By HIIT, I do not mean that it is popular (although, finally and thankfully, it is certainly becoming so), but rather, that sprint interval training (SIT) is also referred to as high-intensity interval training or HIIT. It is of major importance that health care professionals make sure that the general public, as well as many contemporaries, get thoroughly educated about the superior health and fitness benefits of SIT, or "burst" training, as compared to low- to moderate-intensity continuous training (LMICT). This is because, despite research to the contrary, most people still believe that to develop a healthy heart and to lose weight, the best mode of exercise is long and continuous "cardio" exercise, which, inherently, requires a significant investment of time. And, of course, lack of time is the number one excuse given for not complying with an exercise program. Before getting into the health and fitness benefits of SIT or HIIT, I want to make some clarifications about the definitions and also make a few comments about training for individuals that want to take part or compete in endurance events.
Interval training refers to intermittent exercise involving periods of exercise followed by periods of recovery, which enables anyone to increase the intensity of the exercise workload. A pretty simple concept. The problem, however, with the term "high-intensity" is that it is descriptive and, obviously, relative to an individual's level of fitness and dependent upon one's tolerance to exertion. Running at five miles per hour may be an all-out effort for some, whereas, it may be a walk in the park for others. While it is easy to assign relative exercise intensities for both training and research purposes by first measuring maximal capacities, one also needs to understand that the term "high-intensity" is used in the scientific literature to describe intensities ranging from as low as 85% of maximal oxygen uptake (VO2 max or aerobic capacity) to as high as 250%. These intensities also need clarifying for people who incorrectly assume that 100% VO2 max indicates an all-out effort, which is not the case; if it did, studies could not prescribe intensities above 100% VO2 max. These exercise intensities are related to the maximal workload achieved when measuring an individual's VO2 max in a progressively-graded exercise stress test that can last for 10 or more minutes. This is not the same as asking someone to sprint as fast as they can for,say, 20 seconds; the workload or speed, in this case, would be considerably higher than the workload corresponding to 100% VO2 max. For example, if the workload on a treadmill eliciting VO2 max was 8 mph at a 5% grade, then a workload corresponding to 150% VO2 max would be 12 mph at a 5% grade (i.e., one and a half times the VO2 max workload). Furthermore, for many individuals, even this workload would be well short of an all-out effort. When one is exercising above 100% VO2 max, it is termed "supra-maximal" and essentially defines SIT. Because of the wide range of intensities reported in the research for HIIT, the duration of the exercise interval can also range from as low as six seconds to as high as four minutes and sometimes even longer. As a consequence, these issues need to be elucidated when using the term "HIIT" because, when used alone, it can be limiting in terms of understanding the exercise prescription. In contrast, SIT defines a narrower range of intensities that essentially fall within a time constraint of one minute or less. This is simply because, for any human, the intensity of a maximal effort drops off precipitously when the duration goes beyond sixty seconds.
Now, the necessary LMICT that endurance athletes employ in their conditioning programs is irrelevant in the scope of this article since I am not arguing that endurance athletes do not need to do "volume" training. I will, however, argue that endurance athletes, while extremely impressive in their physical accomplishments, are not the healthier athletes when compared to sprinters. Further, while the endurance athlete has a need to maintain a high submaximal intensity for long periods to be successful, the vast majority of athletes, and certainly humans in general, have no need for this type of activity as will be discussed later. It is also noteworthy that interval training is not some new concept to the endurance athlete. They have long employed SIT and HIIT and, for many elite endurance athletes today, HIIT can comprise as much as 50 to 75% of their total training volume. I do not think the average person starting an exercise program realizes this; rather, I believe they think athletes such as Lance Armstrong simply ride their bikes nonstop to achieve their incredible levels of cardiovascular endurance. While there is considerable debate as to the magnitude of the intensity and how much interval training an endurance athlete should include in his or her conditioning program, all successful endurance athletes employ a significant amount of HIIT in their training protocols. However, the HIIT many typically employ is, relatively, at the low end of the intensity spectrum with work intervals often lasting four or more minutes in length. As will be demonstrated in this article, I believe even the endurance athlete would benefit considerably by reducing their volume training and supplementing their "low- to moderate-intensity" HIIT with a significant amount of supra-maximal HIIT. This thought is supported by a study that investigated the relationship between tests of anaerobic (non-oxidative metabolism) power and 10K running performance and which demonstrated that all tests of anaerobic power were significantly correlated with 10K run time and just one plyometric leap test accounted for nearly 75% of the variance in 10K run time(1). When combined with the 300-m sprint time, the variance in 10K run time increased to nearly 80%. However, the wisdom of my recommendation to the endurance athlete is relatively unimportant when one considers that the most pressing issue for society is getting the general public to engage in a successful exercise program that improves their overall health, not to help them cross the finish line in April near Fenway Park.