Interval exercise training has been practiced by athletes for more than a century as a means to enhance performance. While research on the topic dates back many decades, the last 10 to 15 yr have seen a resurgence of interest into the potential for interval training to improve cardiometabolic health in a wide range of individuals. Renewed scientific inquiry has been accompanied by increased attention from fitness enthusiasts, as evidenced by the fact that high-intensity interval training (HIIT) was again recently named the top fitness trend worldwide in an annual survey by the American College of Sports Medicine. The widespread interest in HIIT is due in part to a perceived time savings, given that intermittent exercise can elicit physiological adaptations similar to traditional moderate-intensity continuous training (MICT) despite a reduced total exercise volume and time commitment (1). This brief commentary attempts to place the relevant research in context for medical practitioners and health professionals.
Characterizing HIIT and the Role of Intensity Versus Intermittency
HIIT is commonly defined as relatively intense bouts of exercise that elicit ≥80% of maximal heart rate, interspersed by periods of lower intensity exercise or rest for recovery (2). It can be distinguished from a more intense version — sprint-interval training (SIT) — in which bouts are performed in an “all out” manner or at an absolute intensity that exceeds the workload required to elicit maximal oxygen uptake (V˙O2max) (2). At this point, it is worth remembering that interval exercise does not have to be especially intense to elicit health benefits, although a relatively high volume of exercise is required. Some have called for an increased focus on moderate-intensity interval training, arguing it may be more suited for the general population (3), although this tends to overlook the reality that a perceived lack of time is one of the most commonly cited barriers to regular physical activity. From a mechanistic standpoint, however, there is evidence that the physiological adaptations elicited by interval training may not be solely attributable to intensity per se, but rather the intrinsic nature of the intermittent exercise stimulus may contribute in this regard. For example, some of the cellular signaling pathways that regulate skeletal muscle mitochondrial biogenesis are activated to a greater extent after interval compared with continuous exercise, even when relative intensity is moderate and total work is matched (4).
The potential for moderate-intensity interval training to elicit health benefits superior to matched-work continuous exercise is evidenced by the work of Karstoft and colleagues (5). These authors conducted a small, randomized controlled trial on the effects of free-living interval walking training in older, overweight and obese individuals with type 2 diabetes. Four months of interval walking, involving 1 h·d−1, 5 d·wk−1, was superior to energy expenditure-matched continuous walking for improving cardiorespiratory fitness, body composition, and glycemic control. The interval protocol was more effective even though it involved only slight variations in intensity that corresponded to ~69% and ~63% of maximal heart rate, respectively, during alternating 3-min periods of “fast” and “slow” walking. The authors’ pragmatic conclusion was “[Interval walking training] may therefore be a good option when considering which type of training type 2 diabetic patients should be offered in primary care.” Work by Masuki et al. (6) also shows the feasibility and effectiveness of interval walking training for improving fitness and reducing lifestyle related diseases in older individuals. The researchers suggested that by incorporating factors that enhance adherence, such as an information technology network that required only minimal staff support, the regimen could be successfully implemented in large numbers of individuals over a prolonged period of time.
HIIT Is a Time-Efficient Strategy to Improve Cardiometabolic Health
A considerable body of evidence now suggests that HIIT can elicit cardiometabolic health benefits comparable or superior to traditional endurance training despite reduced time commitment. This is evidenced by a recent systematic review and meta-analyses based on 65 intervention studies that concluded, “HIIT may serve as a time-efficient substitute or as a compliment to commonly recommended MICT in improving cardiometabolic health” (7). High-intensity interval training is particularly effective for improving cardiorespiratory fitness, and the report by Batacan et al. (7) is supported by other systematic reviews and meta-analyses that concluded HIIT is superior to traditional moderate-intensity continuous exercise for improving V˙O2max in both healthy young to middle-aged individuals (8) and people with cardiometabolic diseases (2). Cardiorespiratory fitness is as strong a predictor of mortality as established risk factors such as cigarette smoking, hypertension, high cholesterol, and type 2 diabetes (9), and relatively small increases (equivalent to 1 to 2 metabolic equivalents, which is commonly observed after 6 to 12 wk of various HIIT interventions) are associated with considerably (10% to 30%) lower adverse cardiovascular event rates (9). The potential for HIIT to robustly improve cardiorespiratory fitness is noteworthy given that lower-intensity exercise — equivalent to 150 min·wk−1 at an intensity of approximately 60% of maximal heart rate — may not be sufficient to improve cardiorespiratory fitness for a substantial proportion of sedentary adults (10).
