So we’re going to assume you’re a masochist and you actually perform your HIIT intervals at 80%+ of your max.

HIIT can be a really time efficient to achieve fairly substantial health and fitness gains.

Or it can kill the gains you’ve made and make you weaker and smaller.

It just depends.

HIIT: The Good

When compared to MOD (moderate intensity continuous exercise) High Intensity Interval Training (HIIT) appears to be the bees knees.

It’s been shown time and again to

Time Commitment?

You can get a lot of fitness done in not a lot of time,

“As little as 3 HIIT sessions per week, involving≤10 min of intense exercise within a time commitment of ≤30 min per session, including warm-up, recovery between intervals and cool down, has been shown to improve aerobic capacity, skeletal muscle oxidative capacity, exercise tolerance and markers of disease risk after only a few weeks in both healthy individuals and people with cardiometabolic disorders.”- Is high-intensity interval training a time-efficient exercise strategy to improve health and fitness?

“Mean weekly training time was 55±10 minutes (HIT) and 128±44 min (MICT) (p<0.05), with greater adherence to HIT (83±14% vs. 61±15% prescribed sessions attended, respectively” –Low-Volume High-Intensity Interval Training in a Gym Setting Improves Cardio-Metabolic and Psychological Health

“short-term interval training, using a protocol that involved only 1 min of very intense exercise within a total time commitment of 10 min, (3x’s a week) was a potent stimulus to induce physiological adaptations that are linked to improved health in overweight and obese adults.” – Three Minutes of All-Out Intermittent Exercise per Week Increases Skeletal Muscle Oxidative Capacity and Improves Cardiometabolic Health

Vo2MAx?

It looks like HIIT is at least as effective, and maybe slightly more so, than traditional endurance training in improving aerobic power, especially in the sedentary.

“Our meta-analysis confirms that endurance training and HIT both elicit large improvements in the VO2max of healthy, young to middle-aged adults with the effects being greater for the less fit. Furthermore, when comparing the two modes of training, the gains in VO2max are greater following HIT.”- Milanovic, Weston, Sporis

“data suggest that 2 weeks of HIIT improves V̇o2max and peak and mean power output”- Effect of High-Intensity Interval Training on Cardiovascular Function, V̇o2max, and Muscular Force

“in young individuals with average cardiorespiratory fitness, 20 sessions of periodized HIIT led to significant increases in V˙ O2max which were attendant with increases in maximal SV and CO.” – High-Intensity Interval Training Increases Cardiac Output and V˙O2max

“We found that aerobic capacity increased earlier (after 6 weeks), and more importantly, at 12 weeks in players performing additional short-short HIIT compared with those doing ordinary training.” – Effects of a high-intensity intermittent training program on aerobic capacity and lipid profile in trained subjects

“HIIT could be considered a more effective and time-efficient intervention for improving blood pressure and aerobic capacity levels in obese youth in comparison to other types of exercise.”- Is high-intensity interval training more effective on improving cardiometabolic risk and aerobic capacity than other forms of exercise in overweight and obese youth? A meta-analysis.

“The main finding of this meta-analysis is that interval training produces improvements in VO2max slightly greater than those typically reported with what might be described as adult fitness based continuous training “-VO2max Trainability and High Intensity Interval Training in Humans: A Meta-Analysis

HIT both elicit large improvements in the VO2max of healthy, young to middle-aged adults, with the gains in VO2max being greater following HIT when compared with endurance training.- Effectiveness of High-Intensity Interval Training (HIT) and Continuous Endurance Training for VO2max Improvements: A Systematic Review and Meta-Analysis of Controlled Trials.

Training Motivation, Enjoyment and Adherence to an Exercise Program?

Seems counterintuitive to me, but people DO report, or at least be willing to perform, High Intensity Interval Training more so than  (MOD) continuous training,

“High‐intensity functional circuit training is a time‐saving, effective and motivating training method for healthy, untrained adults.” –Jan Wilke ,Stefanie Kaiser, Daniel Niederer, Kristin Kalo, Tobias Engeroff, Christian Morath, Lutz Vogt, Winfried Banzer

“performing HIIT either two or three times per week is an enjoyable exercise modality that promotes a sustainable increase in habitual PA (Physical Activity) levels and improves HRQOL (health related quality of life).” – Effects of high intensity interval training frequency on perceptual responses and future physical activity participation

“Adherence to the training intervention was significantly greater in the HIT group (83±14% prescribed sessions attended; n = 42) compared to the MICT group (61±15% of prescribed sessions attended” –Low-Volume High-Intensity Interval Training in a Gym Setting Improves Cardio-Metabolic and Psychological Health

“Our data showed greater enjoyment of HIIT compared to MICT despite higher metabolic stress in the form of significantly higher RPE (rate of perceived exertion), HR )(heart rate), and BLa (blood lactate)….. Despite greater metabolic stress, over 90% of participants preferred HIIT versus MICT”- High-Intensity Interval Training Elicits Higher Enjoyment than Moderate Intensity Continuous Exercise

WTF is wrong with you?

