspective, when considering that “lack of time” remains one of the most commonly cited barriers to regular exercise participation79,87-89).
SIT, which takes less training time, has been reported to be as efficient as MICT in improving V・
O2peak49,85,90).
On the contrary, SIT might be inappropriate for a largely sedentary population because SIT is a strenuous “all-out” protocol53). High RPE of SIT, even with a very short time
bout, might cause disinclination (dislike) due to fatigue and discomfort, leading to low long-term adherence53).
SIT could be more time-efficient than 4 × 4-min HIIT; however, the total time commitment would still be long due to the longer rest time of SIT than that of HIIT, with a total time commitment of 30 min including rests between bouts, warm-up, and cool-down. Considering this back-ground, the concept of low-volume SIT was developed recently. One possible alternative strategy could be to define the minimum volume of exercise required to im-prove health indices with the aim of increasing exercise adherence. Vollaard et al. reviewed SIT protocols with the shortest duration and least amount of work91). They
also constructed a modified SIT aiming for the most time efficient and effective protocol with high adherence for sedentary subjects and diabetic patients92-94). To date, this
training protocol, named reduced-exertion HIIT (REHIT) (10-min SIT sessions, 3 sessions a week for 6 weeks, involving only two 20-s Wingate sprints), represents the smallest volume of exercise (per session) that has been shown to induce positive effects on health. This proto-col was sufficient to improve V・
O2max by 10%–13%92,94).
Vollaard et al. also found that after performing only two maximal sprint intervals, each additional sprint in a train-ing session reduced the overall improvement in fitness by around 5%91). This result might raise questions about
the previously held “common sense” idea that performing more repetitions of high-intensity exercise would produce greater improvements in CRF. The effects of the REHIT protocol were confirmed by other groups of patients with type 2 diabetes mellitus95) and metabolic risks96). The
main finding of this study was that 8 weeks of REHIT elicited more potent and time-efficient improvements in CRF and cardiometabolic health compared to traditional MICT in work place settings96). REHIT was also
effec-tive with two sessions/week in improving health in the workplace under unsupervised conditions97), showing the
most effectiveness in a dose-response relationship. It was shown that one session of REHIT elicits positive affective
without an exercise habit. In the Generation 100 study, which was an epidemiological study, frequency, inten-sity (HIIT or MICT), type (running or cycling), location (indoor or outdoor), and social setting (alone vs. together with others) of exercise were assessed using exercise logs recorded after each exercise session from 618 older adults (aged 70–77 years) randomised to MCT or HIIT over a one-year period37). Approximately 300 healthy older
adults, with some initial instruction and a few sessions of supervised training, showed good adherence to unsuper-vised HIIT over the course of one year, meeting the goal of averaging two sessions per week. Exercise logs were kept to evaluate adherence to the protocol, the level on an RPE scale, and location and mode of performance based on personal preferences (Fig. 3). This novel information may help researchers and clinicians to develop tailored exercise programs in an ageing population.
High-intensity stair climbing. Allison et al. showed that
all-out cycling could be substituted by the stair climbing as an alternative exercise mode105). This method could
be a practical one as it can be applied easily in daily life anywhere, such as in the buildings and subway sta-tions. Danford et al. adapted this type of low-dose HIIT as an unsupervised CR for patients with coronary artery disease, in comparison to MICT106). In this study, after 1
month of a supervised protocol (two times/week), partici-pants performed unsupervised SIT for two months (three times/week) with no difference in exercise intensity (96 ± 8% vs. 87 ± 8% of HRpeak) or number of exercise sessions/
week (3.0 ± 3.2 vs. 3.2 ± 2.2) between the high-intensity stair climbing and MICT groups.
The Norwegian Trial of Physical Exercise after Myo-cardial Infarction (NorEx). Maintaining high PA or
having a long-term increase in PA from before to after MI was associated with a lower post-MI risk of all-cause and CVD mortality107). Despite the substantial evidence
for the benefits of CR, only a small percentage of patients with MI are referred to, participate, or complete CR pro-grams in the inpatient and home health setting108). NorEx
is a registry-based multicentre RCT across Norway, which is the largest exercise intervention ever performed for secondary prevention and rehabilitation of patients who have suffered MI109). The research involves ensuring
exercise adherence of approximately 4,500 participants over a period of four years. The primary objective is to determine the efficacy of physical exercise using the personality activity intelligence (PAI) system110,111) and a
two-way communication system (coach-patients-study administration) on mortality and cardiovascular morbidity in patients with MI. PAI might have the potential to con-tribute to improvement in adherence to home-based train-ing in CR, sheddtrain-ing new light on this medical field. PAI is a personalised metric developed for PA tracking with the aim to make it easier to quantify how much PA per valence and similar pleasure: displeasure and enjoyment
responses in adults having diverse CRF in an untrained population98). The exercise intervention was delivered
on a commercially available cycle ergometer (CAR.O.L, Integrated Health Partners Ltd, London, UK) which can be delivered unsupervised. The exercise V・
O2max
signifi-cantly improved in the exercise group (+ 7.4%) compared to the control group (− 2.3%). Participants considered the REHIT intervention acceptable and enjoyable and were confident in their ability to continue to perform REHIT. REHIT could be a promising low-dose SIT even for an untrained and unfit population with potential long-term adherence.
