Although the exact mechanisms behind HIIT’s fat-burning advantages aren’t fully understood yet, scientists have isolated several factors, including the following:4
Increased metabolic rate for up to 24 hours
Increased insulin sensitivity in the muscles
Increased fat burning in the muscles
Increased growth hormone levels
Increased catecholamine (fat-burning chemicals) levels
Decreased postexercise appetite
These days, most people have heard about high-intensity interval training and its special fat-burning powers, but don’t know how to do it correctly.
You yourself have probably wondered things like how “intense” the high-intensity intervals need to be, how “restful” the rest periods should be, how long the workouts should be, and how frequently you should do HIIT sessions.
Let’s find out, starting with how hard your high-intensity intervals should be.
When you review the research on HIIT, you’ll often find that exercise intensity is discussed in terms of percentage of “VO2 max.” This is the maximum rate of oxygen consumption during exercise and a major factor in determining your endurance.
In most studies on HIIT, people reached between 80 and 100 percent of their VO2 max during their high-intensity intervals. This isn’t a very practical insight, however, because VO2 max is hard to approximate while exercising.
It’s tough to know with any certainty whether you’re at, let’s say, 60 or 80 percent of VO2 max without being hooked up to a fancy machine (metabolic cart).
A more useful way of prescribing intensity in your HIIT training is basing it on your ventilatory threshold (VT). This is the level of intensity where breathing becomes labored and you feel like you can’t bring in as much air as your body wants. It’s about 90 percent of your “all-out” effort.
Your goal during your high-intensity intervals is to reach your ventilatory threshold.
That is, you need to get moving fast enough that your breathing becomes labored and you can’t quite suck in air as quickly as you feel you need to. And then you need to hold that speed for a period of time.
As you can imagine, this requires a significant amount of effort.
Repeatedly achieving and sustaining this level of exertion is the whole point of HIIT. If you don’t do this—if you can chat away on the phone during your “high-intensity” periods—you’re not doing HIIT.
You should also know that the total amount of time you exercise at your VT determines the overall effectiveness of your HIIT workouts.
If an individual workout racks up just a minute or two of VT-level exertion, it’s not going to be as effective as one that involves double that amount.
How to Create an Effective HIIT Routine
Now that you know how HIIT works, let’s talk about making it work for you. Specifically, how to create a HIIT routine that will amplify your fat loss in just an hour or two per week.
There are five things to consider when creating a HIIT routine:
The type of cardio performed
The duration and intensity of the high-intensity periods
The duration and intensity of the rest periods
The duration of the workouts
The frequency of the workouts
Let’s look at each point individually.
1. The type of cardio performed
While you can use HIIT principles with any type of cardio, if your goal is to preserve muscle and strength, your best choices are biking and rowing.
The reason I’ve chosen these exercises is studies show that the type of cardio you do has a significant effect on your ability to gain muscle and strength in your weightlifting.5
There are two likely reasons for this:
Cardio that mimics the movement patterns of muscle-building exercises can enhance weightlifting performance.The reason why muscles fatigue during exercise is extremely complicated, but we know that both aerobic (with oxygen present) and anaerobic (without oxygen present) capacities are major factors.6
Even when you’re doing a highly anaerobic activity like sprinting or weightlifting (which outstrip your body’s ability to supply sufficient oxygen for energy), your body’s aerobic system is still producing a significant amount of energy.
Thus, if you improve a muscle’s aerobic capabilities through certain aerobic exercises, you’ll see an improvement in your anaerobic capabilities as well.
Cardio that is low impact doesn’t require much recovery.Low-impact cardio causes very little soft tissue damage and adds little additional stress for the body to cope with.
Biking and rowing check both of these boxes. They imitate the movement patterns of the squat and deadlift, respectively, and involve no impact.
If you can’t or don’t want to bike or row, however, feel free to use other low-impact methods of cardio for your HIIT workouts. Swimming, jump roping, elliptical, and bodyweight circuits are popular choices.
I don’t recommend sprinting, though, as it causes too much muscle and joint soreness, which will get in the way of progress in your lower-body workouts.
2. The duration and intensity of the high-intensity periods
As you now know, the intensity target of your high-intensity intervals is your ventilatory threshold, and the total number of minutes spent at this level of exertion dictates the effectiveness of your HIIT workouts.
Too little time at this near-peak effort level results in a “kinda-high-intensity” workout, and too much can lead to exhaustion and overtraining.
Don’t slowly build up to this level of effort when you launch into a high-intensity interval. Give it everything you’ve got right out of the gate. You should be breathing hard within 10 to 15 seconds.
It’s also worth noting that you want to adjust your speed in your training more than the resistance settings offered by various machines.
The goal of HIIT is to go fast and hard, not slow and hard. (And there I go again. . .)
In terms of the duration of your high-intensity intervals, 50 to 60 percent of the total amount of time that you can maintain VT intensity is sufficient if your goal is losing fat and improving metabolic health.
