Historically, some things within the fitness industry have been glamourised or had the result of a study exaggerated, beyond the parameters in which it was conducted. When exercise scientists first began establishing the benefits of high intensity interval training (HIIT) – i.e., higher fat burning efficiency and muscle building power in a fraction of the time? (Daussin et al., 2008) – to some, it felt like the holy grail of workouts had been unearthed.
With so many benefits spruiked, it’s easy to see how the popularity of HIIT grew so fast; with many members of the general population including it in their exercise regimes. However, like all things in life, is there a point in which it can do more harm than good?
Before we dive in we must understand what HIIT is and how it differs from its abandoned cousin, steady state training (SST).
HIIT workouts can be performed in all exercise modes including cycling, walking, swimming, and in many group exercise classes. It is often marketed as an efficient workout to achieve fat loss and muscle gain.
HIIT workouts are periods of very high-intensity work above 80 per cent heart rate (HR) followed by a period of less intense or no work until you recover. A workout typically lasts around 30 minutes, which is sufficient time to have you at maximal heart rate for about 10 to 15 minutes. The ‘work’ set should take you to your limits so as to engage your ‘fight or flight’ response, and flood your body with stress hormones. This, in short, burns fat and helps build muscle.
A common fault in attempting HIIT however, is insufficient recovery during rest periods. Timed group training doesn’t cater to all fitness levels, so it does not necessarily allow your HR to lower; thus removing the very important ‘interval’ component, making the workout high-intensity training (HIT), which can increase the risk of injury or illness.
In contrast, SST is defined as exercise (e.g., running, cycling, swimming, etc.) that is held at a steady intensity for 20 minutes or longer. Sadly, this method of training has been disregarded by the masses, as it is assumed to take longer to generate results, is thought to be ideal only for endurance athletes, or for fear it may reduce muscle mass gains. However, this could not be further from the truth.
Let’s discuss and compare the cardiovascular, skeletal muscle, and metabolic adaptations to HIIT versus continuous endurance exercise.
Daussin et al. (2007) measured VO2max responses among men and women who participated in an eight-week HIIT and a continuous cardiovascular training program. VO2max increases were 6 per cent higher with the HIIT program (15 per cent), compared to the continuous aerobic training (9 per cent).
Increased mitochondria (the energy factory of the cell) size and number is becoming a hallmark adaptation to HIIT (Gibala, 2009). Mitochondria uses oxygen to manufacture ATP (the energy molecule of the muscle cell) at high levels through the breakdown of carbohydrates and fat during aerobic exercise. With increased mitochondrial density there is more energy available for the working muscles to produce greater force, and for a longer period of time. An increase in these mitochondrial oxidative enzymes leads to more effective fat and carbohydrate breakdown for fuel.
In a six-week training study, Burgomaster et al. (2008) showed similar increases in oxidative enzyme levels (proteins in mitochondria that liberate ATP) among subjects who performed a HIIT program 3 days/week and subjects who completed steady cycling at 65 per cent VO2max on 5 days/week.
Increasing mitochondrial density can be considered a skeletal muscle and metabolic adaptation. One focal point of interest for metabolic adaptations is with the metabolism of fat for fuel during exercise.
Perry et al. (2008), Talanian et al. (2007) both showed that fat oxidation, or fat burning was significantly higher and carbohydrate oxidation (burning) significantly lower after six weeks of interval training.
Another metabolic benefit of HIIT training is the increase in post-exercise energy expenditure referred to as Excess Post-exercise Oxygen Consumption (EPOC). Following an exercise session, oxygen consumption (caloric expenditure) remains elevated as the working muscle cells restore physiological and metabolic levels in the cell to pre-exercise levels. This translates into higher and longer post-exercise caloric expenditure. LaForgia, Withers and Gore (2006) showed that exercise intensity studies indicate higher EPOC values with HIIT training as compared to continuous aerobic training.
The major goals of most gym goers is to improve cardiovascular, metabolic, and skeletal muscle function in the body. For years continuous aerobic exercise has been the chosen method to achieve these goals. However, research shows that HIIT leads to similar and in some cases better improvements in shorter periods of time. Incorporating HIIT (at the appropriate level of intensity and frequency) into a client’s cardiovascular training allows exercise enthusiasts to reach their goals in a very time efficient manner. And, since both HIIT and continuous aerobic exercise programs improve all of these meaningful physiological and metabolic functions of the human body, incorporating a balance of both programs for clients in their training is clearly the ‘win-win’ approach for successful cardiovascular exercise improvement and performance.
Maybe not, according to this study of the molecular effects of HIIT, as almost everyone who worked out strenuously everyday, developed severe declines in mitochondrial dysfunction, metabolic health, and glucose intolerance. And, all they did was increase from 4 to 8 minute intervals over 36 minutes to 152 minutes by the fourth week on a stationary sprint bike. Even then, their results came back with insulin resistance similar to a person developing diabetes, and mitochondrial respiration fell by an average of 40 per cent from week 1. Interestingly, their metabolic issues reversed once they dialled back on the number of workouts in the week, suggesting that the benefits of HIIT may just depend on just how much we do.
One of the most remarkable (but perhaps not surprising) findings of this study comparing high-intensity to moderate or low intensity workouts, is that over the course of an eight-week training block the level of enjoyment dropped significantly for those that participated in the Tabata HIIT protocol (Bartlett et al. 2011, Jung et al. 2014, Kilpartrick et al., 2012). Of course, regardless of how effective an exercise training program might be, adherence over any meaningful period of time is unlikely in programs that are not enjoyable.
Les Mills even came in with their own study of overtraining, showing that HIIT has a recommended dose of 30 to 40 minutes above 90 per cent maximum heart rate per week. The researcher behind the Les Mils study, Dr Jinger Gottschall, Associate Professor at Pennsylvania State University, also states that she suggests only introducing HIIT after six months of prior training five days a week. At this point HIIT can be introduced one to two times a week with two sleep cycles between them.
With these results suggesting such seemingly low bouts of HIIT, how would a professional athlete get away with it? Surely they’d do more than four weeks of more than two high-intensity workouts.
After all, HIIT is often marketed to look like a pro athlete’s workout routine. The problem is, everything a professional athlete does around the training session or the length of the session isn’t shown or talked about at the general population level. An athlete knows the risks of being such a high performer and is willing to sacrifice long-term health for their sport. But even with the high risk, big measures are put in place to reduce burnout, injury and optimise training.
Professional athletes spend most of their time in recovery strategies, with the average athlete spending four hours recovering (using different methods), to every one hour of training. Without effective recovery, fitness diminishes, and all the hard work in the gym is lost. Of course, in addition to this, is the actual training; as gaining fitness is completely different to maintaining it.
You may have heard of a ‘pre-season’ or comp-prep stage of training. This is the limited amount of time an athlete will work hard using HIIT in order to increase their fitness levels. In contrast, they’ll undertake lower intensity workouts, as a way to maintain their fitness; this also helps them minimise the risk of injury or burnout. When you think about it, this means that what an athlete will do a few times a week for 4 to 12 weeks in preparation for a season or competition, some members of the general population are doing every day (or sometimes multiple times a day), all year-round!
When it comes to healthy training, like so many other things in life, balance is key; as is understanding the impact of stress imposed on your body from different types of workouts; and understanding how to recover effectively.
Here is an example of how to work smarter not harder, and how to keep it real:
Tim is the Education Manager at Functional Training Institute. In recent times, Tim coaching focus has changed from bodybuilding, to corrective strength and sports science. He is committed to enhancing the knowledge and practises of other fitness coaches and allied health professionals.