Exercise Benefits, Part 4: Why You Should Do Intervals (If You Can)

I might be a bit of an anomaly (after all, I love burpees—and I don’t say that sarcastically; I truly love them), but I love HIIT (high-intensity interval training). I love pushing my body to the limit for short bursts and then recovering my breath, only to do it all over again. It’s incredibly intense and fatiguing, but it’s also exhilarating. It just makes me feel so fierce! And it's a great way to get a quick and killer workout at home

But it’s more than just the feeling, though; I also love what it has done to my body. I was in pretty good shape before I started doing HIIT regularly, but doing HIIT a few times a week took my fitness to the next level. And it could for you, too.

Warning: as its name would suggest, HIIT is intense, so it’s not recommended for everybody. Or, better said, it’s not for every body. Based on experience or health concerns, some people's bodies may not be ready for HIIT.

  • If you have certain medical concerns (high blood pressure, pregnancy, injury, etc.), check with your doctor first. 
  • If you’re new to fitness, you might want to wait, letting your body get used to less intense workouts first.
  • If your joints don’t handle impact well, this may not be for you, as HIIT often involves plyometric moves. (Though you could go for a no-impact option like cycling, rowing, or battle ropes [image below].)

But if you and your body are up for it, adding HIIT to your weekly workouts would be excellent way to kick your fitness up a notch.

What Is HIIT?

For those that aren’t familiar with HIIT, I’ll give you a quick low-down on how it works. If you are familiar with this workout style, feel free to skip to the next section on the benefits of this type of training.

What HIIT Entails

As its name would suggest, HIIT involves short intervals of high-intensity work. Depending on how you choose to structure your workout, these intervals can be as short as 20 or 30 seconds or as long as a few minutes

The goal is that, for those short, high-intensity intervals, your heart rate (HR) is in one of the anaerobic zones: 4 or 5. (See the chart below for a reference to the zones.) These zones are challenging and uncomfortable, so by the end of the interval, you should feel pretty spent, like you’ve got nothing left to give and like you couldn’t go any longer.

The other crucial piece of HIIT is that you allow your HR to recover after you push. So after working all-out on your high-intensity interval, give yourself 30 seconds to a few minutes of low-intensity work to allow your HR to recover, ideally all the way down to Zone 2 or lower. This recovery is crucial for getting the full health and fitness benefits of HIIT.

How To Do It

HIIT often involves plyometric movements. Plyometric (a.k.a., plyo) moves are quick, explosive movements that start by lengthening the muscles and then quickly contracting them, such as with a squat jump: the quads and glutes lengthen as you squat down, and then they quickly contract to propel you into the air. These kinds of big, explosive movements using the big muscles are usually what it takes to get the HR into the upper zones in such a short amount of time. Other examples of these kinds of moves include burpees (my favorite!), tuck jumps, lunge jumps, box jumps, and more. 

For a non-impact but still plyo option, you can throw sandbags or medicine balls. For non-plyo options, try rowing or cycling at a fast pace and with a heavy resistance. Sprinting is also a great option for pushing into the anaerobic zones. While it’s technically plyometric (there’s some recoil as you plant your feet and push off again), it’s less so than something like box jumps so that might be easier on your joints.

Lastly, there are a lot of ways to structure a HIIT workout, so feel free to experiment to find what works for you and your body (and with the equipment and space you have at home). Some people might do a three-minute push of intense rowing or sprinting and then recover for a minute or few with a light row or jog/walk.  The way I like to do it (and the way I structure my HIIT classes) is to push all-out for 40 seconds (sometimes less, if I burn out early) and then recover with 40 seconds of low-intensity strength training. Then I repeat that for a total of 18 high-intensity intervals (with a roughly one-minute break after three pairs of high-low intervals), lasting about 30 minutes in total (not counting the warmup and cool-down, which are as important as the interval workout itself).

Benefits of HIIT

As you may have gathered from the description of HIIT, it sounds like a combination of cardio and strength. Experientially, it even feels like a combination of the two: if you’re really going for it on those high-intensity intervals, you’ll get breathless, like you do with cardio, and you’ll feel a muscle burn, like you do with strength.

That’s because it essentially is both. HIIT is basically strength-based cardio. As such, the benefits of HIIT are similar to those you get from cardio and strength, sometimes even to a better degree, but sometimes not. It is also activity, so the universal exercise benefits apply: improving mood, boosting cognitive function, improving various disease risk factors, etc. But let’s take a closer look at the more unique benefits of HIIT.