This total amount of potential VT exertion is known as your “Tmax.”
So, for example, my Tmax on the bike is about three minutes, so my high-intensity intervals are 90 to 120 seconds long (yeah, it’s tough!).
To determine the proper length of your intervals, you can either test your Tmax (all you need is your phone’s clock app), or if you’re new to HIIT and want to keep it simple, start with one-minute high-intensity periods.
If you also want to significantly improve your conditioning, you’ll need to make your HIIT workouts progressively tougher. As you get fitter, your Tmax is going to improve, and as it improves, the duration of your high-intensity intervals will need to increase if you want to continue increasing your cardiovascular capacity.
As you can imagine, this can get pretty dang intense for experienced athletes. In several HIIT studies conducted with highly trained cyclists, for instance, high-intensity intervals were five minutes long and resulted in improved performance.7 In contrast, other research conducted with endurance athletes found that two- and one-minute intervals (hard!) weren’t enough to improve performance.8
3. The duration and intensity of the rest periods
Your rest periods should consist of active recovery, which means you should keep moving, not come to a standstill.
Studies show that this helps you reach your ventilatory threshold easier during your high-intensity intervals, which makes your HIIT workouts more effective.9
As far as duration goes, start out with a 1:2 ratio between high- and low-intensity intervals. For example, one minute at high-intensity and two minutes at low.
As yo
ur conditioning improves, you can work toward a 1:1 ratio.
4. The duration of the workouts
The great thing about HIIT is how much you can get out of relatively small amounts of it. It can be quite stressful on the body, though, which means you don’t want to do too much (especially when you’re lifting weights as well, and especially if you’re also cutting).
So, start your HIIT workouts with 2 to 3 minutes of low-intensity warm-up, and then do 20 to 25 minutes of intervals, followed by 2 to 3 minutes of warm-down, and you’re done.
5. The frequency of the workouts
How often you should do HIIT workouts depends on your goals and what other types of exercise you’re doing.
I’ve found that four to seven hours of exercise per week is plenty for losing fat quickly and efficiently, and of course, you want to spend most of that time on resistance training, not cardio.
Thus, when I’m cutting, I like to do four to five hours of weightlifting and one and a half to two hours of HIIT per week. This allows me to get as lean as I want without risking overtraining or burnout.
The Second-Best Type of Cardio for Losing Fat Faster
If your goal is maximum fat loss and you have the energy and will to endure an hour or two of HIIT per week, that’s your best bet.
If you can’t or don’t want to do that much HIIT, however, or any at all, you have another option.
You’re not going to need any special equipment, gadgets, or skills. You’re not going to need to track your heart rate, time your intervals, or log your miles. All you’re going to do is something that you’ve been doing every day since you were a toddler, and that you’ll do for the rest of your life.
It’s walking, of course, and while it’s not the best way to lose fat rapidly, it’s definitely the easiest way to burn additional calories and lose fat faster.
That’s why many people don’t think walking qualifies as a bonafide “cardio workout.” When it comes to exercise, “easy” is usually equated with “worthless.” That’s true in certain contexts, but not this one.
For instance, a study conducted by scientists at California State University found that people who ran a 10-minute mile burned about 190 calories.10 People who walked a 19-minute mile burned fewer calories, of course, but not as few as you might think—about 111.
This isn’t going to impact your fat loss like HIIT will, but if you go for several walks per week, it can make a significant difference over time.
Walking has other benefits too.
Walking Is Very Easy on the Body
Managing stress levels is an important part of minimizing muscle loss while restricting calories to lose fat.
Walking is great in this regard because, unlike more intense forms of exercise, it places very little stress on the body. In fact, research shows that walking can counteract the effects of stress and reduce cortisol levels.11
Thus, if walking were your only form of exercise while dieting, you probably couldn’t do enough to risk overtraining. Moreover, you can also safely add several hours of walking per week on top of an already rigorous exercise schedule.
Walking Minimally Impacts Muscle Gain
As you know, cardio workouts can directly impair strength and muscle gain.
This is why strength athletes dramatically reduce or eliminate cardio altogether leading up to a competition, and why many bodybuilders generally keep cardio to a minimum while lean bulking.
We also recall that not all forms of cardio are equally detrimental to weightlifters. For example, running clearly impairs muscle and strength building but cycling and rowing don’t seem to.12
In the case of walking, it doesn’t mimic a muscle-building movement and so won’t likely improve your performance in the gym, but it’s as low impact as you can get.13 And that means you can use it to burn calories without getting in the way of your progress in your weightlifting.
Walking Preferentially Burns Fat
Walking may not burn many calories, but the calories it does burn come primarily from fat stores.