The Ability to Respond to Stress (Heart Rate Variability or HRV)?

“data suggest that for healthy people either continuous or discontinuous aerobic training is effective in improving measures of fitness; however discontinuous is better able to improve supine indices of vagal activity on heart rate variability.“- Effects of Continuous vs Discontinuous Aerobic Training on Cardiac Autonomic Remodeling

“The exercise, compared to the control group, recorded a significant reduction in heart rate accompanied by an increase in stroke volume. For the exercise group forearm vasodilatory capacity was significantly enhanced, P < 0.05. Arterial stiffness, determined by pulse wave velocity and augmentation index, was also significantly improved, after the 12-week intervention. For the exercise group, heart period variability (low- and high-frequency power) and baroreceptor sensitivity were significantly increased. High-intensity intermittent exercise induced significant cardiac, vascular, and autonomic improvements after 12 weeks of training.” – High-intensity intermittent exercise and cardiovascular and autonomic function

*One of the reasons I like writing these is that I learn stuff every time, I had NO IDEA that HIIT had a greater impact than MOD on HRV. Had I been asked before researching this article I would have guessed 100% the exact opposite would have been true do to the stress level (allostatic load) I believed is associated with HIIT (more on this later).

Weight Loss, Insulin Resistance and other Nasty Metabolic Stuff Related to Excess Fat/ Weight?

“The HIIT group showed greater decrease in body fat percentage as well as the improvement of overall anthropometric indices in overweight females”- The Effects of High-Intensity Interval Training and Continuous Training on Weight Loss and Body Composition in Overweight Females 

“The primary finding is ~10 weeks of high-intensity or moderate-intensity exercise training can reduce BFkg by~2 kg and waist circumference by ~3 cm in the absence of body mass changes. These values indicate a modest improvement in body composition from short-term exercise training, with body fat mass decreasing by ~6% from initial levels.” – The effects of high-intensity interval training vs. moderate-intensity continuous training on body composition in overweight and obese adults: a systematic review and meta-analysis: Exercise for improving body composition

“HIIE appears to have a dramatic acute and chronic effect on insulin sensitivity.” – High-Intensity Intermittent Exercise and Fat Loss

“HIIT may have beneficial effects on body composition and cardiometabolic health in overweight adolescents.”- Cardiometabolic risk markers, adipocyte fatty acid binding protein (aFABP) and the impact of high-intensity interval training (HIIT) in obese adolescents

“Modified HIIT reduces liver fat and improves body composition alongside benefits to cardiac function in patients with NAFLD (non alcoholic fatty liver disease)”- Modified high-intensity interval training reduces liver fat and improves cardiac function in non-alcoholic fatty liver disease: a randomized controlled trial

Just put High Intensity Interval Training into Google Scholar and you’ll get back more positive outcomes from HIIT than you can imagine.

Much like creatine, the VAST number of studies done using HIIT find a beneficial effect and generally find it to be slightly superior to MOD or moderate intensity continuous exercise training in most of the tested indices.

Wait? There’s No Downside to HIIT?

Like everything, there’s a limit.

Context needs to be added to the conversation.

What are the actual STRUCTURAL physical adaptations to HIIT vs MOD?

Is there a limit to the adaptations HIIT evokes?

How do you scale HIIT? More intervals? Longer intervals? More frequent training?

HIIT, The Bad:

Well, not really the bad, but the limits and limitations of a good thing.

Here’s the thing, when it comes to the human body you cant just add.

There are limits, or as the Soviets used to call it, CAR (current adaptive resourses).

No one has an unlimited pool of resources to pull from and the body can not adapt to an unlimited amount of stressors.

Every stressor applied to the body is like a debit from a bank account.

Some big, some small, all take from the physical capital you have available.