Improved Cardiorespiratory Fitness

Aerobic cardio definitely improves cardiorespiratory fitness, but to get the last bit of optimization, you need anaerobic training, because it puts a higher demand on the heart and lungs. Higher demand means greater changes. Just like curling 20-lb weights is going to do more to build your biceps than will curling 3-lb weights, spending time in Zone 4 can do more to strengthen your heart than will spending time in Zone 2. As such, HIIT is arguably the best way to improve your cardiorespiratory fitness.

HIIT improves your HR variability (HRV), which is how well your HR adapts to your body’s current needs. If you have a high HRV, your HR will quickly climb during your high-intensity intervals, but will also drop pretty quickly during your recovery intervals. A high HRV signifies a fit, healthy body and a healthy, resilient stress response.

HIIT also improves the heart’s stroke volume (the amount of blood pumped with each contraction). A higher strike volume means the heart is more efficient. Because your heart is working more efficiently and adapting more effectively, HIIT can have a positive effect on one’s HR zones (more on that below).

But it’s not just the heart: HIIT works the respiratory system, too, with all that fast breathing in those high HR zones. As such, it's arguably the best way to boost your VO2 max (i.e., lung capacity).

Optimized Metabolism and HR Zone Profile

HIIT makes for amazing metabolic conditioning and is, very probably, the best way to improve your HR zone profile (yes, that can change). It benefits our metabolism in some interesting—and diverse—ways.

  • First of all, it improves insulin sensitivity, perhaps even more so than aerobic cardio does.
  • It also enhances fat burning, as it shifts your body’s metabolism towards burning more fat and less sugar. But, to get this benefit, you have to make sure you're recovering down to the lower aerobic zones after you push on the high-intensity intervals. Otherwise, if you stay in the higher, sugar-burning zones, you might actually train your body to burn more sugar and less fat. Recover is key.
  • Lastly, HIIT keeps your body’s energy (calorie) expenditure elevated even after your workout. This EPOC (excess post-exercise oxygen consumption), or “afterburn”, is an elevated energy usage (to help your body recover) that usually lasts for 24 to 48 hours after your workout. Yes, that’s right: you keep burning extra calories for the next day or two. But remember, that’s not license to eat whatever you want; you can’t out-exercise a bad diet.

HIIT puts a considerable stress (a healthy stress) on your heart and takes you through pretty much the full spectrum of HR zones. As such, it’s no surprise that HIIT can actually help you optimize your HR zone profile. This can also mean a more optimal metabolism. In the lower zones (1 and 2), we primarily burn fat as fuel. However, as we move into Zone 3 and need more energy more quickly, we rely more on carbs (e.g., sugar) for fuel, because they metabolize more quickly.  In Zones 4 and 5, which require a lot of energy very quickly, we’re burning exclusively carbs as fuel.

With effective metabolic conditioning (which you could also call zone training), you can shift the beats per minute at which your zones change. You can also shift the fuel usage of those zones, burning even more fat in the lower zones. There are two key elements to effective zone training.

  • First, do HIIT smartly. As mentioned above, this means pushing hard during the intervals, but then fully recovering before pushing again. (Otherwise, you overtrain in the anaerobic zones, which could mess up your metabolism.)
  • Second, spend your aerobic cardio workouts in the lower, fat-burning zones (1 and 2) to train your body to use fat as fuel.

Together, using these two zone training techniques will help maximize the fat-burning of your lower zones and optimize your overall metabolism and HR zone profile.

More Power

In order to send your HR into Zones 4 or 5 in a matter of minutes (or less), you’re almost certainly going to have to do explosive, plyometric movements. These movements are one of the best ways to build muscle power. And power is different from strength.

  • Strength refers to how much load you can lift and is best improved by strength training (e.g., controlled, slow-ish squats or presses).
  • Power refers to how quickly you can move a load and is best improved by plyometric training (e.g., jumping up from the bottom of a bodyweight squat, picking up and throwing a medicine ball quickly).

The power you get from training with plyo moves is going to give you more speed and explosiveness in your movements. This is why HIIT makes for very effective training for athletes who need to sprint fast or jump high.

