Why HIIT

This is an excerpt from Intense by Irene Lewis-McCormick.

As mentioned, HIIT can offer amazing health and fitness benefits using workouts that are shorter and performed less often than aerobic activities, in which improvements come with a greater volume of training, meaning more time running, swimming, cycling, or using an aerobic machine such as a treadmill. The greatest appeal of HIIT is its time-saving attributes, but the benefits go much further to include a cumulative, broad range of physiological gains in both health and performance.


HIIT Versus Discontinuous Interval Training

For several years, research has consistently shown that interval training increases overall levels of fitness and burns more calories over a short period of time as compared with steady-state aerobic exercise. In the past, the traditional approach to interval training typically consisted of cardio workouts that alternated steady-state exercise with higher workloads (intervals) for short periods and provided positive recovery periods in a variety of time frames. For example, in the traditional interval approach, you determine the length and speed of each high-intensity interval based on how you feel that day. After warming up, you might increase the intensity for 30 seconds and then resume your normal aerobic pace. The next burst of more intense activity may last 2 to 3 minutes. The intensity and how often you change or add an interval and for how long are largely determined by you.


This personally established approach to interval training is referred to as discontinuous interval training because, unless otherwise specified, the approach to each interval and recovery period is neither systematic nor controlled. Such an approach to interval training is useful, because it offers exercisers the flexibility to intersperse harder bouts of discontinuous loads of high-intensity movements with lower-intensity recovery periods and helps improve anaerobic as well as aerobic capacity, but in much longer training sessions with much lower microbursts of intensity. However, unlike HIIT, traditional discontinuous interval training does not consist of precise, specific time frames in which to perform the higher-intensity workloads and is not necessarily systematic.


In the HIIT protocols presented in this book, the interval ratios are clearly prescribed, detailed, and specific. Additionally, the concept of negative recovery is a major difference between the more random approach of discontinuous interval training and HIIT. Because of the negative aspect of recovery in precise ratios, HIIT is harder and can yield greater training results.


HIIT Versus Steady-State Training

Research has demonstrated the effectiveness of HIIT protocols by contrasting them with steady-state endurance, or aerobic, exercise. To appreciate the power of HIIT, it’s important to understand the difference between steady-state, aerobic endurance activities and high-intensity exercise.


Steady-state aerobic activity, or endurance exercise, is simply a form of cardio exercise paced at a continuous, steady rate. This can be defined as exercise performed continuously, such as walking or running for at least 20 minutes at a pace at which oxygen supply meets oxygen demand; the heart rate stays at a constant pace and you do not become breathless. During a HIIT protocol, on the other hand, you vary your energy output and become breathless, or close to it, for short periods of time. V over timeO2max is also considered the body’s upper limit for consuming and distributing oxygen for the purpose of energy production and is considered a good predictor of exercise performance. V over timeO2max is considered the gold standard for determining peak power output, or the maximal physical work capacity a person is capable of. For most healthy people, the V over timeO2max during a steady-state workout is somewhere between 50 and 70 percent. The rate of oxygen consumption increases as the level of intensity increases - for example, from rest to easy, from easy to difficult, and from difficult to maximal effort.


Additionally, the cardiovascular system adapts to aerobic stressors by increasing in functional capacity. The application of a stressor in exercise science terminology is referred to as overload. When a system of the body (e.g., the cardiovascular system) is overloaded through aerobic activity, it responds by becoming stronger, more resilient, and better able to handle the stress of greater activity and at more intense levels. Aerobic overload results in a stronger heart muscle, improved lung capacity, and better overall cardiorespiratory performance. These parameters are measured by heart rate, stroke volume, and the contractility (the ability to contract with force) of the heart muscle. These factors also assist in blood flow, which allows the oxygen supply to meet the oxygen and energy demands of the working muscles during aerobic activities.


However, there are more than just heart health rewards to be gained from aerobic exercise. The list of benefits is actually quite long. They include increased contraction of skeletal muscles, which also boosts blood flow, making venous blood return to the heart more efficient. This quicker return of blood to the heart increases how quickly blood can refill in the ventricles (the chambers of the heart), which results in an increased preload. This elevated preload adds to the heart’s ability to expel blood quickly, which in turn contributes significantly to enhanced aerobic performance. The following physiological markers are just a few more of the many benefits of aerobic activity:

  • Increased size of heart muscle (stronger heart)
  • Increased stroke volume (more blood flows out with each heartbeat)
  • Increased rate of oxidized enzyme efficiency (creates ATP energy with greater efficiency)
  • Increased rate and efficiency of oxygen and fuel getting into muscle
  • Greater endurance of slow-twitch muscle fibers (Type I, slower to fatigue)
  • Increased reliance on fat as an energy source
  • Increased number of mitochondria (energy factory of a muscle cell)
  • Better ability to dispose of waste products created in the muscles during exercise (onset of blood lactate accumulation, or OBLA)


But even with all this evidence demonstrating the positive effects of aerobic activities, much of the newer research is demonstrating that HIIT protocols provide the same health and performance benefits, and more. A study performed by Helgerud and colleagues (2007) showed that HIIT was significantly more effective in improving maximal oxygen uptake (V over timeO2max) and stroke volume (the amount of blood that pumps out of the left ventricle each time the heart beats) than steady-state aerobic activity.


Another study examined participants who performed a HIIT walking workout (walking on a treadmill at 80 to 90 percent of V over timeO2max) 3 times a week for 10 weeks. These participants were compared with a control group who performed an aerobic walking program at 50 to 60 percent of V over timeO2max. The HIIT group had a 12 percent increase in the size of the left ventricle of the heart as well as improved heart contractility when compared with the control group. This research is particularly significant because the participants in this study were coronary artery disease patients going through rehabilitation, yet they were able to safely improve their health and performance using a HIIT walking protocol (Slordahl et al., 2004).

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Learn more about The HIIT Advantage: High-Intensity Workouts for Women.

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