You burn both fat and carbohydrate when you exercise, and the proportions vary with the intensity.14 As intensity increases, so does the reliance on muscle glycogen (carbohydrate) for energy over fat stores.15
This is why a very low-intensity activity like walking taps mainly into fat stores, whereas high-intensity exercise pulls much more heavily from glycogen stores.
This is also why some people think low-intensity steady-state cardio is best for fat loss. You now know it’s not (HIIT is), but it’s certainly the easiest and least stressful way to augment weight loss.
How Much Walking Should You Do?
The biggest downside to walking is it doesn’t burn all that much energy (about 300 to 350 calories per hour).
That means you’d need to do quite a bit of walking (several hours per week) to see noticeable changes in your body composition. Every calorie burned matters, however, so even relatively small amounts of walking will help you reach your fat loss goals faster.
This is especially true if you do other exercise as well. For example, if you add weightlifting to the mix, you can dramatically increase fat loss. Just four heavy sets of deadlifts can burn over 100 calories, and that’s not taking into account the additional energy expenditure that occurs after (the “afterburn effect”).16
I’ve worked with many people who’ve used a combination of weightlifting, high-intensity interval training, and walking to lose fat rapidly. The most successful approach looks like this:
Three to five one-hour weightlifting sessions per week
One to three 25-to-30 minute HIIT sessions per week
Two to three 30-to-45-minute walks per week
•••
Medicine has known the value of regular exercise for thousands of years, but only recently have we gained a better understanding of how much is enough and how much is too much.
If you do at least a few hours of resistance training per week, you should view cardio as supportive, not essential, and you should do enough to reach your goals (and enjoy yourself, if you like endurance exercise) but not more.
For most people, that means doing no more than one to two hours of HIIT or three to four hours of walking per week, or some combination thereof.
Not only will this regimen help you get into the best shape of your life, it’ll also leave you with plenty of free time to invest in other activities and pursuits, including relationships, family, and hobbies.
Remember that your health and fitness routines should enhance you and your life, not consume them!
Key Takeaways
If you just want to build muscle, lose fat, and be healthy, cardio is far less important than you might realize.
If you don’t particularly enjoy cardio, you should do as much as it takes to achieve your goals and no more.
A 17-to-27-minute session of high-intensity cardio, which mostly consists of low-intensity cooldown periods, burns more fat than 60 minutes of traditional bodybuilding cardio. This high-intensity style of cardio is known as high-intensity interval training, or HIIT, and the science is clear: it’s significantly more time effective for losing fat than traditional low-intensity steady-state cardio (LISS).
HIIT involves alternating between periods of (almost) all-out-intensity sprinting and low-intensity recovery.
Your goal during your high-intensity intervals is to reach your ventilatory threshold.
The total amount of time you exercise at your VT determines the overall effectiveness of your HIIT workouts.
While you can use HIIT principles with any type of cardio, if your goal is to preserve muscle and strength, your best choices are biking and rowing.
If you can’t or don’t want to bike or row, however, feel free to use other low-impact methods of cardio for your HIIT workouts like swimming
, jump roping, elliptical, and bodyweight circuits.
In terms of the duration of your high-intensity intervals, 50 to 60 percent of the total amount of time that you can maintain VT intensity is sufficient if your goal is losing fat and improving metabolic health.
Your rest periods should consist of active recovery, which means you should keep moving, not come to a standstill.
Start out with a 1:2 ratio between high- and low-intensity intervals.
Start your HIIT workouts with 2 to 3 minutes of low-intensity warm-up, and then do 20 to 25 minutes of intervals, followed by 2 to 3 minutes of warm-down, and you’re done.
Four to seven hours of exercise per week is plenty for losing fat quickly and efficiently, and of course, you want to spend most of that time on resistance training, not cardio.
If you can’t or don’t want to do that much HIIT or any at all, you can walk instead.This isn’t going to impact your fat loss like HIIT will, but if you go for several walks per week, it can make a significant difference over time.
I’ve worked with many people who’ve used a combination of weightlifting, high-intensity interval training, and walking to lose fat rapidly, and the most successful approach looks like this:Three to five one-hour weightlifting sessions per week
One to three 25-to-30 minute HIIT sessions per week
Two to three 30-to-45-minute walks per week
Macpherson REK, Hazell TJ, Olver TD, Paterson DH, Lemon PWR. Run Sprint Interval Training Improves Aerobic Performance but Not Maximal Cardiac Output. Med Sci Sport Exerc. 2011;43(1):115-122. doi:10.1249/MSS.0b013e3181e5eacd.
Boutcher SH. High-Intensity Intermittent Exercise and Fat Loss. J Obes. 2011;2011:1-10. doi:10.1155/2011/868305.
Trapp EG, Chisholm DJ, Freund J, Boutcher SH. The effects of high-intensity intermittent exercise training on fat loss and fasting insulin levels of young women. Int J Obes. 2008;32(4):684-691. doi:10.1038/sj.ijo.0803781.
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