Most competitive bodybuilders do hours of aerobics with the goal of adding new striations and cuts at each session. Well, let's face it: Cardio can be boring and monotonous. In fact, it's often thought of as a necessary evil for achieving road-map veins, shredded quads and delineated abs.
The debates about cardio are vast, and the subjects include high-intensity interval training vs. long-duration work; optimal frequency and timing of cardio; and precardio and postcardio nutrition. Because of their complexities, we're dividing the discussion into a series. We begin with one of the most heated debates of all: HIIT vs. long-duration cardio.
The first way of enhancing fat metabolism from aerobic workouts is during the session itself, in which the goal is to increase total calorie expenditure and fat burning. The second way is to actually produce metabolic effects in your body after the workout-for example, by improving insulin sensitivity, decreasing fat storage after a meal and speeding metabolism.
HIIT advocates claim that the latter effect of cardio is much more important and is maximized following HIIT sessions, while long-duration-cardio advocates suggest that the former effect is more important and that long-duration cardio will burn more calories and use more fat during the workout than sprints.
So who's right? Well, as you'll discover shortly, it may very well be both.
Optimizing Fat Oxidation During Cardio Workouts
You can optimize fat oxidation, or burning, during cardio workouts by manipulating exercise intensity. By intensity we mean the percentage of maximum aerobic capacity (VO2 max) at which you work, as determined by oxygen consumption.
Two variables determine how to maximize fat oxidation during cardio:
1) How many calories you expend at a given intensity.
2) What types of calories you're actually expending.
A classic study compared the body's fuel use during 25 (low), 65 (moderate) and 85 (high) percent VO2 max intensity.1 While the highest percentage of fat was used during the 25 percent exercise intensity, the highest total amount of fat oxidation occurred during the 65 percent intensity. Moderate-intensity cardio uses a lower percentage of fat, but it metabolizes more total calories. So more total fat and calories are expended at moderate intensity. That means if you want to maximize total fat use during your cardio workouts, you should exercise at 65 percent of your VO2 max.
Calculating Your Cardio Intensity
In the lab we measure exercise intensity by the amount of oxygen you can take in and use, but that’s clearly impractical for you. A second way is to estimate the percent of your VO2 max at which you’re doing cardio, which is your heart rate reserve.
Heart rate reserve is calculated by subtracting maximum heart rate by resting heart rate. To figure your maximum heart rate, use what’s called the Karvonen formula, which is 220 minus your age. If you’re 20 years old with a resting heart rate of 60 beats per minute, your maximum heart rate would be 200 (220 – 20), and your heart rate reserve would be 140 (200 – 60).
Say you want to train at 65 percent of your VO2 max. Simply multiply your heart rate reserve by .65, which would be 91 beats per minute.
Maximizing the Metabolic Benefits of Cardio Through High-Intensity Interval Training
The second issue is how to maximize metabolic benefits after your cardio session. While HIIT doesn't optimize fat burning during the cardio session, it apparently does so afterward. You get five benefits from HIIT on postcardio metabolic adaptations:
1) Excess postexercise oxygen consumption
2) Increased fat oxidation
3) Improved muscle glucose disposal
4) Enhanced mitochondrial density
5) Enhanced muscular hypertrophy
High-intensity interval training is often said to use more calories after exercise than during it. Because of that, advocates of HIIT suggest that a 10-to-15-minute HIIT session will use more calories over a 24-hour period than a 60-minute moderate-intensity session.
While excess postexercise oxygen consumption is a clear metabolic benefit of HIIT, its effects have been greatly exaggerated. Even the most grueling HIIT session will use only the number of calories found in a cup of orange juice.2 So excess postexercise oxygen consumption in and of itself is not an adequate reason to perform HIIT.
However, fat oxidation increases after an HIIT session. That's because HIIT causes a greater increase in hormones that speed fat metabolism, including growth hormone and the catecholamines, epinephrine and norepinephrine.
Another effect of HIIT is enhanced glucose uptake in muscles. After cardio your muscles can take in more carbohydrates from the blood-muscular contraction stimulates the insertion of glucose transporters, such as glut-4 proteins, in the muscle cell membrane. That's important because if they can use a higher portion of the carbohydrates you eat, then fewer carbs will enter fat tissue and become unwanted fat storage.
