HIIT Training - The latest craze that people can't seem to get right
High intensity interval training (HIIT) seems to reign king in the latest fitness trends (RIP to Zumba, P90X, Jane Fonda aerobics at home videos) and people cannot get enough of it. Many boutique gyms have opened up solely operating HIIT style training and have been extremely successful in the process. But, HIIT training seems to have this massive grey area with nuances unlike strength, hypertrophy, endurance, and power training which have clearly distinct variables validated in scientific research on how to optimize results. This blog is aimed to highlight the ins and outs of HIIT training, particularly
Defining HIIT training
Claims/benefits of HIIT training via research
How frequent HIIT should be performed
Examples of HIIT done correctly/incorrectly
Yes, I am a bit biased towards traditional strength/hypertrophy training but of course, I always encourage people to find a training regimen that they enjoy the most. This post isn't to slander HIIT training as I try to highlight the pros and cons. But a lot of trainers/gyms claim they do/don't perform HIIT training so this should hopefully serve as a general reference for identifying and making an informed decision.
HIIT Training Defined
HIIT training is defined as a protocol with alternating rounds of high-intensity and low-intensity exercise in an acute bout of training aimed at improving health, particularly cardiovascular measures (1). The structure for this type of exercise is simple. During the high-intensity intervals where you are performing an exercise, you are training at an intensity level that should be on average ~80% HR max and just shy of VO2 max and volitional fatigue for roughly 60s, although some research has tested up to 3-5 minutes of high-intensity rounds (1). Further, during the low-intensity portion of the interval, you are in full recovery mode and during that time allowing the body to go through its processes of clearing out metabolic byproducts (breathing), allowing the body time to decrease HR and breath rate, and rehydrating to prepare for the next round of high-intensity intervals. Generally, the high and low-intensity intervals alternate with an established period that allows for the maximization of the highlighted processes. The total duration of a HIIT session should last anywhere from 15-45 minutes (1,2,3). An example structure could look something like this
40 minutes - 15-20 rounds of exercise/rest
During those 40 minutes, the workout is predominantly bouts of high and low intensity intervals with differing exercises completed in either stations or chunks of 2-5 exercises. What makes HIIT different than traditional training methods such as an open gym session aimed at hypertrophy is
HIIT targets multiple muscle groups/full body within a single session while traditional training generally does 1-3 muscle groups per session
Total tonnage (sets x reps x weight) is higher than in an open gym session
More constant movement throughout the session
Less focus on specialty methods such as eccentric overload, positional isometrics, etc
which is likely why a lot of the general population enjoys it. You can burn a f*ck ton of calories and for those who gauge how good a workout is by looking at their apple watch and how much they sweat after a session, it's a great workout.
Claims/benefits of HIIT training via scientific research
Let's look at some research to see what has to be said about HIIT training. The ACSM claims that HIIT training can improve blood pressure, insulin sensitivity, and cholesterol profiles among others (2). A literature review from Schoenfeld and Dawes mentions HIIT improves VO2max, cardiac contractility (harder/more quality heart pumps each beat), and insulin signaling (1). And another review from Atakan et al. highlights improved endurance capacity, substrate metabolism, and insulin sensitivity (3). That all sounds great right? Yes! Those are great benefits to HIIT. But there's a common denominator for most of those adaptations listed above and it's that they are primarily cardiovascular adaptations, not hypertrophy/strength. This is where the disconnect occurs in trainers/gyms promoting HIIT telling clients that they will get stronger and more muscle by doing their classes/workout compared to traditional strength training methods and that's what motivated me to write this blog. There will be a very small percentage of clients taking those classes/workouts that will see improvements in strength/hypertrophy and that is likely due to
Their prior training status. If they are detrained you could give them bodyweight squats and push-ups and they will see strength/hypertrophy improvements because it is a stimulus they have not been exposed to before.
The client's ability to go through the "working" efforts and actually going heavy and take the exercise prescribed to near failure (~1-2 reps in reserve post set is most optimal for muscle growth/strength) but again HIIT is not the best method if your goal is muscle growth/strength.
Takeaway: HIIT is a great option for endurance/conditioning and all of the great adaptations that come with it which are primarily cardiovascular with a slight improvement in strength/hypertrophy. The claims of "you will get stronger than traditional training methods" are just absurd. So for ladies who want big glutes, HIIT is not for you. Guys who want big shoulders, HIIT is not for you. Do you want to add 20 pounds to your squat? HIIT is not for you. We live in a day of disinformation and it's hard seeing trainers/gyms promoting HIIT for anything other than cardiovascular benefits.