“The Minute you Take the bar out of the Rack You’ve Introduced a Stressor to the Body”- Buddy Morris

Allostasis:

“Allostasis refers to the adaptive processes that maintain homeostasis through the production of mediators such as adrenalin, cortisol and other chemical messengers. These mediators of the stress response promote adaptation in the aftermath of acute stress, but they also contribute to allostatic overload, the wear and tear on the body and brain that result from being “stressed out.” – Stressed or stressed out: What is the difference?

Done Correctly HIIT Is a MAJOR STRESSOR

Too many stressors = too high an allostatic load = you break.

If I write a program HIIT is ALWAYS treated as weight training NOT aerobic training. .

“Because of the intense nature of the routine, those engaging in HIIT need to be cognizant of the potential for overtraining………….given the high-intensity nature of the protocol, HIIT may be associated with an increased potential for overtraining, especially when combined with regimented resistance training. ” – High-Intensity Interval Training: Applications for General Fitness Training

In my experience, when this isn’t the case the intensity of HIIT isn’t taken into account and HIIT is programmed as the primary from of “cardio” or worse “recovery work” more than 3x’s a week BAD THINGS HAPPEN.

“six-week HIIT can induce large improvements in absolute and relative PPO (peak power output), and small increases in free-T in male masters athletes. Taken together, this indicates there is a place for epochs of HIIT in training regimes of masters athletes, which may result in an improved anabolic environment. Given our previous work detailing that recovery of older adults takes five days to recover PPO following HIIT, carefully timed HIIT may be a pragmatic approach for maintaining athletic capability during periods of time restriction.” – HIIT produces increases in muscle power and free testosterone in male masters athletes

Granted, that’s in reference to Masters Athletes (60 years and older in this study), and younger/ fitter people do generally recovery faster, but 5 days to recover Peak Power Output from HIIT?

I would argue anything that needs 5+ days to recover from is more like intense weight training and less like aerobic conditioning.

If that’s the case, HIIT, when used too often and in too much volume leaves the user open to overtraining,

“Interval training at Vo2Max three times instead of once a week induced a great elevation of plasma norepinephrine measured at the end of the all-out run at Vo2Max which is recognized as an overtraining marker.” –Interval training at VO2max: effects on aerobic performance and overtraining markers

The Hole(s) in the Research:

Most research so far has been done on the sedentary. 

We know that going from nothing to something is going to have a MASSIVE positive effect.

HIIT probably beats continuous aerobic training in these instances because the participants are not as bored and are able and actually want to perform the training sessions and because HIIT is more like weight training the stressor and therefore the stress response is generally greater.

They’re inducing a larger allostatic load to be compensated against.

One of the markers of aerobic exercise is that, all things being equal, until you achieve really high volumes, it’s relatively less sympathetically demanding on the body and therefore easier to recovery from.

Hell, aerobic work, when managed properly, IS recovery work.

The length of the studies done on “Trained” participants. 

How much HIIT is too much and leads to overtraining?

The longest study I found was 12 weeks (I think I saw a 16 week protocol, but I can find it again) and to use the word “trained” is arguable in some.

A famous quote in the strength coach biz is,

Q: What’s the best program?

A: The one you’re not on. 

What the hell does that mean?

All Programs Have Holes

No program can account for everything.

Remember, we have a limited amount of adaptive resources and ALL training (stressors) applied are fighting over those resources.

“Stress is cumulative and non-differentiated”

Adaptation is specific to the stressor(s) applied.

“Specificity is a core principle of exercise training to promote the desired adaptations for maximizing athletic performance. The principle of specificity of adaptation is underpinned by the volume, intensity, frequency and mode of contractile activity and is most evident when contrasting the divergent phenotypes that result after undertaking either prolonged endurance or resistance training.“- Concurrent exercise training: do opposites distract?

You get what you do.

As the length of time you’ve been training increases (training age) you’re adaptability decreases and it takes larger and more intense volumes of work to continue to increase performance.

It also means you need to become more specific in your training due to the limited adaptive resources available for this increased workload.

Eventually, out of necessity, some physiological qualities will almost always be undertrained.

So when you take what are primarily endurance athletes and subject them to a 8 weeks of HIIT they improve in some indices because those are the qualities that had been undertrained.

