HIIT [High‐intensity Interval Training] and Cardiovascular Health Research

HIIT [High‐intensity Interval Training] and Cardiovascular Health Research. Okay, so let us take a look at some of the research that should help direct people in a more successful direction and might also bring some “cardio” junkies to discover a more effective and time‐efficient exercise regime. One of the first points I like to emphasize is that in 1995, the American College of Sports Medicine (ACSM) and the Centers for Disease Control and Prevention (CDCP) convened to assess the, then, current research and to produce a position statement with respect to exercise guidelines. Much of the motivation came from the lack of success that the guidelines of that time were producing.

A pertinent conclusion from the study was "accumulation of physical activity in interval, short bouts is considered an appropriate approach to achieving the activity goal" and that activity goal should be to utilize between 120 and 210 calories a day due to exercise. It is worth noting here that this calorie goal is easily accomplished with HIIT in short order. Perhaps the most positive outcome of this position statement was that it appeared to trigger a much needed increase in research comparing HIIT with LMICT. Coincidence or not, in 1996, Tabata et al. examined the effect of six weeks of moderate‐intensity endurance training (70% VO2 max, 60 minutes per day, five days per week) compared to six weeks of HIIT (170% VO2 max, 7‐8 sets of 20 seconds with 10 seconds recovery between bouts; so SIT in this case).

Both training methods significantly increased VO2 max (aerobic capacity); however, while the endurance training had no impact on the anaerobic capacity, HIIT increased it by 28%. It was, therefore, concluded that the HIIT imposed intensive stimuli on both energy systems. A two for one then, anyone out there not wanting that? Of course, unless you understand the importance of having a well developed anaerobic metabolism one might not be impressed by these findings. Although one should, simply by the fact that the HIIT had the same improvement in the aerobic capacity in a fifteenth of the time. Yes, you did read that correctly.

Compared to 60 minutes of endurance training, a high‐intensity interval workout, lasting less than four minutes, produced a comparable increase in VO2 max while simultaneously increasing the anaerobic capacity. And, having a high anaerobic capacity makes your overall functional capacity and resultant quality of life, as well as performance, much greater.

Tabata’s study demonstrates an extremely time‐effective approach to exercise, particularly for those individuals who lack the time or desire for lengthy workouts. It also has obvious implications upon the training programs of athletes from interval‐based sports, since they require a high anaerobic capacity and the endurance to reproduce multiple repetitions of high energy output.

Studies such as this, that compare training protocols side by side, are particularly powerful. A very recent and similar study by Gibala, et al. compared the effect of low volume SIT with high volume endurance training (ET) upon exercise performance and muscle adaptations. Both training protocols utilized six training sessions over the course of just two weeks, on the Monday, Wednesday and Friday of each week. The SIT consisted of four to six repeats of 30 second “all out” cycling (approximately 250% of VO2 max) with four minutes of recovery between each repetition and the ET consisted of 90 to 120 minutes of continuous cycling at approximately 65% VO2 max. This resulted in a 90%, yes 90%, lower training volume for the SIT (630 kJ) versus the ET (6500 kJ) and nearly a fifth of the time commitment.

Omitting the recovery intervals, the actual exercise time for the ET (630 minutes) was 42 times greater than for the SIT (15 minutes). Despite the large disparity in the time commitment of the two training protocols, both groups had similar improvements in two self‐paced cycling time trials that required approximately two and 60 minutes to complete.

Additionally, similar increases were seen in muscle oxidative capacity, buffering capacity (ability to process lactic acid) and glycogen content (muscle fuel). While it is impressive that similar physiological and performance adaptations took place; despite a large difference in the training volume, I suspect that a time trial lasting only one minute may well have produced a better performance from the SIT group than the ET group because of the greater anaerobic contribution in an activity lasting one minute as compared to two minutes. Either way, the study clearly demonstrates, again, the superior time efficiency of SIT to LMICT and that SIT is effective at improving the ability to perform LMICT when required.

In a previous study from the same research group, it had been shown that a very similar SIT protocol elicited a twofold increase in cycle endurance capacity, measured as time to fatigue at 80% VO2 max (an average of 26 minutes pre training to an average of 51 minutes post training)18. This study by Burgomaster et al. further demonstrated that muscle oxidative potential, as measured by an increase in citrate synthase (CS) maximal activity (a marker for oxidative metabolism) increased 38% and resting muscle glycogen content increased by 26%, another benefit for improving performance. But the ability to double one’s endurance capacity in just two weeks utilizing only 16 minutes of SIT is worthy of some major publicity. While the results of the study did reach the media, the news was typically lost in the vast sea of “cardio” dogma.