This section of the blog is a bit subjective because there are a few important factors to consider when recommending frequency which are
Training age (years of experience with exercise and technique, different from chronological age)
Training status (detrained, aerobically/hypertrophy/strength trained)
Time availability during the week
What the research says is a bit vague and has some limitations to consider. Most studies analyzed a 2 vs 3 days/week HIIT model of training and they all found the 3-day groups to have better markers of improvement which were skinfold tests, total cholesterol, LDL, and mental well-being (1,4,5). Also, most studies looked at HIIT methods for cycling, running, and jogging/walking when the context for this conversation is more for a gym with weights, bars, bands, etc. What I think is important here is HIIT seems to be a method that a lot of people enjoy because you can get a great workout in a shorter amount of time vs going to an open gym during rush hour and struggling to find equipment open. Most HIIT workouts are performed in a classroom setting and the average adult doesn't want to go to the gym after a long day of work and then fight over equipment. Plus, there's a social side that needs to be considered because the community aspect is hard to find anywhere else, especially if you are someone who is trying to surround yourself with healthy like-minded individuals. What I will highlight in the limitations section is the negative side effects of doing too many HIIT sessions on a weekly basis which is the most important piece of this blog but what I would recommend is to find a sweet spot of frequency weekly with HIIT classes that allows you to have time to recover between workouts which will then allow for some super-compensation (see my blog post section for a detailed explanation of that term) to occur thus have better quality workouts when you go back for the next lift. An ideal frequency if I had to lay it on a week timeline would look like this
Strength/hypertrophy open gym
Strength/hypertrophy open gym
Active recovery (yoga/pilates/light jog)
Takeaway: Start with HIIT 1x per week, slowly add in a second to third day with ample time between the sessions spent doing another form of exercise. If you are truly doing HIIT in it's intended form sustaining 5-6x per week could be a potential for overtraining (see limitations section for more on that).
Work: Rest Ratios
There are numerous scientific papers that explain what protocols are optimal in order for HIIT training to serve its full effect(1,2,3,5). The X factor in order to create a successful HIIT workout is the work:rest ratios. Work:rest ratios defined is a ratio of how much time is spent performing the exercises (work) and how much time is spent recovering between rounds (rest). The reason is, if you are truly doing HIIT efforts you need to maintain energy system output from the phosphocreatine and glycolytic systems and for those two systems to remain in the driver seat (vs a shift to the aerobic systems) REST is crucial and upwards in the range of 60s-300s between rounds. In order to maximize the benefits of HIIT training, work:rest ratios are performed in either 1:1-1:4 (1,2).
For example, using 60s as the "working" effort
Increase work capacity
Increase submaximal effort
Increase near maximal effort
Increase maximal effort
The table above shows examples of times that are applicable to use for work:rest ratios in a HIIT session by selecting 60s as your "working" effort. As the "rest" number increases, that is implied that the intensity of the "work" effort increases. When creating a HIIT based workout this is the first question you need to answer before even thinking about writing the first exercise. "What are the ratios I want to use for this workout?" Then you would select exercises that are challenging but not complex enough for you to deviate away from. Again research ranges from 10-300s of "working" efforts but for the general population, a range of 30-60s would be most ideal (1,2,3,4).
Now here's the problem you see in most gyms/training sessions claiming they do HIIT, the ratios are flipped
High as f*ck
Realistically, this is what most gyms do thinking they're doing HIIT when in reality they're setting classes up for failure and it becomes sloppy. Another question to ask is "is this sustainable for a 45-60 minute period of exercise?" absolutely not. Say you are doing a workout with the 3:1 example of 180s of work and 60s of rest, you might be able to maintain the intensity and whatever weights you pick for the first couple of rounds, but as the workout goes on you will eventually experience a dropoff thus no longer doing high intensity efforts with weights/movement but increasing the rate of perceived exertion and now you're in survival mode just trying to do the exercise by any means possible and likely having to drop weights/do fewer reps/decreasing jump height/etc. What comes with that is dogshit form and increased risk of injury. I look at training through the lens of prioritizing the setup of the workout first (training split, set/rep schemes, rest periods, THEN exercise selection). For gyms/trainers aimed at solely trying to have people limp out of the class, it's flipped and it's a way of thinking that needs to be changed.
