Introduction

According to the Centers for Disease Control and Prevention (CDC), more than one third of the American adult population are obese (1).  Often there is a correlation between obesity and sedentary behavior (2, 3). Consequently, obesity and inactivity are known to be linked to health related problems such as cardiovascular disease (CVD), stroke, insulin resistance, and type 2 diabetes, as well as, certain types of cancer, reproductive abnormalities, and osteoarthritis (2-8). Furthermore, according to a study published by the American Diabetes Association, risk factors associated with CVD are increased 30 years before the individual is diagnosed with type 2 diabetes (43).  This suggests that control of the risk factors, particularly glycemic control, hypertension, dyslipidemia and smoking cessation, should start early in life, and improvements on these factors could potentially decrease instances of both CVD and type 2 diabetes. 

In 1948, the National Heart, Lung, and Blood Institute (NHLBI) partnered with Boston University to learn more about the causes of CVD, resulting in what is now an on-going study known as the Framingham Heart Study (42). The original cohort had 5,209 participants and followed individuals over the course of several years to identify what common characteristics resulted in CVD. Over the course of the study, risk factors such as dyslipidemia, hypertension, physical inactivity, obesity, type 2 diabetes, insulin resistance were identified. This valuable information lead to the development the Framingham Risk Calculators and Scores. The algorithms rate the risk of different outcomes, such as atrial fibrillation, CVD, congestive heart failure, coronary heart disease, diabetes, hypertension, intermittent claudication, and stroke and determines an individual’s risk of developing the conditions in the next 10 or 30 years.

Improved fitness has proven benefits to cardiovascular health and the prevention of metabolic diseases such as CVD and type 2 diabetes (2, 3, 11-15). Furthermore, exercise is considered a direct, cost-effective way of improving overall health (5-7, 9, 10) and can “reverse” some of the adverse effects associated with metabolic syndrome (16).  According to Bird and colleagues, there is “no single intervention with greater efficacy than physical exercise to reduce the risk of virtually all chronic diseases simultaneously” (17). Benefits of exercise training include reduced blood pressure (BP), increased high-density lipoprotein cholesterol (HDL-C), improved body composition, and improved VO2max. 

Despite exercise being a cost-efficient and effective response to many of the health epidemics that Americans face, only about 15% of American adults participate in vigorous regular exercise (three times a week for at least 20 minutes) and only 22% participate in regular exercise at any intensity (five times a week for at least 30 minutes) (21). The most cited reason for poor participation in exercise programs, regardless of age, sex, ethnic background, or health status (17), is a lack of time (7, 9, 13, 22-24).  As a result, more research has focused on reducing the time factor involved in physical activity, while maintaining and/or improving aerobic and metabolic benefits seen with traditional exercise programs (7, 9, 13, 22-24). 

High intensity interval training (HIIT) is an exercise protocol that has recently received attention from researchers (2, 5, 7-10, 16, 22, 25-37). HIIT is defined as intense periods of intermittent exercise above anaerobic threshold (85-95% maximal heart rate or %HRmax), separated by periods of rest or low intensity exercise that does not allow for full recovery (10, 38).  Repeated bouts at a high intensity have been shown to improve lipid oxidation, insulin sensitivity, and vascular endothelial function as well as increased maximal oxygen uptake and decreased subcutaneous fat (2, 17). 

HIIT encompasses several different exercise interventions that manipulate levels of intensity, interval duration, recovery times, and total work volume (10, 38). Additionally, it encompasses various modalities, including running and cycle ergometers.  Sprint Interval Training (SIT), a type of HIIT, incorporates maximal or near maximal all-out sprint intervals (10-30 seconds) with relatively long periods of active recovery between the intervals (approximately four minutes). The major difference between HIIT and SIT is in the ‘all-out’ effort expected of the participants. The higher intensity results in shorter intervals and total duration (10, 37, 38). Generally, HIIT protocols are completed at high (but not maximal) intensities for longer periods of time (two-four minutes) (38). Both HIIT and SIT have shown rapid, beneficial adaptations that may help with cardiovascular health and type 2 diabetes prevention in both pre-clinical and clinical populations (17), however, to our knowledge no studies have evaluated how these changes might affect an individual’s risk of developing CVD in the next ten years.

The purpose of this study was to determine if a reduction of the 10-year risk of CVD (determined by Framingham CVD risk calculator), is greater in an R-SIT experimental group compared to a MIT of sedentary prediabetic women, after a 16-week intervention.

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