Chronic Diseases
High-Intensity Interval Training: A Sprint or Nine Saves Time?
By Nancy J. Selfridge, MD
Associate Professor, Department of Integrated Medical Education, Ross University School of Medicine, Commonwealth of Dominica, West Indies
Dr. Selfridge reports no financial relationships relevant to this field of study.
Strong evidence supports regular exercise as a lifestyle habit and intervention that lowers risk for a myriad of diseases including coronary heart disease, high blood pressure, type 2 diabetes, obesity, and breast and colon cancers.1 In addition, exercise has gained treatment status for cardiometabolic risk factors, type 2 diabetes, osteopenia and osteoporosis, and rehabilitation for existing cardiac disease. Current guidelines for healthy adults recommend a minimum of 150 minutes per week of moderate aerobic exercise or 75 minutes per week of intense aerobic exercise.1 However, only 64.5% of U.S. adults are able to meet these federal recommendations.2 Guidelines also state that additional time accrued during exercise is associated proportionally with increasing health benefits. For maximum benefits, muscle strengthening, stretching, and balance exercises are recommended in addition to this baseline time commitment to aerobic exercise.1,3 It is no surprise that “lack of time” is one of the main barriers to regular exercise cited by adults.4
High-intensity interval training (HIT) is characterized by “burst” type exercise efforts at 90% or more of VO2 max or maximum aerobic capacity for short periods of time interspersed with periods of low-intensity exercise or rest. This kind of exercise has been studied and used for decades to improve performance in competitive athletes, but recently there has been a growing research interest into the possible physiologic benefits of HIT, both for healthy and clinical populations. Because of the short lengths of intense exercise interposed with recovery periods, untrained individuals are able to work harder than they would otherwise during more prolonged continuous exercise. It has been shown that the physiologic adaptations to HIT appear to be superior to continuous moderate exercise (CME). HIT protocols offer additional potential advantages in that the total exercise volume and time commitments are 10-90% less than matched-work CME protocols. Thus, if HIT offers metabolic and health benefits equal to or greater than continuous moderate activity and in less time, it could be an acceptable solution to the “time excuse” that prevents patients from exercising.
Physiologic Effects of HIT
To assess the potential benefits of HIT, the physiological effects of such interventions need to be elucidated. In a series of such studies, Gibala et al showed a variety of physiological and biochemical adaptations using a Wingate HIT protocol, consisting of 4-6 sets of intervals of 30 seconds of all-out cycling effort against a supra-maximal workload followed by 4 minutes of recovery. These authors showed that as little as 2 weeks of this protocol, totaling 18-27 minutes of total exercise per session performed three times weekly, resulted in increased maximal activity and concentration of mitochondrial enzymes, consistent with increased skeletal muscle oxidative capacity. After several weeks of this HIT protocol, compared to a CME program designed to be consistent with current public health recommendations for aerobic exercise, the authors found similar improvements in skeletal and cardiovascular markers for adaption, including increased resting muscle glycogen content, increased capacity for lipid oxidation, decreased glycogen utilization and lactate production, enhanced peripheral vascular function, and increased exercise performance and VO2max. Subsequently, the same authors, in a series of small “proof of principle” studies using a modified protocol that was more practical and tolerable, consisting of 10 sets of 10-second, high-intensity cycling using a constant workload at about 90% maximal heart rate alternating with 60 seconds of recovery, demonstrated the same effectiveness at producing skeletal muscle metabolic adaptations as shown in their previous HIT protocol and higher volume endurance training.5 This latter protocol clearly offers a time advantage requiring only about 12 minutes per exercise session. The authors claim that the metabolic changes occurring in muscle with both CME and HIT are mediated by similar mechanisms; both induce activation and nuclear concentration of PGC-1α, a master regulator protein of muscle mitochondrial biogenesis.6 A modest increase in muscle PGC-1α appears to have a beneficial effect on oxidative capacity, antioxidant defense, anti-inflammatory pathways, glucose utilization, and prevention of age-related muscle loss. Thus, a significantly lower volume of exercise than is currently recommended, performed at higher intensity, appears to have significant potential health benefits.
Clinical Research
Although physiological and clinical evidence is vital, this needs to be translated to clinical relevance. In support of this approach, Kessler et al reviewed research evidence for HIT in reducing the risk of cardiometabolic disease and separated HIT protocols into two distinct types performed on a cycle ergometer or a treadmill.7 Using young healthy subjects (because of potential health, safety, and motivational concerns), sprint interval training (SIT) was used to represent the Wingate HIT protocol described above. Aerobic interval training (AIT), on the other hand, had protocols using 4-6 sets of 4 minutes of high-intensity work at 80-95% VO2max followed by 3-4 minutes of recovery. AIT studies in this review included young healthy subjects as well as older and clinical populations as compared to the SIT studies.
