Functional Training Institute

Is HIIT making you sick or fit?

HIIT workout benefits risks training

HIIT workouts promise better results in less time, but they can take a toll on your body.


Main topics discussed:

  1. The difference between HIIT and SST
  2. When is HIIT too much? Plus Healthy Prescription of HIIT & Recovery
  3. How do pro-athletes do it?


The fitness industry has a bad habit of glamorizing things that aren’t actually good for you, or exaggerating the result of a study far beyond the parameters which it was conducted.

When exercise scientists first began establishing the benefits of High-Intensity Interval Training – AKA HIIT, it felt like we’d unearthed the holy grail of workouts.

Higher fat burning efficiency and muscle building power in a fraction of the time? (Daussin et al., 2008)


With so many benefits, I understand the popularity of HIIT and why more people are including it in their exercise regimen.

However, is it always beneficial for everyone to indulge in HIIT or are there cases when it can do more harm than good?

Before we dive in we must understand what HIIT is and how it differs from it’s abandoned cousin, steady state training (SST).

HIIT workouts can be performed on 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%  HR) followed by a period of less intense or no work till you recover. The workout lasting about 30 mins should have you at maximal heart rate for about 10-15  mins. The ‘work’ set should have you at your limits to engage your “fight or flight” response to flood you with your stress hormones, which in short, burn fat & help build muscle.

A common fault in attempting HIIT is not recovering enough during rest periods. Timed group training doesn’t cater to all fitness levels, not allowing your HR to lower makes your workout become High-Intensity Training (HIT). This removes the very important ‘interval’ component making the risk of injury & illness much greater.

Whereas Steady State training (SST) is defined as exercise (e.g., running, cycling, swimming, etc.) lasting greater than 20 minutes and held at steady intensity during the entire bout.

This method over recent years has been left to the side lines as it takes longer, assuming only endurance athletes require it, or a fear it may lose muscle mass gains.

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.

Cardiovascular adaptations

Daussin et al. (2007) measured VO2max responses among men and women who participated in an 8-week HIIT and a continuous cardiovascular training program. VO2max increases were higher with the HIIT program (15%) as compared to the continuous aerobic training (9%), 6% difference.

Skeletal muscle adaptations

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 6-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% VO2max on 5 days/week.

Metabolic adaptations

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 6 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 (E.P.O.C.). 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, & Gore (2006) showed that exercise intensity studies indicate higher E.P.O.C. 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.

So, if it’s these improvements we want from HIIT, how much do we do?

Does doing more give us better results?

Maybe not, according to this study of the molecular effects of HIIT, almost everyone that worked out strenuously everyday developed severe declines in mitochondrial dysfunction, metabolic health, and glucose intolerance. And, all they did was increase from 4-8 minute intervals over 36 minutes to 152 minutes by the 4th 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% from week 1.

Their metabolic issues reversed once they dialed 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 8-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, Kilpartirck et al., 2012)

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.

Even Les Mills comes in with their own study of overtraining. Showing that HIIT has a recommended dose of 30-40 minutes above 90 percent maximum heart rate per week. The researcher behind the Les Mils study, Jinger Gottschall, Associate Professor at Pennsylvania State University also states that she suggests only introducing HIIT after 6 months of prior training 5 days a week. At this point HIIT can be introduced 1-2 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 4 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 gen pop level.

An athlete knows the risks of being such a high performer and is willing to sacrifice long-term health for their sport. How many athletes do you hear about living well after 40?! But even with the high risk, big measures are put into place to reduce burn-out, injury and optimize training.

The component of fitness athletes spend most of their time in is recovery strategies. Recovering from these high-intensity workouts is hard & a full-time job! Something the gen pop is seriously lacking. Without effective recovery, fitness diminishes & all the hard work in the gym is slowly stepping people backwards more than forwards. The average athlete will spend 4 hours to every 1 hour of training using various recovery methods.

But next to this is the actual training, 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 at HIIT to gain fitness but stop to do low-intensity workouts to maintain fitness due to the extremely high risk of injury, or burnout. So what an athlete will do a few times a week for 4-12 weeks, the gen pop does year round every day, sometimes multiple times a day.

It all comes back to that balance, understanding the stress imposed on your body during a workout and your lifestyle, then understanding how you can effectively recover.

Work smarter, not harder.

As an example:

  • 2x per week HIIT
  • 2x per week low intensity strength training
  • 2x per week low intensity movement based activities

And of course, lots of rest…

Keep it real

Tim Sunderland
Education manager at Functional Training Institute

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