High-Intensity Interval Training

High-intensity interval training (HIIT) is an intense exercise workout. It is becoming more and more popular as a way to get a high return on investment within the world of exercise and physical fitness. HIIT periods can range from a few seconds long to eight minutes long, and are performed at 80 to 95% of a person’s estimated maximum heart rate.^[1] The recovery periods are just as long as the workout periods, but are performed such that the heart rate achieved is much lower, usually in the range of 40 to 50%. These periods of high intensity and relatively low intensity by comparison are alternated for the duration of the complete workout, which can last on average anywhere from 20 to 60 minutes.

HIIT is purported to have numerous health benefits.^[2] Aerobic exercise has an impact on cardiovascular disease risk, including benefits to lipid profiles, blood pressure, and inflammatory markers. Exercise also decreases the risk of events like stroke, acute coronary syndrome, and overall cardiovascular mortality. Most exercise programs utilize continuous aerobic exercise as a model for fat reduction. However, there are numerous benefits that can be found with HIIT in particular as opposed to general aerobic exercise. One article found that certain benefits, especially weight loss, are amplified with HIIT. The authors found that HIIT was associated with increased patient compliance, meaning people were more likely to actually follow through with their exercise plans. It also found that there were improved cardiovascular and metabolic outcomes, even in populations at risk for such problems.^[2] However, it did identify that some patients may require specific assessments or instructions before commencing a HIIT program.

A HIIT exercise program can be developed in a number of ways, as long as the key principles are adhered to. Some sources suggest using the talk test as a guide; the idea behind the talk test is that the exercise you are doing when performing the high-intensity activity is that it should be difficult to talk while performing the activity.^[1] However, when performing the lower-intensity part of the workout, talking should be comfortable and you should be able to carry a conversation. Very generally speaking, that tells you that you are correctly performing a HIIT workout program. This article will discuss HIIT and its health benefits.

Physical Fitness and Blood Pressure in Middle-Aged and Older People

One study examined whether HIIT as a walking exercise could increase thigh muscle strength and peak aerobic capacity and reduced blood pressure more than moderate-intensity continuous walking training.^[3] In this study, 60 men and 186 women with a mean age of 63 were divided into three groups: no walking training, moderate-intensity continuous walking training, and high-intensity interval walking training. Participants in the moderateintensity continuous walking group were instructed to walk at approximately 50% of their peak aerobic capacity for walking, using a pedometer to verify that they took 8000 steps or more per day for four or more days per week. Those in the HIIT group, who were monitored by accelerometry, were instructed to repeat five or more sets of threeminute low-intensity walking at 40% of peak aerobic capacity for walking, followed by three minutes of high-intensity walking above 70% of peak aerobic capacity for walking per day for four or more days per week. In addition, other items were measured, including isometric knee extension and flexion forces, peak aerobic capacity for cycling, and peak aerobic capacity for walking. All items were measured both before and after the training.

The study results showed that targets were met by nine of 25 men and 37 of 59 women in the no-walking-training group, by eight of 16 and 43 of 59 in the moderate-intensity continuous-walking-training group, and by 11 of 19 men and 31 of 68 women in the high-intensity interval-walking-training group. In the HIIT group, isometric knee extension increased by 13%, isometric knee flexion by 17%, peak aerobic capacity for cycling by 8%, and peak aerobic capacity for walking by 9%. All were statistically significant changes, and all were greater than the increases observed in the moderate-intensity continuous-walking-training group. In addition, the reduction in resting systolic blood pressure was higher for the HIIT group. The authors concluded that interval walking, specifically the HIIT, may protect against age-associated increases in blood pressure and decreases in thigh-muscle strength and peak aerobic capacity.^[3]

Reducing Cardiometabolic Disease Risk

One article looked at the potential of HIIT to reduce cardiometabolic disease risk.^[4] Over a third of adults in some countries (like the United States) meet the criteria for metabolic syndrome, including cardiometabolic factors like elevated waist circumference, high triglycerides, issues with blood sugar or blood pressure, and decreased HDL (the good cholesterol). Lifestyle modification is strongly recommended as a first-line approach for these cardiometabolic factors.^[4] In this study, the authors reviewed the effect of physical activity specifically on the ability to incorporate HIIT, and whether or not it resulted in a superior or equal improvement of fitness and cardiovascular health.

The premise of using HIIT in healthy and clinical populations is that the vigorous activity segments promote greater adaptations through increased cellular stress, yet the short time periods and the following recovery intervals allow even untrained individuals to work harder than they would otherwise through ongoing continuous activity. The authors looked at the health outcomes of 24 separate studies on HIIT. A minimum duration of 12 weeks was necessary to demonstrate improvement in fasting blood sugar in 57% of the studies. A minimum of eight weeks was required for improvements in HDL in 30% of the studies. No studies reported that HIIT resulted in improvement of total cholesterol, LDL, or triglycerides. At least 12 weeks were required for changes in blood pressure and for improvements in measures of obesity and overweight/obese individuals. HIIT was shown to be safe and effective in patients with a range of cardiac and metabolic dysfunction. The authors of this study conclude that HIIT appears to promote superior improvements in aerobic fitness and similar improvements in some cardiometabolic risk factors, when performed by participants for at least 8–12 weeks.^[4]

Take-Home Messages

In this article, we’ve discussed HIIT. The exact types of exercise and exact durations of the high-intensity and low-intensity bouts can vary and should depend on your current fitness level and what you are trying to accomplish. There are some safety concerns with HIIT. For example, people who have been sedentary for long amounts of time have an increased risk of coronary disease due to high-intensity exercise. It is worth getting clearance from your practitioner prior to starting any HIIT. You may also want to consider doing regular exercise for a while before moving into an HIIT regimen, so that you can establish a good baseline fitness level. Also, if you are exercising in groups, it’s important not to compare yourself to others and just focus on an intensity level that is right for you.

We’ve highlighted some of the key benefits of HIIT. HIIT can help with weight loss, above and beyond what regular aerobic exercise can accomplish. We’ve also seen that it can help with fitness, range of motion, and numerous metabolic factors like blood pressure, blood sugar, and high cholesterol. Particularly if you suffer from metabolic syndrome or a combination of any of the above risk factors, HIIT may be a good choice for you, along with healthy dieting and lifestyle modifications such as smoking cessation. In terms of how long you need to be using HIIT to see a benefit, the studies suggest that at least 8–12 weeks, exercising at least four days a week, is a minimum time frame. It is likely that the longer HIIT is used, the more the benefits increase. If you’d like to find out more about HIIT, please consult your naturopathic doctor.