Mark Kelly by Mark Kelly

High Intensity Interval TrainingWith 34 percent of adults in the U.S. currently meeting the criteria for metabolic syndrome, it has never been more important to identify a solution that will reverse this increasingly prevalent condition. High-intensity interval training (HIIT), which has become extremely popular in the past few years, has proven to be effective in improving athletic performance and enhancing weight loss. As HIIT becomes more widely practiced, more and more studies are being done on it—thus, more discoveries are being revealed, and some in areas not originally suspected. This article discusses the benefits of HIIT on health and disease prevention, as well as mental aspects such as adherence.

Health Benefits of Going Hard

The benefits of sustained, moderate-intensity exercise on cardiovascular symptoms is well known, but interval training is just as effective as continuous, moderate activity at improving factors related to metabolic syndrome. It helps improve insulin resistance, lower blood pressure, reduce body fat and raise HDL-cholesterol levels. In addition to increasing maximum exercise capacity, there also are greater shifts in metabolic rate and muscle mass gain. A small study focusing on older (55 to 71 years old) patients with chronic heart failure was performed to see if a HIIT approach could be safely and effectively used with this group. The results showed increased aerobic capacity in all patients; researchers also found that quality of life was improved and no adverse effects were seen.

Short and Fast Exercise Sessions, Longer and Slower Results

As is often the case, important things such as beneficial shifts in metabolic and cardiac risk factors take time and energy to achieve. Researchers in Oklahoma performed a meta-analysis of studies comparing continuous moderate exercise (CME) and high-intensity interval training (HIIT). Exercise programs ranged from two weeks to six months. All 17 studies measuring aerobic fitness and all seven that measured insulin sensitivity found significant improvements among HIIT subjects over control groups; however, those who performed HIIT did not always experience greater improvements than those who performed CME.

The same researchers sought to identify the minimal duration of exercise necessary to experience significant improvements. They found that a minimum of eight weeks was needed to show improvements in HDL-C in three of the 10 studies. Additionally, 12 weeks of training was required for improvements in fasting glucose in four of seven studies, as well as for improvements in anthropometrics in overweight and obese people in six studies. Twelve weeks also was needed to show reductions in blood pressure in five studies. Improvements in aerobic fitness were equal (in five studies) or greater (in eight studies) in response to HIIT when compared to CME. The researchers concluded that HIIT yielded superior results for both cardiometabolic factors and aerobic fitness compared to CME in both healthy and clinical subjects exercising for eight to 12 weeks (Kessler et al., 2012).

Another study looked at how different exercise programs worked on people diagnosed with metabolic syndrome. Norwegian researchers divided subjects into three groups: aerobic interval, continuous-moderate exercise or a control group. After a 10-minute warm-up, the aerobic-interval group exercised at 90% VO2max for four minutes followed by three minutes at 70% VO2max. The subjects repeated this four times, followed by a 5-minute cool-down. The continuous-moderate group exercised at 70% VO2max for 45 minutes. Again the exercise treatments were designed to expend equal calories. While both conditions improved many factors, the aerobic-interval group showed greater improvements in insulin sensitivity, aerobic capacity and HDL levels, and lower blood glucose levels than the other groups. Of all the risk factors, low aerobic capacity is the strongest predictor of death. Interestingly, the aerobic-interval group improved their capacity 35%, while the continuous-exercise group improved their capacity 16% during the 16-week training study (Tjonnas et Al., 2008).

Working Hard Can Be Considered Hardly Working For Those Who Enjoy It

A review study by a group of French researchers France looked at the benefits of HIIT versus moderate-intensity continuous exercise (MICE). In people with high-risk cardiovascular symptoms, HIIT was safe and better tolerated than MICE and it was associated with many long- and short-term physiological adaptations. Factors such as peak oxygen uptake and ventricular and endothelial function were all improved significantly more among those who performed HIIT. The review also found an improved adherence to exercise training, which is particularly significant for those who are rehabilitating from coronary artery disease (Guiraud et al., 2012).

Another study looked at how much subjects enjoyed interval training (Bartlett et al., 2011). Researchers recruited active men who ran for three minutes at a treadmill speed that elicited 90% VO2max, followed by a three-minute recovery period at 50% VO2max; this pattern was repeated six times for a total of 50 minutes (including the warm-up and cool-down). The control groups performed 50 minutes of moderate-intensity, continuous exercise at 70% VO2max. Despite reporting a slightly higher RPE (14 vs. 13), the interval condition received a significantly higher rating on the Physical Activity Enjoyment Scale (88±6 vs. 61±12). Another review found that exercise adherence was actually enhanced with high-intensity interval training and disease symptoms and conditioning were significantly improved. Thus, it appears that less is more when it comes to the volume of training versus the benefit bestowed to the exerciser when performing HIIT.


All exercise programs have potential risks and benefits. Many people avoid high-intensity exercise because of the fear that they will get injured or that it will be too hard. In reality, HIIT is safe if an individual’s conditioning is gradually built up and the joints are appropriately strengthened to tolerate the stresses. The benefits of HIIT on health and conditioning are significant and the higher intensities can be exhilarating. Of course, it is important to rest appropriately between work intervals and to recover adequately between sessions. Many experts advocate following a HIIT session with an easy recovery workout the next day. Like all aspects of training, they key is to start slowly and increase the intensity over time.


Bartlett, J.D. et al. (2011). High-intensity interval running is perceived to be more enjoyable than moderate-intensity continuous exercise: implications for exercise adherence. Journal of Sports Sciences, 29, 6, 547-553.

Guiraud, T. et al. (2012). High-intensity interval training in cardiac rehabilitation. Sports Medicine, 42, 7, 587-605.

Kessler, H., Sisson, S. and Short, K. (2012). The potential for high-intensity interval training to reduce cardiometabolic disease risk. Sports Medicine, 42, 6, 489-509.

Tjonnas, A.E., Lee SJ, Rogonmo O et al. (2008). Aerobic interval training vs. cContinuous moderate exercise as a treatment for the metabolic syíndrome: Aa pilot study. Circulation, 2008;doi:10.116/circulationaha.108.772822

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