It's important to understand that only the muscle fibers that are activated during cardio increase glucose uptake. Therefore, HIIT maximizes postexercise muscle glucose deployment because it activates more muscle fibers.3
Chronic HIIT training may change your muscle tissues' ability to use fat and metabolize the meals you eat. When you train aerobically, your muscle fibers adapt by creating new mitochondria,4 the cell structures responsible for using fat for fuel. The greater the number of mitochondria, the greater your capacity to use fats.
The problem with low-to-moderate-intensity cardio is that it uses only a moderate fraction of your muscle fibers to power the activity, whereas HIIT training requires your body to recruit many of the larger muscle fibers. That leads to several adaptations, such as mitochondrial biogenesis, that enhance fat metabolism.4
Another chronic adaptation from HIIT is muscular hypertrophy, or growth. HIIT very similar to weightlifting in stimulating muscle growth-just look at the difference between sprinters and long-distance runners. Need more be said?
We're left at a standoff: Long-duration, moderate-intensity cardio will optimize total calorie expenditure and fat burning during the workout, but HIIT clearly has superior metabolic benefits afterward.
So which should you use? How about both?
Combining HIIT With Moderate-Intensity Cardio
For years we've advocated a concept that suggests the cardio debate is based on a false dichotomy. In other words, we don't have to choose between HIIT and moderate-intensity cardio because we can use both.
When you perform an HIIT session, several fat-metabolizing hormones increase. The problem is that at the same time the nervous system decreases the diameter of the blood vessels that carry blood to your bodyfat, meaning that you don't get the benefit of high fat metabolism during the session. Immediately after the high-intensity session, however, when intensity is lowered, blood flow returns to fat tissue at a high rate, meaning epinephrine and norepinephrine have a chance to work their magic on fat cells.
We suggest performing 10 to 20 minutes of HIIT cardio to raise the secretion of hormones known to stimulate the breakdown of fat, followed by 30 to 40 minutes of moderate-intensity cardio, which can maximally use the fatty acids for fuel. Studies clearly demonstrate that traditional cardio burns more fat when it's performed following high-intensity exercise than when it's done alone. One study found that both the actual energy cost and rate of fat metabolism of moderate-intensity running were higher when it was performed following interval training.5 Another indicated that performing moderate-intensity cardio after resistance training resulted in maximum fat burning during the cardio session.6 Note that it's important to perform the moderate-intensity cardio immediately after your high-intensity workout, as the metabolic effects are short lived and quickly dissipate.
Cardio is a brutal but necessary evil for any bodybuilder seeking shredded glutes and washboard abs. With our recommendations, you'll at least have the satisfaction of knowing you're getting the most out of every brutal second.
Editor's note: Gabriel Wilson is completing his Ph.D. in nutrition with an emphasis on optimal protein requirements for muscle growth and is a researcher in the Division of Nutritional Sciences, University of Illinois, Urbana. He is vice president of the Web site ABCBodybuilding.com. Jacob Wilson is a skeletal-muscle physiologist and researcher in the Department of Nutrition, Food, and Exercise Science, Florida State University, Tallahassee. He is president of the Web site ABCBodybuilding.com.
1 Romijn, J.A., et al. (1993). Regulation of endogenous fat and carbohydrate metabolism in relation to exercise intensity and duration. Am J Physiol. 265(3 Pt 1):E380-391.
2 Laforgia, J., et al. (1997). Comparison of energy expenditure elevations after submaximal and supramaximal running. J Appl Physiol. 82(2):661-666.
3 Burgomaster, K.A., et al. (2007). Divergent response of metabolite transport proteins in human skeletal muscle after sprint interval training and detraining. Am J Physiol Regul Integr Comp Physiol. 292(5):R1970-1976.
4 Talanian, J.L., et al. (2007). Two weeks of high-intensity aerobic interval training increases the capacity for fat oxidation during exercise in women. J Appl Physiol. 102(4):1439-1447.
5 James, D.V., and Doust, J.H. (1998). Oxygen uptake during moderate intensity running: Response following a single bout of interval training. Eur J Appl Physiol Occup Physiol. 77(6):551-555.
6 Goto, K., et al. (2007). Effects of resistance exercise on lipolysis during subsequent submaximal exercise. Med Sci Sports Exerc. 39(2):308-315. IM