Chances are you’ve heard of HIIT – high intensity interval training. If you’re someone that comes to my classes, you’ve definitely done it. But what is it, really? And where should it fit into your exercise regimen?
Let me give you a quick run-down of the basic physiology of exercise so that you can have a better understanding of what is going on with your body when you train. Your body has two basic pathways to generate energy while you’re working out: aerobic metabolism (this one requires oxygen) and anaerobic metabolism. Traditionally, we think of aerobic activity as our fat-burning state while anaerobic activity uses your body’s stored glycogen (the byproduct of carbohydrate breakdown). Aerobic activity uses oxygen, so theoretically you should be able to speak a full sentence without gasping for air. Anaerobic activity on the other hand, is not sustainable and gets you breathless. The point at which your body switches from aerobic to anaerobic work is known as your VO2 max, which is the point at which your body no longer can utilize oxygen to fuel your activity. Meaning: you lose your breath. Maybe you’ve heard of the “talk test” as a simple way to gauge your intensity. If you can talk, you’re using your aerobic system. If you go all out and are breathing so hard that you can’t speak more than 3-5 words, you’ve gone anaerobic. The purpose of HIIT is to maximize your energy expenditure and improve your performance by getting you into that anaerobic zone (or, at least approaching it) and then allowing yourself to recover. Oh, and HIIIT revs your metabolism up for hours afterward. Excess post-exercise oxygen consumption (EPOC) occurs when you get into that anaerobic state; afterward your body has to work return to its pre-exercise state, which requires energy. This means that you continue to burn more calories at rest than you normally would. Bonus!
HIIT workouts can be any duration and can involve any work to rest ratio. HIIT is appropriate for everyone! Yes, everyone. It has benefits for endurance athletes, strength-based athletes, as well as for older athletes. I would argue that it may be the most effective and efficient way to train, regardless of your goals. Read on.
Where exactly did HIIT come from? Ever hear of Tabata training? If you’ve been doing any sort of HIIT workouts, you’ve probably heard of it, but maybe you don’t actually know what it means. My biggest pet peeve is when I go to a class and someone has a 30-second interval and calls the set a “Tabata interval.” Izumi Tabata (yes, it’s named after a person!) did a study in 1996 during which he compared people doing 60 minutes of work at 70% of their VO2 max versus groups that did 8 sets of 20 seconds all-out work on a cycle ergometer followed by 10 seconds of rest. That’s it. 4 minutes of work versus 60 minutes of steady-state, endurance-based work. These groups did their workouts 5 days/week for 6 weeks. At the end of the training period, the steady state group did show an increase in VO2 max, but no change in their anaerobic power. The Tabata group, on the other hand, showed an improvement in VO2 max AND a 28% increase in anaerobic capacity (how hard they could work after they’d lost their breath).
To see how effective it is, look at it this way: the Tabata group worked for 120 minutes total, while the steady-state group worked for 1800 minutes total over that 6 week period. That is quite a discrepancy, huh? To me, I’d rather make those bigger performance improvements in a shorter time period, but that’s just me…
My boyfriend and I have been incorporating a Tabata finisher on the air bike at the end of our lifting sessions and they are KILLER (in the best way possible, of course).
If you want to check out Tabata’s research article, here it is:
I’ve worked with a ton of endurance athletes throughout my career, and in my experience the vast majority of them are reluctant to let go of their long-ass training sessions. For example, a cyclist training for a century ride may feel like they have to log several rides that approach that 100-mile mark so that they’re prepared for race day. But, what if I told you that it wasn’t about the volume of training, it was about the intensity? Enter HIIT training. Research has shown that people training for a marathon perform better on race day when they prepare by doing 1 long run, 1 track/treadmill HIIT run, 1 tempo run and 2 days of cross-training per week instead of just going for those long endurance runs. And this makes perfect sense if we consider how the human body works. The body is meant to be adaptable and as efficient as possible. For example, if you train by doing 10-mile runs you’re going to have to continually up your mileage to get the same bang for your buck because your body adapts to that form of exercise.
Endurance athletes traditionally fear that lactate threshold, because they think it means they’ve gone too far. However, when you get efficient, lactate can be taken up and recycled by the liver to make more glucose, meaning – we recycle our energy, which helps to improve our endurance. What’s more, when you train this way, you have an increase in oxidative enzymes. Those enzymes allow you to use fat as a fuel source, which really improves your endurance because we all have some body fat that can be used as fuel. So get comfortable with being uncomfortable.
Conversely, I know that a lot of strength-based athletes are reluctant to engage in HIIT and would rather focus on slower-paced lifting. Don’t get me wrong, I’m not saying we need to be treating every strength-training session as a WOD. But, I know that if building strength is your goal, you would benefit from incorporating this type of training into your regimen. A lot of strength-based athletes HIIT to be a form of cardio and they avoid it because they don’t want to diminish their muscle mass. But, literature shows otherwise. Incorporating 2 days/week of HIIT training compared to regular strength training has been shown to increase your Glut4 proteins and testosterone levels, increase your VO2 max and increase motor fiber and motor unit recruitment. What this means for you, is more work economy. You will be able to produce more power when needed and also recover more quickly. Meaning: you can lift heavier shit.
I even recommend HIIT training (modified, of course) for my older clients. Because guess what? It’s been shown to increase levels of testosterone, iGF, growth hormone and brain-derived neutrophic factor (BDNF), which means you get increases in muscle mass (SO important to ward off the atrophy associated with aging) AND improved brain health. A great example of the efficacy of HIIT training at an older age is Robert Marchand. Have you heard of him? If not, look him up, he’s pretty impressive. He set the world record for the most laps done in the velodrome in the over-100 age group in 2012; then he beat his own record in 2014. And how did he do it? By incorporating HIIT! Initially, Marchand was doing moderate intensity training 7 days/week. However, after two years of incorporating HIIT training he increased his VO2 max by 13% and increased his power output of his pedal stroke by 40%. And he did all of this over the age of 100. Pretty incredible.
Here is a link to the article that appeared in the Journal of Applied Physiology if you’re so inclined:
But, just like anything else, there is such a thing as too much of a good thing. The signs that you’re overdoing it: Decreased performance when you’re doing HIIT and a higher heart rate at rest. Just remember that intense exercise leads to cortisol release, so we don’t want to be doing this every single day. Along that same vein, let’s be sure to actually let our heart rates come down and recover. Undoubtedly, one of my biggest pet peeves (ok, maybe I have more than one biggest pet peeve) as a coach is seeing someone continue to work while they’re supposed to be recovering. The point of HIIT is to work hard enough that you NEED to recover.
So get out there and HIIT it! Your body will thank you.