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Complete Cardio Part 1: HIIT vs. Steady State

Complete Cardio Part 1: HIIT vs. Steady State

If you’re interested in blasting body fat while holding onto your hard-earned muscle, you can’t beat HIIT.

Written By Jim Stoppani, PhD

Updated August 31, 2019

There was a time when bodybuilders would only consider doing low- to moderate-intensity steady-state cardio, such as fast walking or pedaling a stationary bike at a moderate intensity. Anything more intense would be considered a big no-no. There were two main reasons for this: First, they believed that more intense cardio would “burn up” muscle tissue (meaning that muscle tissue would be broken down to fuel the exercise). Second, lower-intensity cardio was reported to put you in the optimal fat-burning zone.

Today, we know that both of those lines of thinking are flawed. The concept that high-intensity cardio will burn muscle while low-intensity cardio will spare muscle is quite wrong. In fact, if you just compared the muscle mass of long-distance runners (who spend a good deal of their training at a slower pace for longer periods) to sprinters (who spend a good deal of their training at higher intensities for short periods), you get a good idea of just how flawed that logic is.

When you train at a slow and steady pace for a longer period of time, you’re training your muscle fibers to be more aerobic and have greater endurance. There’s some evidence that suggests that muscle fibers adapt to becoming more aerobic by becoming smaller and weaker, because the smaller a muscle fiber is, the less time it takes for nutrients to travel within the muscle fiber. This speeds up the rate at which nutrients can be burned for fuel.

Another way to consider the misconception that low-intensity cardio performed for longer periods will better spare muscle mass than high-intensity cardio is to compare a high-intensity squat workout done for 5 sets with a weight that limits you to 10 reps per set and a low-intensity squat workout done with a weight that allows you to complete 100 reps per set. Would the higher intensity leg workout of 10-rep sets “burn up” muscle tissue while the lower intensity leg workout of 100-rep sets would better maintain muscle? No. If anything, it would be quite the opposite. In fact, doing higher-intensity cardio, particularly HIIT (discussed below) may actually help to increase muscle mass.

While lower-intensity cardio has been shown to burn a higher percentage of calories from fat, you actually burn fewer total calories with this method. To burn an equivalent amount of calories and fat as high-intensity cardio, you would have to exercise for considerably longer. One obvious problem with doing excessive cardio is time. Most of us barely have time to fit in a 60-minute weight workout, let alone another 60 minutes or longer of cardio. However, another problem with excessively long cardio, particularly for men, is that it has been found to lower testosterone levels.

Yet focusing on just how many calories, and how many calories you burn from fat, during a workout is also a flawed approach. The real benefit of cardio for fat loss is the amount of calories (and calories from fat) you burn after the workout. This is due to the process known as EPOC (excess post-exercise oxygen consumption), which refers to the boost in your metabolism and calorie burning that comes after the workout is over. When you workout, you burn calories to fuel your muscles during the workout. But when the workout is over, your body keeps burning more calories than normal, despite the fact that you’re doing nothing. This is due to the processes involved in recovery from exercise. After exercise, your body must repair damaged muscle fibers, restock muscle glycogen levels and remove lactic acid from the muscles, among other things. All these processes require calories, with a lot of those calories coming from fat. And when it comes to EPOC, this is where HIIT really trumps steady-state cardio done at a lower intensity.

The Science Behind HIIT
High-intensity interval training (HIIT) is a form of cardio that involves intervals of high-intensity exercise (such as running at a very fast pace) interspersed with intervals of low-intensity (walking at a slow pace) or complete rest. This is in sharp contrast to the typical continuous steady-state (slow and steady) cardio done at a moderate intensity, such as walking at a fast pace or jogging for 30 to 60 minutes.

Although HIIT seems to have gained popularity in the last few years, the concept is actually quite old. The origin of HIIT can be traced back many decades to a technique called Fartlek training that was used by track coaches to better prepare runners. The term “Fartlek” is Swedish for "speed" (fart) and "play" (lek), so it means speedplay, which is essentially what HIIT is. Today, HIIT has crossed over to the fitness industry due to beneficial results that have been established through anecdotal reports and published research studies. In fact, studies comparing HIIT to continuous steady-state cardio have shown that HIIT is far superior for fat loss, despite the fact that it requires much less time.

One of the first studies to discover that HIIT was more effective for fat loss was a 1994 study by researchers at Laval University (Ste-Foy, Quebec, Canada). It reported that young men and women who followed a 15-week HIIT program lost significantly more body fat than those following a 20-week continuous, steady-state endurance program, despite the fact that the steady-state program burned about 15,000 calories more than the HIIT program.

A 2001 study from East Tennessee State University demonstrated similar findings with subjects who followed an 8-week HIIT program (subjects dropped 2% in percent body fat) compared to those who followed a continuous, steady-state program (subjects had no drop in body fat). A study from Australia reported that females following a 20-minute HIIT program that consisted of 8-second sprints followed by 12 seconds of rest lost six times more body fat than a group who followed a 40-minute cardio program performed at a constant intensity of 60% of their maximum heart rate.

A recent study from the University of Western Ontario suggests that you can burn off more body fat than slow and steady cardio with even less than 15 minutes of HIIT. The Canadian research team had male and female subjects follow one of two cardio programs for six weeks. One group of subjects ran slow and steady for 30-60 minutes, three times per week. The other group did four to six 30-second sprints with a 4-minute rest period between sprints three times per week – that’s basically HIIT with an extended rest period between the high-intensity exercise intervals. They reported in a 2011 issue of the journal Medicine and Science in Sports and Exercise that the group doing the sprint intervals lost more than twice as much body fat as the slow and steady group despite the fact that they only did 2-3 minutes of total cardio exercise per day and just 6-9 minutes per week! The sprint interval group also gained over one pound of muscle. This shows that HIIT cardio not only burns off body fat and spares muscle, but it may even help build it.