With respect to other indices of cardiometabolic health, recent systematic reviews and meta-analyses have concluded that HIIT may reduce insulin resistance compared with continuous exercise training (11), and HIIT is more effective at improving brachial artery vascular function than MICT (12). Batacan et al. (7) concluded that HIIT may be especially attractive to overweight/obese populations interested in improving cardiometabolic health but with limited time available. From a clinical standpoint, the authors suggested that HIIT may reduce the development and progression of disease-related risk factors that are associated with overweight/obesity and low aerobic fitness. In addition to V˙O2max, their analyses revealed that at least 12 wk of HIIT improves cardiometabolic risk factors such as waist circumference, percent body fat, resting heart rate, systolic blood pressure and diastolic blood pressure in overweight/obese populations. A meta-analysis by García-Hermoso et al. (13) similarly concluded that HIIT can be considered a more effective and time-efficient intervention for improving aerobic capacity and blood pressure as compared with other types of exercise in overweight and obese youth.
Is HIIT Safe?
The most comprehensive study to address the issue of risk examined 4,846 individuals with coronary heart disease who were engaged in supervised HIIT and moderate-intensity training at three cardiac rehabilitation sites in Norway (14). In a total of 175,820 training hours during which all patients performed both types of exercise, there was one fatal cardiac arrest during moderate-intensity exercise (129,456 exercise hours) and two nonfatal cardiac arrests during high-intensity interval exercise (46,364 exercise hours). While the absolute rate of complications was higher for high-intensity compared with moderate-intensity exercise (1 per 23,182 compared with 1 per 129,456 h), the overall risk of a cardiovascular event was low after both types of exercise. It must be emphasized that the individuals were engaged in a supervised training program after having undergone a full medical screening prior to participation. The risk of acute myocardial infarction and sudden cardiac death is known to be increased after vigorous activity in susceptible individuals, which emphasizes the need for appropriate individual medical prescreening as commonly recommended (15). Nonetheless, as summarized by Cassidy et al. (16) in a recent review, “On the whole, however, mounting clinical evidence supports HIIT as a safe therapy for the majority of individuals with elevated cardiometabolic risk.”
How Low Can You Go?
The potency of brief, very intense exercise is illustrated by the work of Gillen et al. (17), who showed that 12 wk of sprint interval training improves indices of cardiometabolic health similar to traditional endurance training despite a five-fold lower exercise volume and time commitment. The sprint protocol involved a total of 1 min of “all out” intermittent exercise set within a 10-min time commitment, whereas moderate training consisted of 50 min of continuous exercise, and both groups training three times per week. V˙O2max increased similarly by 19% (almost two metabolic equivalents) in both groups, and there were comparable improvements in insulin sensitivity as determined by intravenous glucose tolerance tests. The sprint exercise was performed on a specialized ergometer that limits translation beyond a laboratory setting. A recent study, however, used a more practical, constant-load cycle protocol that requires <15 min per session, and reported that 18 sessions of training over 6 wk improved V˙O2max and other cardiometabolic risk factors (18). Another recent report by Allison et al. (19) showed that brief bouts of intermittent stair climbing is a practical and accessible model of sprint training that can improve cardiorespiratory fitness in a time-efficient manner. Bodyweight style interval training is another option that has been shown to enhance cardiorespiratory fitness, and muscular strength, without the need for specialized equipment (20).
What’s Next for HIIT?
Considering the Exercise is Medicine® initiative and by way of analogy, HIIT is like a promising new treatment on the market: it is showing considerable efficacy in small scale, early phase trials, but larger, longer and more comprehensive studies are warranted. Efforts to elucidate the role of exercise intensity in promoting healthy aging and longevity are of particular significance. Generation 100 will be the first randomized controlled trial to determine the effect of exercise training on morbidity and mortality in the elderly, and it will specifically explore the relationship between exercise intensity and health benefits. Given that most interval training studies have been conducted in a laboratory setting under controlled conditions, translational studies also are warranted to more firmly establish the effectiveness of practical, time-efficient HIIT approaches in the “real world.” Additional work also is warranted from a behavior standpoint, although emerging data support the viability of interval exercise as an alternative to continuous exercise from a psychological perspective (21).
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