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Understanding high-intensity interval training (HIIT)

This is an excerpt from Laboratory Manual for Exercise Physiology 2nd Edition With HKPropel Access by G. Gregory Haff & Charles Dumke.

Many sports are, for the most part, undertaken in an intermittent fashion. It is well documented in the scientific literature that speed, agility, strength, explosive power, and the ability to repeat brief supramaximal exercise is highly related to performance in sports that are dominated by intermittent activities. In preparing for intermittent sports, various forms of high-intensity interval training (HIIT) are often used to improve cardiorespiratory and metabolic function as well as physical performance. Typically employed with running or cycling training, HIIT involves high-intensity exercise performed in short to long bouts interspersed with periods of recovery.

Commonly associated with sport performance, HIIT has begun to be seen as an effective training tool for combating cardiometabolic diseases, improving vascular function and cardiovascular fitness, reducing insulin resistance, and improving metabolic health when compared with low- to moderate-intensity continuous endurance training:

Increased

• Adiponectin
• Availability of nitric oxide
• Beta-cell function
• Cardiac function
• Enjoyment of exercise
• High-density lipoproteins
• Insulin sensitivity
• Maximal rate of Ca²⁺ reuptake
• PGC-1α
• Quality of life
• O₂peak

Decreased

• BP
• Fasting glucose
• Fatty acid synthase (FAS)
• Fatty acid transport protein (FATP-1)
• Inflammation
• Oxidative stress
• Triglycerides

Adapted from British Journal of Sports Medicine, "High-intensity interval training in patients with lifestyle-induced cardiometabolic disease: a systematic review and meta-analysis," K.S. Weston, U. Wisloff , and J.S. Coombes, 48: 1227-1234, ©2014, with permission from BMJ Publishing Group Ltd.

Though HIIT is a useful training tool, it should be used cautiously with clinical populations. The safety of HIIT training is a controversial topic among health professionals who work with clinical patients or at-risk populations. In a study by Rognmo et al. examining the cardiovascular risk of HIIT in coronary heart disease patients, both HIIT and moderate-intensity training exhibited a low risk. However, Halle suggests that the data presented by Rognmo et al. should be interpreted with caution because the rates of cardiovascular complications calculated to the number of patient exercise hours were more than five times higher during HIIT. As such, the risks of adverse effects from HIIT may outweigh the benefits for some clinical populations. To avoid contraindications to HIIT, Weston et al. suggest that careful screening must be conducted when working with clinical populations to ensure that HIIT is used in a safe and appropriate fashion. Potential contraindications to HIIT include the following:

• Unstable angina pectoris
• Uncompensated heart failure
• Recent myocardial infarction (
• Recent coronary artery bypass graft or percutaneous coronary intervention (
• Heart disease that limits exercise (valvular, congenital, ischemic, and hypertrophic cardiomyopathy)
• Complex ventricular arrhythmias or heart block
• Severe COPD, cerebrovascular disease, or uncontrolled peripheral vascular disease
• Uncontrolled diabetes mellitus
• Hypertension with BP >180/110 (or uncontrolled)
• Severe neuropathy

Adapted from British Journal of Sports Medicine, "High-intensity interval training in patients with lifestyle-induced cardiometabolic disease: a systematic review and meta-analysis," K.S. Weston, U. Wisloff , and J.S. Coombes, 48: 1227-1234, ©2014, with permission from BMJ Publishing Group Ltd.

Regardless of the population using HIIT, it is necessary to use a testing method that allows interval training to be programmed. Classically, intense interval training has been programmed based on maximal aerobic speed (MAS),which is the lowest speed that elicits O₂max and is typically determined via gas exchange analysis. Traditionally composed of shuttle runs, these tests often incorporate accelerations, decelerations, and changes of direction, which reflect the demands associated with many intermittent sports but also can enhance peripheral aspects of cardiorespiratory function. To make these tests more practical, numerous field-based tests have been developed to determine the MAS and indirectly reflect O₂max. Typically, these tests are based on continuous linear runs or shuttle tests and are used to determine the maximal running speed (MRS), which is similar to the MAS, at the end of the test. However, as noted by Buchheit, these tests determine the MRS via efforts that are fundamentally different from intermittent sports, the methods typically used to develop individualized HIIT training programs, and the physiological determinants of performance associated with intermittent or shuttle test efforts.

Careful inspection of the scientific and applied literature suggests that the most effective test protocol for evaluating this type of endurance needs to allow the simultaneous inclusion of intermittent and shuttle runs. Two intermittent tests that are often used by practitioners are the interval shuttle run test (ISRT) and the Yo-Yo test. Although these tests are commonly performed, they only provide an index of intermittent aerobic performance and do not yield an MRS that can be used for developing HIIT programs. To address the inability of these tests to yield an MRS that could be used for programming, Buchheit developed the 30-15 Intermittent Fitness Test (30-15 _IFT-40m ),which is tested on a 40 m field. The test has been modified to be performed on a 28 m court and is called the 30-15 Intermittent Fitness Test (30-15 _IFT-28m ). The strength of this test is that it incorporates physiological variables similar to those seen in interval training, including explosive expressions of power when changing directions, aerobic qualities, and the ability to recover between efforts. This is accomplished by simultaneously using aspects of intermittent and shuttle tests to establish an MRS.

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