Two groups developed the SIT protocols for sedentary older individuals99,100). Although the number of
partici-pants was small, these protocols were well tolerated and showed no adverse events. These studies might show the potential of SIT for older adults in the future.
It might be beneficial to listen to music during SIT. Stork et al. determined whether listening to self-selected music can reduce the potential aversion to an acute ses-sion of SIT by improving affect, motivation, and enjoy-ment, and to examine the effects of music on perfor-mance101). Peak and mean power output, RPE, affect, task
motivation, and perceived enjoyment of the exercise were measured during four 30-s “all-out” Wingate Anaerobic Test bouts. Music enhanced in-task performance and en-joyment of an acute bout of SIT.
Improved adherence to non-supervised high-intensity session participation after lab-/gym-based HIIT. One of
the important aspects of HIIT/SIT is the long-term main-tenance of protocols in a non-supervised way. Although lab-/gym-based training would be desirable in terms of ef-ficacy and safety, future studies should explore long-term adherence102). Some feedback on long-term adherence has
already been accumulated from recent studies.
Long-term follow-up after CR. For home-based
train-ing (followtrain-ing a supervised period), HIIT, compared to MICT, has resulted in greater exercise adherence in the long-term (30 months) following CR103). Aamot et al.104)
showed long-term adherence (1 year), which was evalu-ated by estimating change in V・
O2peak and self-reported
objectively measured physical activity in a multicentre randomised study. There was no significant difference between homed-based and lab-based (treadmill or group exercise) 12-week HIIT CR. This result implies that the 12-week HIIT protocol experience has the potential to improve long-term adherence to PA regardless of whether it is home-based or lab-based.
The Generation 100 study: 1-year follow-up37). Under-standing how exercise participants prefer to exercise may help in developing tailored exercise programs and increase sustained exercise participation in populations
Fig. 1 Literature search for “high-intensity interval training” by PubMed globally and in Japan.
The number of articles has gradually increased both globally and in Japan. The total number is 2,857 globally and 94 in Japan through June 30, 2020. The increase is dramatic, particularly in the last decade. This increase might reflect the popularity of high-intensity interval training protocols in the general population and its wide adaptation across various fields.
Fig. 2 Relatively high intensity prescribed for two individuals in high-intensity interval training.
This figure shows two individuals performing high intensity interval training (HIIT). They have different absolute peak heart rates (HRpeak) and energy consumption, but exercise at identical relative intensity (% of HRpeak). Red line: HIIT performed as
4 × 4 min of roller ski training by a highly trained Olympic athlete with an HRpeak of 188. Target HR zone is 169–179 beats/
min (90–95% of HRpeak). Blue line: HIIT performed as 4 × 4 min of outdoor uphill walking by patient with coronary artery
disease (CAD) with an HRpeak of 130. Target HR zone is 117–124 beats/min (90–95% of HRpeak).
Adapted from Karlsen T et al. Prog Cardiovasc Dis 2017; 60: 67-77.
Fig. 3 Representative 4 × 4 min of HIIT practice sessions in cardiac rehabilitation at St. Olav’s University Hospital and The Generation
100 Study in Trondheim, Norway.
A: Phase 2 cardiac rehabilitation at St Olav’s University Hospital in Trondheim.
Participants can choose modes of exercise which they prefer during high-intensity interval training. B: The Generation 100 Study (indoor)
C: The Generation 100 Study (outdoor). Participants run uphill during interval and walk or jog downhill during active rest. D: The Generation 100 Study (outdoor). Borg scale measurements during exercise are kept between 16 and 18.
Adapted from Ito S. Jpn J Clin Sports Med 2018; 25: 79-89. A D C B A D C B A D C B A D C B A B C D ,Q-DSDQ 1XPEHURISXEOLFDWLRQV <HDU *OREDOO\
when prescribing HIIT to patients with cardiometabolic disease”116).
Although HIIT appears to provide a time-efficient alter-native to MICT, additional long-term studies assessing the safety of HIIT are needed before it can be widely adopted in individuals with known or suspected CAD, particularly in unsupervised, nonmedical settings31,114).