“A closer look at the data supports the principle of specificity, suggesting that the implementation of a repeated supramaximal sprint interval training program improves the capacity for adenosine triphosphate production through glycolytic means (sup-ported by greater changes in Lac2levels during the TT40and increases in ANC), whereas the implementation of a more prolonged peak aerobic power interval training program improves the capacity of adenosine triphosphate production via oxidative phosphorlytic means“- Influence of High-Intensity Interval Training on Adaptations in Well-Trained Cyclists

That’s a lot of nerd speak, but in short the longer the HIIT phase the primary means of ATP (energy) production in the cell changed.

Basically, these athletes were not adapted to working at those higher intensities.

Initially, the body was producing energy anaerobically.

As they gained the ability to better buffer the acidic environment (and no, it’s not lactic acid, its hydrogen ions) in the cells the body returned to producing energy aerobically.

The body adapted to the stressor applied.

But as we know, adaptation isn’t finite.

At what point does HIIT no longer deliver an effect?

Or worse, an adverse effect due to the accumulation of the stress associated with it?

Truth is, we don’t know.

What we Do Know is HIIT Comes With a Cost

HIIT has a fairly high residual fatigue associated with it.

Other than easy aerobic work, you ain’t doing sh@t after a HIIT session,

“it can be concluded that strength endurance is impaired after high-intensity intermittent aerobic exercise and that this decrease is similar in athletes with aerobic, concurrent and strength training backgrounds.” – Effects of interval time between high-intensity intermittent aerobic exercise on strength performance: analysis in individuals with different training background

“aerobic and strength trained men who performed a high-intensity intermittent running exercise presented compromised strength endurance performance up to 1-h after the HIIE, with concomitant decreases in RR and lnRMSSD. After a 4-h recovery interval the interference of HIIE on strength endurance performance was suppressed and HRV returned to basal values. – High-Intensity Intermittent Exercise and its Effects on Heart Rate Variability and Subsequent Strength Performance

Fatigue from chronic HIIT accumulates and needs to be accounted and planned for.

“based on the cyclists’ perceived wellbeing in their legs, it seems that fatigue was cumulated during the HIT block. The cyclists reported that their legs were heavy after the HIT block, while they returned to normal during the following weeks when focus was on the low-intensity training, although still performing one HIT session a week. In contrast, TRAD group reported normal legs during the entire training period, indicating that the concurrent focus on HIT and low-intensity training provided smaller peaks of training stimulus.” – Block periodization of high-intensity aerobic intervals provides superior training effects in trained cyclists

Basically, doing nothing but HIIT sessions chronically has the ability to accumulate massive fatigue and can potentially make the rest of your training craptasitic.

How to Program HIIT

Here’s my suggestions on how to use HIIT in a program.

1. After weight training, no more than 15 minutes of HIIT.
2. I like smaller intervals 5-60 seconds, and usually more in the 10-30 second range.
3. Recover. Don’t be afraid to allow the heart rate to fall back to 100-120bpm (bpm) in between intervals.
4. Don’t cross 90% of Max HR often. The higher % heart rate, the more difficult to recover and if combined with inadequate rest intervals (see #3) the better chance you’ll go glycolytic and destroy mitochondria.
5. No more than 3 HIIT sessions per week
6. No more than 60 minutes of HIIT in a week if you’re ONLY doing HIIT and no weights that week. I’ve found more than 30 minutes a week of true HIIT (when combined with weight training) will impact your lifting
7. Stand alone HIIT sessions should be no longer than 30 minutes.
8. If you do a stand alone HIIT session, the next day should be a “recovery/ aerobic” day.
9. HIIT on an aerobic day should be done first to ensure the quality of the intervals.

*For my time recommendations I’m including the entirety of the working segment of the training session. What I mean is I’m including the interval and the rest period, but not any warmup or cooldown. 

Personally, because I treat HIIT like weight training.

For most of my clients I program HIIT in relatively small doses (10-15 min blocks) after weight training, using self-limiting modalities like ropes, sleds, bear crawls, spin bikes, even Kettlebell swings if they’re capable.

Things that are hard to really screw up when fatigued and I have limited time with people and need to get the “most out of the least”.

HIIT can be a fantastic tool.

It can also be a gainz killer.

Know the tools in your toolbox, and more importantly,

When and How to Use That Tool

and when it comes to HIIT, it’s not always the best tool.

*the study referenced in the video can be found here. The Effects of High Intensity Interval Training vs Steady State Training on Aerobic and Anaerobic Capacity

In that study they used three protocols, one of which was the “least enjoyable” Tababta protocol, but all were fairly intense and often more intense than other HIIT studies I used earlier to show HIIT had a higher enjoyment and that probably lead to the participants hating every minute of  the study.