Of the studies reported by Kessler, 17 examined the impact of HIT on exercise capacity, with all showing improvement in VO2max.7 Four of the HIT studies were of 4-8 weeks duration and compared HIT to CME; the improvements in VO2max were similar in the HIT and CME arms. In contrast, differences were noted in the studies of longer duration. In nine AIT studies of 10 weeks to 6 months duration, all but one resulted in greater improvement in VO2max compared to CME. When reported, P values ranged from < 0.05 to < 0.001, demonstrating statistical significance. The one study failing to demonstrate any improvement in VO2max over an 11-week training period used an alternate AIT protocol of eight sets of 2-minute duration, high-intensity intervals. This somewhat surprising finding could be due to small sample size and the fact that the ideal HIT protocol proportion of exercise and rest is as yet unknown. In contrast, an SIT study of only 2 weeks’ duration and an AIT study of only 4 weeks’ duration showed significant improvements in VO2max induced after only a few training sessions. In Kessler’s review, 13 studies examined HIT impact on biochemical parameters such as glucose metabolism, insulin sensitivity, and fasting glucose as well as oral glucose tolerance. Of these 13 studies, seven measuring insulin sensitivity after HIT showed significant improvement.7 Three of these studies included subjects who were obese, had a family history of hypertension, or had metabolic syndrome. In those studies described in the review that included a CME arm, improvements with HIT were similar in both exercise groups. Eleven studies also assessed fasting glucose levels. Three 2-4 week duration SIT studies showed no changes from baseline values in fasting plasma glucose; the studies of longer duration revealed results that were inconclusive. For example, seven AIT studies of 12-16 weeks’ duration reported inconsistent results in fasting plasma glucose after AIT, with four trials reporting reductions and three reporting no change in subjects with normal, borderline, and elevated baseline values. However, all five HIT studies that reported results for oral glucose tolerance testing showed significant improvement in the 2-hour postprandial glucose or glucose area under the curve (AUC) measurements. Measuring oral glucose tolerance could be a more reliable marker than measuring fasting plasma glucose alone and needs further investigation.
From a biochemical perspective, the association of blood lipids and cardiovascular disease under these exercise conditions also would need to be considered. In this respect, HIT was shown to have less of an impact on lipid measurements, unfortunately. Of the lipids measured, only high-density lipoprotein cholesterol (HDL-C) was beneficially affected by HIT, and only three of 10 studies lasting at least 8 weeks showed any improvement. HIT was comparable to CME in this regard, though.7
A reduction in elevated blood pressure with exercise training is indeed a desirable outcome. In this respect, of 12 studies that examined the effect of HIT on blood pressure, most that were of 2-10 weeks’ duration showed no change. A single, 2-week SIT study showed a transient decrease in systolic blood pressure 24 hours after exercise that did not persist at 72 hours. Five studies of AIT of longer duration (12-16 weeks), in subjects who were not on blood pressure medication, showed a variable decrease in blood pressure. In AIT studies of subjects on antihypertensive medications, no significant change in blood pressure was noted, though all study subjects had well-controlled blood pressure at baseline.7
From a broader perspective, biochemical and clinical parameters should be viewed together with morphological characteristics, as these are also predictors of health outcomes. Seventeen studies measured the impact of HIT on anthropomorphic measurements such as body weight, BMI, body-fat percentage, lean body mass percentage, waist-to-hip ratio, and waist circumference. Improved measurements were observed only in studies of > 10 weeks’ duration, except for one 2-week study using SIT that reported an average 2.4 cm reduction in waist circumference in obese young men. In studies comparing HIT to CME, such outcomes were comparable.7
In a meta-analysis of six AIT randomized, controlled trials including 153 patients with cardiometabolic disorders, Huang et al reported similar conclusions. These authors also reported a high adherence rate of > 70% in three trials and > 90% in two trials. A total of only 10 patients withdrew, five from the AIT group, four from the CME group, and one from an undetermined group, and all for reasons not directly related to the exercise intervention. No exercise-related adverse events were reported in this meta-analysis,8 nor in the larger review by Kessler.7
For long-term exercise compliance, subjects need to be motivated by enjoying the activity. Surprisingly, only a few studies have addressed subjects’ perception of HIT. Though HIT is often given a higher rate of perceived exertion by patients, Bartlett et al reported higher ratings of perceived “enjoyment” for HIT compared to CME.9 This finding was corroborated in a study by Wisloff et al, who showed that heart failure patients experienced HIT as more motivating than CME, which in turn was perceived as “boring.”10
Conclusion