Currently, guidelines by Taylor et al.75) could help
judge the adoption of HIIT among participants with clini-cal problems. These guidelines summarised the cliniclini-cal considerations for HIIT based on ACSM and AHA state-ments, including an initial assessment117), absolute
contra-indications to participants118,119), monitoring checklist, and
indications for avoiding/ceasing HIIT119). Medical
clear-ance should be sought for all patients with clinical condi-tions from medical specialists or general practitioners and for patients after surgery or percutaneous coronary inter-vention, from the respective surgeon or interventionalist, prior to commencing HIIT75).
SIT and low-dose/shorter SIT. Systematic reviews on
the safety and risk of injury for SIT are very limited1).
Su-pra maximal sprints used in protocols such as the Wingate protocol are associated with blood pressure elevation, as well as blood flow increase, which could pose the risk of dislodging unstable plaque, potentially leading to acute
MI120,121). Redistribution of blood flow (increased flow in
muscle followed by decreased flow in visceral organs) also might pose a risk to patients with CVD and chronic kidney disease. However, to date, SIT has been adopted in healthy young people, irrespective of their athletic or sedentary lifestyle. For these subjects, the cardiovascular risk could be low due to the low incidence of hypertension and/ or atherosclerotic disease. For individuals with lifestyle-related and/or cardiovascular disease, the potential risk of the SIT protocol has not been evaluated. Thus, for the time being, it should not be adopted for individuals with clini-cal issues without proper clearance18). Ruffino et al
investi-gated REHIT among 16 patients with type 2 diabetes, and neither risk nor cardiovascular event was reported.93).
Conclusions
For generally unfit target populations, with lifestyle-re-lated disease and/or chronic disease, to benefit from HIIT effectively and safely, the optimisation of HIIT protocols for adherence to high intensity and high participation rates during long-term use should be considered.
Financial Disclosure
The study did not require financial support. Conflict of Interests
The author declares no conflict of interests. week is needed to reduce the risk of premature mortality
from non-communicable diseases112). PAI can be used by
integrating the individual’s gender, age, and resting time and HRmax. Among individuals ranging from the general
population to subgroups of patients with CVD, a PAI ≥ 100 per week at baseline, maintaining ≥ 100 PAIs and then increasing the PAI score over time, was associated with multiple years of life gained112). It is expected that
PAI has the potential to contribute to improved adherence to home-based training in CR, shedding new light on this medical field. Furthermore, participants will be able to self-learn which intensity is necessary for which exercise duration by spending time using PAI. The dose and inten-sity of PA/exercise achieving 100 PAI depend on the indi-vidual baseline data, and change according to the updated HR data. As an example, 30 minutes of vigorous exercise per week yielded scores of 100 PAI, demonstrating that it is the most time-efficient method.
Safety and risks of HIIT and SIT
HIIT. There is little risk of HIIT for young adults without
any known cardiac disease. By contrast, there is an under-standable concern regarding the safety of HIIT in adults with known or occult CAD and/or CHF. At present, the evaluation of the safety of HIIT among adults with varied health and disease characteristics is compromised by the limited availability of relevant data, which is due to the low proportion of studies reporting adverse events1). There
are limited data on safety regarding CR under supervision. In stable and selected patients in CR, HIIT/MICT can be performed with relatively low risk113). Rognmo et al.
ex-amined the risk of cardiovascular events during organised HIIT and MICT among 4846 patients with CAD in three CR centres113). One fatal cardiac arrest during
moderate-intensity exercise (129,456 exercise hours) and two non-fatal cardiac arrests during HIIT (46,364 exercise hours) were reported. No myocardial infarctions were reported. These low event rates preclude a definitive quantitative determination of the risk associated with HIIT114). More
recently, a systematic review examined the cardiovascular complications associated with HIIT in CR sites for pa-tients with CAD or CHF115). Based on 23 studies involving
547 participants completing 17,083 HIIT sessions (equiv-alent to 11,333 training hours), there was only one major, non-fatal cardiovascular event. Another systematic review reported no deaths or cardiac events requiring hospitali-sation among 465 HIIT patients and 488 MICT patients within a medically supervised CR setting69).
Meanwhile, a recent systematic review also evaluated the safety of HIIT among 156 patients with cardiometa-bolic disease and found the incidence of adverse acute re-sponses within 24 hours post exercise, to a single session of HIIT to be around 8% and “mild in nature” and only “somewhat higher compared to previously reported risk during MICT”, and concluded that “caution must be taken
Author Contributions
SI designed the study and collected and analysed the data. He also drafted and edited the manuscript.
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