Metabolic Boost
One of the major reasons that HIIT works so well to drop body fat as compared to steady-state cardio appears to be due to the greater increase in resting metabolism following HIIT. Baylor College of Medicine (Houston, TX) researchers reported that subjects who followed a HIIT workout on a stationary cycle burned significantly more calories during the 24 hours following the workout than those who cycled at a moderate steady-state intensity. The East Tennessee State University study mentioned above also found that subjects following the HIIT program burned more calories during the 24 hours after exercise than the steady-state cardio group. A study presented at the 2007 annual meeting of the American College of Sports Medicine by Florida State University (Tallahassee) researchers reported that subjects who performed HIIT burned about 10% more calories during the 24 hours following exercise than those who performed continuous steady-state exercise, despite the fact that the total calories burned during the workouts were the same.

In addition to the increase in resting metabolism, research confirms that HIIT is effective at enhancing the metabolic machinery in muscle cells that promote fat burning and blunt fat production. The Laval University study that found a decrease in body fat with HIIT discovered that the HIIT subjects’ muscle fibers had significantly higher markers for fat oxidation (fat burning) than those in the continuous steady-state exercise group.

A study published in a 2007 issue of the Journal of Applied Physiology reported that young females who performed seven HIIT workouts over a two-week period experienced a 30% increase in both fat oxidation and levels of muscle enzymes that enhance fat oxidation. Research shows that this may be due to an increase in the number of mitochondria in muscle cells (mitochondria is the machinery in cells that burns fat to produce energy). A study from the Norwegian University of Science and Technology (Trondheim) reported that subjects with metabolic syndrome who followed a 16-week HIIT program had a 100% greater decrease in content of the fat-producing enzyme fatty acid synthase as compared to subjects who followed continuous moderate-intensity exercise. In other words, HIIT enhances your body’s ability to burn fat and prevent the storage of fat.

Another way that HIIT appears to work has to do with getting fat to where it will be burned away for good. One study published in the American Journal of Physiology reported that six weeks of HIIT increased the amount of special proteins in muscle cells that are responsible for carrying fat into the mitochondria (where fat is burned away for fuel) by up to 50%. Having more of these proteins in muscle means that more fat can be burned up for fuel during workouts and when resting.

HIIT for More Muscle
As I’ve already mentioned, HIIT will not only help you maintain your muscle, but can actually help build muscle mass. A study published in the Journal of the International Society of Sports Nutrition reported that male subjects following a six-week HIIT program (done for 15 minutes per day at a 2:1 work-to-rest ratio, 3 days per week) while supplementing with beta-alanine gained over two pounds of muscle, despite the fact that they never lifted weights during the program. The 2011 study on sprinting from the University of Western Ontario discussed above reported that those performing 30-second sprint intervals actually gained some muscle mass, while the slow and steady cardio group did not.

Another study from the UK reported that obese subjects following a low-carb diet lost muscle, yet those performing HIIT along with the low-carb diet were able to maintain muscle mass. This makes sense when you consider that weight training is technically a form of HIIT – you have short periods of high-intensity exercise interspersed with periods of rest.

One reason why HIIT may lead to greater gains in muscle mass may be due to the anabolic hormone testosterone. New Zealand researchers had competitive cyclists complete four weeks of HIIT training involving 30-second sprints on a stationary cycle separated by 30 seconds of rest. One group sprinted with high resistance on the pedals, making it harder to pedal, while the other group used a lighter resistance, which was easier to peddle. Both groups peddled as fast as they could during the 30-second sprints. They found that the men peddling at the highest resistance increased their testosterone levels by almost 100%, while the group peddling at a lighter resistance only increased test levels by about 60%.

Another reason for both the health benefits of HIIT and its benefits on muscle mass, not to mention fat loss, has to do with improved insulin sensitivity. When you improve insulin sensitivity, not only does this help to keep you lean and prevent diabetes, but it can also aid muscle growth. Insulin is an anabolic hormone that acts on the muscle cells to increase muscle protein synthesis, decrease muscle protein breakdown and drive more glucose, amino acids, creatine and carnitine into muscle cells.

Here’s a study to back these claims up: Watt University (Edinburgh) researchers had subjects follow a two-week training program with workouts consisting of just four to six 30-second sprints on a stationary cycle. Sprints were separated by four minutes of rest. They discovered that at the end of the two weeks, the subjects’ blood glucose and insulin levels were reduced by almost 15% and 40%, respectively, following the consumption of 75 grams of glucose. Insulin sensitivity, which is the measurement of how well insulin does its job at the muscle cells, improved by about 25%.

Without a doubt, there’s a mountain of research that says HIIT is a better option for getting lean and maintaining (and even building) muscle mass than your typical low-intensity 45-minute treadmill session. But this isn’t to say that you should never do steady-state cardio. If you enjoy jogging, hiking and/or cycling, then by all means include that exercise in your program. However, you should still consider adding a few days of HIIT into your routine. This will not only improve your physique, but will also improve your performance when you do steady-state cardio.

Now that I’ve sold you on HIIT cardio, it’s time to look at some actual HIIT workout programming. That’s what Part 2 of this Complete Cardio series will offer. In that installment, you’ll find my “beginner-to-advanced” HIIT routine, which is great for anyone still getting used to the high-intensity nature of HIIT training. In the meantime, keep hitting the weights hard!

Part 2 HIIT Programming

Part 3 Frequency, Timing & Fasted Cardio

Part 4 Tabata & Cardioacceleration

Part 5 Power HIIT

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