The findings to date on HIT for health maintenance and for prevention and treatment of cardiometabolic diseases are intriguing. HIT appears to be as effective as CME, if not more so, for increasing maximum exercise capacity, improving insulin resistance and blood pressure, reducing body fat, and raising HDL-C. Many questions and concerns still need to be addressed. The ideal dose of HIT in terms of exercise and recovery ratios and total volume of HIT exercise has not been clearly established, and may differ greatly depending on patient goals and therapeutic targets. The absolute minimum amount of exercise needed to reduce cardiometabolic risk still needs to be established. Epidemiologic evidence from a Norwegian study suggests that a single weekly bout of high-intensity exercise is associated with a significantly lower risk of cardiovascular disease.11 Further, Metcalfe et al recently found that cycling 10 minutes at low intensity with just one or two burst efforts of sprints lasting 10-20 seconds, three times per week, resulted in improved insulin sensitivity and increased VO2 max.12 The studies to date have had heterogeneous patient populations. Even though results look promising for HIT as a therapeutic intervention, studies of larger samples of more homogeneous populations with cardiometabolic risk factors or disease will help support these initial data. Studies of longer duration will help determine if additional benefits accrue and if HIT is sustainable for various populations. HIT is a highly structured and individualized exercise program and requires at least some initial education and supervision. For example, to reach 90% of maximal exercise capacity, some healthy individuals will need to perform all-out cycling against a significant load, while a patient with heart failure may achieve the same exercise intensity with slow treadmill walking up a slight incline. Though using rate of perceived exertion can help with self-monitoring of exercise intensity, it seems prudent to initiate an HIT program with close supervision and heart rate monitoring, at least in clinical populations. Most studies reported that subjects were verbally encouraged during the high-intensity intervals, which might artificially increase motivation, ability, and adherence compared to a home program. Many of the HIT trials to date have been performed in the same laboratory, introducing possible investigator bias.
Recommendation
HIT may be an ideal form of exercise for people with time constraints who wish to be fit and reduce disease risk. Certainly, it may be a way to help inactive patients segue into a more active lifestyle since it so rapidly increases exercise tolerance. The fact that heart failure patients in one study found HIT more motivating than constant moderate exercise suggests that sedentary inactive people with low baseline exercise capacity may find HIT a quite acceptable way to start a fitness program. However, it is clear that education and supervision are indicated for this kind of exercise to ensure safety and effectiveness, especially for sedentary people, those with risk factors for cardiac disease, or for those with established disease. Large multicenter trials of longer duration, including home programs, are essential before HIT can be included in our exercise prescriptions for patients with confidence.
References
1. U.S. Department of Health and Human Services. Physical Activities Guidelines for Americans; 2008.
2. Center for Disease Control and Prevention. State indicator report on physical activity, 2010. Atlanta GA: U.S. Department of Health and Human Services; 2010.
3. Garber CE, et al. Quantity and quality of exercise for developing and maintaining cardiorespiratory, musculoskeletal, and neuromotor fitness in apparently healthy adults: Guidance for prescribing exercise. Med Sci Sports Exerc 2011;43:1134-1359.
4. Cerin E, et al. Perceived barriers to leisure-time physical activity in adults: An ecological perspective. J Phys Act Health 2010;7:451-459.
5. Gibala M, et al. Physiologic adaptation to low-volume, high-intensity interval training in health and disease. J Physiol 2012;590:1077-1084.
6. Little JP, et al. An acute bout of high-intensity interval training increases the nuclear abundance of PGC-1α and activates mitochondrial biogenesis in human skeletal muscle. Am J Physiol Regul Integr Comp Physiol 2011;300:R1303-R1310.
7. Kessler HS, et al. The potential for high-intensity interval training to reduce cardiometabolic disease risk. Sports Med 2012;42:489-509.
8. Huang CL, et al. Effect of aerobic interval training on exercise capacity and metabolic risk factors in people with cardiometabolic disorders. J Cardiopulm Rehabil Prev 2011;31:378-385.
9. Bartlett JD, et al. High-intensity interval running is perceived to be more enjoyable than moderate-intensity continuous exercise: Implications for exercise adherence. J Sports Sci 2011;29:547-553.
10. Wisloff U, et al. Superior cardiovascular effect of aerobic interval training versus moderate continuous training in heart failure pateints: A randomized study. Circulation 2007;115:3086-3094.
11. Wisloff U, et al. A single bout of exercise may reduce cardiovascular mortality: how little pain for cardiac gain? The HUNT study, Norway. Eur J Cardiovasc Prev Rehabil 2006;13:798-804.
12. Metcalfe RS, et al. Towards the minimal amount of exercise for improving metabolic health: beneficial effects of reduced-exertion high-intensity interval training. Eur J App Physiol 2011;112:2767-2775.
Strong evidence supports regular exercise as a lifestyle habit and intervention that lowers risk for a myriad of diseases including coronary heart disease, high blood pressure, type 2 diabetes, obesity, and breast and colon cancers.1 In addition, exercise has gained treatment status for cardiometabolic risk factors, type 2 diabetes, osteopenia and osteoporosis, and rehabilitation for existing cardiac disease.You have reached your article limit for the month. Subscribe now to access this article plus other member-only content.
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