The study also brought to light a very important principle when it comes to SIT. Para et al. had previously shown an increase in muscle oxidative potential with daily SIT but without an improvement in anaerobic work capacity19. It was concluded that chronic fatigue may have contributed to the lack of an improvement in this important physiological parameter; however, the study by Burgomaster et al. was able to demonstrate an improvement in peak power output, a marker of anaerobic performance, while simultaneously improving endurance capacity using less training days and more rest days.

So, when it comes to SIT, less can be more; so, focus on quality not quantity. I have long said, “inactivity is a bad thing but rest is a good thing and there is a big difference between the two.” It is during rest that the body adapts to the stress of exercise and it is particularly relevant when resting from SIT or HIIT where it has been shown that very short time periods of activity can have a significant physiological effect.

There are a vast number of studies that lend considerable support to the argument that SIT, compared to LMICT, has an equal or greater cardiovascular training affect while simultaneously improving anaerobic capacity. A study that examined the anaerobic capacity of untrained, endurance‐trained, and sprint‐trained young men, showed that there was no difference in anaerobic capacity between the untrained and endurance‐trained subjects; whereas, the anaerobic capacity of the sprinters was 30% greater20.

This study demonstrated that the endurance training utilized by the endurance‐trained subjects (namely, LMICT) had little impact on their anaerobic capacity while, SIT, the training employed by the sprint‐trained men, significantly elevated it. Because of the LMICT dogma that exists within the medical field and fitness community, many people are often surprised to learn of the positive physiological effect SIT has on the cardiovascular system and aerobic capacity of an individual. But, studies examining the kinetics of oxygen uptake during short‐term intense exercise reveal that the contribution of oxidative metabolism is early and significant.

This very important finding has been classically misunderstood by the medical and fitness industries and is perhaps one of the main reasons for such a bias toward long‐duration “cardio” exercise. As early as the late 1980s, studies had demonstrated oxidative energy contributions as high as 40% in intense exercise lasting 30 seconds and 50% for exercise lasting a minute. More recent studies have shown an even greater contribution. It has been discovered that a threefold increase in muscle oxygen uptake can take place within only six seconds of intense activity (peaking at 50 seconds) and that oxidative metabolism can contribute as much as 40% within 15 seconds of short‐term exhaustive running (peaking as soon as 25 seconds at 79% of VO2 max). Further, a comparison of the 30‐second Wingate anaerobic power test and a graded VO max cycle ergometer test, showed a significant difference in muscle deoxygenation. During the Wingate test, deoxygenation reached 80% of the established maximum value, whereas, in the VO2 max test, it reached only 36%. Significantly, there was no delay in the onset of deoxygenation in the Wingate test, while it did not occur under low intensity work in the VO2 max test. Another study that examined 10, 6‐second maximal sprints with 30 seconds of recovery between sprints, showed a slow decline in power output without a change in muscle lactate indicating a large contribution from oxidative (aerobic) metabolism.

So what does all this research actually demonstrate? Well, it shows that when you exercise with a maximal effort, your cardiovascular system and aerobic metabolism go on “full burner” to provide you as much energy as quickly as it can – it actually cannot provide all the energy needed (this is where your anaerobic metabolism comes into play), but it will give you everything it has. This is actually easy to appreciate when examined from the perspective of an example. If you were to go hiking up a mountain trail that you anticipated would take approximately one hour to get to the top, you would pace yourself so as to not get fatigued too quickly even if it was your goal to make it as fast as you could.

On the other hand, if a mountain lion arrived on the scene, you had better hope that you can find some form of cover pretty quickly. Let us say you were able to sprint to a tree in approximately one minute and climb out of harm’s way (we are assuming, of course, this lion’s climbing ability is not that good!). Now, a simple question. Are you breathing harder after escaping the lion or after reaching the summit of the trail? I know you know the answer to this and common sense should tell you that escaping that lion would place an immediate stress on your cardiovascular system and aerobic metabolism, which is one reason why you are breathing so hard.

A very basic physiological principle is that the human body adapts to stress; so, if you escape mountain lions on a regular basis or, better still, mimic the running part of it in a more controlled environment, your body is going to change for the better.

The study by Tabata et al., discussed earlier, is a good example. Another reason you would be breathing very hard (and harder than after the reaching the trail summit) is that you are now needing additional oxygen to replenish significant energy stores that were hastily utilized, via non‐oxidative metabolic pathways, to save your life.

You would also be needing to deal with the large amounts of lactic acid that accumulated in your muscles as a result of this anaerobic effort. This additional oxygen consumption following exercise is termed “oxygen debt” or “excess post‐exercise oxygen consumption” (EPOC). EPOC is a particularly important issue when addressing fat loss and it is important to remember from this example that the intensity of an exercise, as opposed to the duration, has the greatest influence on the magnitude of this elevated oxygen consumption. And, this elevated oxygen consumption continues to have a training effect on the body, as well as to simply continue burning calories, even after the actual activity has ceased.