For my visual peeps here's a graph showing sustainability with clear efforts done at the same perceived exertion in a HIIT workout. Here you are able to maintain the level of output and intensity for the entire period of time vs having a burnout where the perceived intensity is so high that you have to compensate (and we can use a 1:1 60s on 60s off as an example, with max rate of perceived exertion and time as the respective axes).
and here's a 3:1 180s on 60s off graph that would look like (I drew this as you can tell I never went to art class in undergrad)
HIIT should be done with the intention of sustaining high intensity (including the weights not just perceived effort) for the entire workout, not going into 6th gear min 0-10 and finishing your last station barely able to move. That's not an effective workout. "But I was so sore the next day!" Soreness is one of the worst indicators of workout effectiveness and should not be considered the method of measurement when in the workout you dropped your weights by 40-60%. Continuing on with the second graph, as the workout goes on, the intensity will get to a point where it is not sustainable and we then have to drop weights, compensate form, and take more rest periods during the "working" efforts, among others. Now multiply this by a couple of sessions in a row or weeks in a row and we then likely experience fatigue and plateauing which leads to overtraining syndrome.
Takeaway: Step one in designing a HIIT workout is to pick the ratio of "working" and "recovery" efforts in the range of 1:1-1:4, then you pick your times of how long you want to go for your "working" effort (recommended 10-60s and under for general population) and your "recovery" time is based off how intense you want to go.
Overtraining syndrome is defined as a condition of maladapted physiology in the setting of excessive exercise without adequate rest over time (6). The common symptoms associated with overtraining are nervous system dysfunction (most important IMO), systemic inflammation, depressed mood, neurohormonal changes, HRV changes, unexplained weight loss, anxiety, anorexia, decreased sleep quality, and unhealthy food cravings (6,7). A lot of those symptoms sound scary and it's hard to think that can happen from exercise when we're always told about the benefits of training and how it is good for us. Like everything in life, there's a spectrum between too little and too much and exercise is not exempt from this. What makes HIIT so effective is the concept of "less is more" by claiming you can do insane efforts in 30-45 minutes vs an old school open gym session for 60-80 minutes, and you can, but only if the workout is set up for success and I truly cannot emphasize how important structuring the workout with the methods highlighted in work:rest ratios can be to avoid these symptoms from happening. There are ~7 hypotheses on why overtraining occurs but I will briefly highlight the importance of the autonomic nervous system (ANS) hypothesis because it is one of the more accepted and all of the symptoms listed above are in a way a side effect of the body's inability of the ANS to serve its purpose when in an overtrained or nonfunctional overreaching state (the state of stress just before considered overtrained).
Exercising is perceived by the body as a stressor, and as a result, our ANS decides what to do to help cope with the stressor and in turn activates our sympathetic nervous system (SNS). The SNS is also referred to as the "fight, flight or freeze" response, and does all these awesome things to get the body's awareness heightened like releasing stress hormones such as adrenaline, norepinephrine, and cortisol among others. Further, the SNS increases HR, blood flow redistribution to the muscles/brain, breath rate, and sense of smell and vision among others. Sounds like what happens when you workout right? But what's important here is as much as we need the SNS to do its job during a stressor like exercising, we need to downregulate as well and that's where the parasympathetic nervous system (PNS) comes in. Think more relaxed state and all of the stress hormones dissipate, HR and breath rate decrease, and we can then begin the recovery process. What happens then? Adaptations from training!
Let's circle this back to HIIT. When HIIT is repeated over consecutive days/weeks/months, we're imposing an environmental stressor on the body and in turn will respond by adapting to the stress (highlighted back in the claims/benefits section) but in order for the body to have time to adapt it needs adequate rest, especially if the intensity in the session was high enough (which it should if you set it up correctly). Adequate rest for HIIT if done correctly should be anywhere from 1-4 days depending on training status and intervals done during the HIIT session (1,2,3). So if you do not allow ample rest for your own body and are gungho on HIIT 5-6x per week or doing two a days, there's a chance you will likely experience some symptoms associated with nonfunctional overreaching and overtraining syndrome. This isn't meant to be a scare tactic but I run into a lot of folks who do multiple HIIT classes a week that are poorly designed and the unfortunate side effect is the client/member getting great benefits at first followed by a plateau of progress and/or symptoms of overtraining. Could this happen from any form of exercise? absolutely. But HIIT runs a higher risk if it is poorly designed because trying to mix volume and intensity within a session then repeated 4-6x per week increases the likeliness of the symptoms listed above.
Takeaway: Overtraining causes a whirl of problems for the body. Often times people aren't aware of the side effects of overtraining and HIIT done incorrectly can speed up the known side effects. Think of HIIT as an exercise tool in a workout toolbox, there are plenty of tools for the job but if you keep using the same tool it'll get dull and the job cannot be completed