Stronger Bones and Muscles

Because there is muscle work and resistance involved in doing plyo movements, HIIT can strengthen your muscles and improve your bone strength. I haven't been able to find studies directly comparing HIIT to resistance training in terms of building muscle mass and improving bone density; however, I do feel confident in saying that strength training is more effective at building muscle mass. You can get toned and strong from HIIT, for sure, but you’ll need to lift heavy if you want to bulk up.

Regarding bone density, this is only my educated guess, but I would surmise that the benefits on bone density are comparable. Yes, loads are heavier with resistance training, which means more stress to the bones. However, there is more impact with plyometric moves, which adds a different kind of stress to the bones, so I would think that would make it comparable. The specifics notwithstanding, the evidence is clear that plyometric-based HIIT (jumping, running, etc.—not cycling or rowing) improves bone density and builds strength (well, more so power).

Better Return on Investment (of Time)

This last point isn’t so much a health benefit as it is a time benefit. Nonetheless, it’s still worth noting. Because of the numerous health benefits that HIIT offers (all of the general activity benefits, plus the ones discussed above, plus some of the benefits from cardio and strength training, which you can get from HIIT, even if to a lesser degree—so, pretty much everything), and because these benefits can be gained from even a short workout (15 to 30 minutes is sufficient), HIIT is the best bang for your buck in terms of time you spend on your workout: you get a lot of benefits from a small time commitment

This makes it a very appealing exercise format, especially when you’re crunched for time. So during those busy weeks, when you can’t find longer chunks of time for your walk/run/bike ride or for your lifting routine, hopefully you can fit in a short 15- or 30-minute HIIT workout at home. And you don't even need equipment. Most of my favorite HIIT moves are bodyweight-only: burpees, plyo lunges, tuck jumps, jumping jacks, squat jumps, kickboxing combos, etc.

Final Remarks

Based on the above evidence, HIIT would seem like the clear fitness winner in terms of benefits. That’s because it kind of is. And especially if you often find yourself short on time, you might think that HIIT is all you ever need to do. 

On the one hand, yes; HIIT is, hands down, the best style of workout for your physical health and fitness: it improves mood and immunity, boots brain growth, optimizes cardiorespiratory fitness and metabolism, enhances power, builds muscle, and strengthens bones. So if you only have 15 to 20 minutes to spare a few times a week, HIIT would be your best choice, especially for a simple workout to do at home. 

On the other hand, HIIT doesn’t build as much muscle as strength training does, and it doesn’t have even close to the same brain-boosting power as cardio. So, really, we need all three.

Plus, like with strength, there's an important caveat: it’s possible to overtrain (which can lead to injury, burnout, etc.) on HIIT. Because it’s such an intense workout, our bodies need time to recover properly, so it’s highly recommended that you give yourself at least a full day of recovery (or, in other words, two nights of sleep) between HIIT workouts. Lighter, less-intensity aerobic cardio or yoga would be a great option for those in-between days.

The other thing to be mindful of is to not spend too much time in the upper HR zones. Extended periods of high-intensity training can actually have a damaging effect on our bodies, including weakening our immune systems. (Yes, exercise is normally good for our immunity, but our immune systems can’t handle that much high-intensity stress.) The recommendation is that you don’t spend more than 30 to 40 cumulative minutes per week in the upper ones. For example, if I do fifteen high-intensity intervals in one workout, with each interval lasting 40 seconds, that makes for 10 cumulative minutes in the upper zones. If I do that three times a week, that’s 30 cumulative minutes. That’s plenty.

So, by all means, do HIIT (if your body can handle the intensity and impact), because it’ll take your fitness to the next level. But be careful to not overdo it. You’ll need time to recover. Consider something less intense on the in-between days. Like walking. Or like barre or yoga, which have a lot of their own great benefits that we’ll look at in the next article in this series.

After reading all that, if you're looking do some online HIIT workouts at home, check out ZentasticFit. We have virtual personal trainers that teach all kinds of exercise types, including HIIT. Find an instructor you like and schedule a one-on-one, online workout at a time that works for you—all from the comfort of your home!

Further Reading

“HIIT vs Continuous Endurance Training: Battle of the Aerobic Titans”, Micah Zuhl and Len Kravitz (University of New Mexico)
“Low-Volume High-Intensity Interval Training in a Gym Setting Improves Cardio-Metabolic and Psychological Health”, Sam O. Shepherd et al. (PLOS One)
“New Study Reveals the Optimal Weekly Dose of HIIT”, Emma Hogan (Les Mills)


By Dustin R. Meriwether

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