My clients often ask which type of cardio training is best for them. My answer, as with most things is that it depends on your goal. Let’s start with some basic definitions and then we can dive into some pros and cons of each type of training.
Steady-state cardio workouts are simple. Perform your activity (jogging, swimming, cycling) at a steady, challenging, but manageable pace (60%-70% of maximal capacity) for 20 minutes or more aiming for a heart rate of 120 – 150 beats per minute.
High Intensity Interval Training (HIIT)
HIIT workouts are slightly more complex. Turn “on” your activity as hard as you can (75% – 90%) of maximal capacity for a brief, set time period (usually less than 2 minutes) then back off for a predetermined rest interval (usually at least twice the “on” time period), and repeat the cycle 4 times or more.
There are 2 ways that muscle can burn glucose (blood sugars): Aerobic work (with oxygen) and anaerobic work (without oxygen). Steady-State cardio is generally aerobic. It requires oxygen and is fueled mostly by stored fat. 60 percent of the calories you burn come from fat stored in your body. This is the classic “fat burning zone” and is generally measured by the absence of dramatic elevation in your heart rate.
Interval work is anaerobic and doesn’t rely exclusively on oxygen and is fueled mostly by stored carbohydrates. When you exercise at a high intensity, your body uses carbohydrates for fuel and only 35% of the calories you burn come from stored fat.
If my goal is to lose fat, which style of cardio do I choose?
It appears that you would want to be in the “fat burning zone” to lose fat, but actually the math works more in your favor if you add higher intensities. Yes 60% of the calories you burn come from fat when doing steady state cardio, but you burn far more total calories (thereby more fat calories) overall when you work at higher intensities. But it is unrealistic to exercise at a high intensity, such as 85% or higher of your maximum capacity for a long duration, such as 30 – 40 minutes. The best result is combining low and high intensity (aka: HIIT) in a workout. This proficient method accelerates fat loss and burns more calories. Remember, the faster you burn 3,500 calories, the faster you will burn one pound of fat.
How do I increase my cardiovascular strength for long-endurance activities such as cycling, running and hiking?
VO2Max is the body’s upper limit for consuming, distributing and using oxygen for energy production. Also referred to as the maximal aerobic capacity. VO2Max is a good predictor of exercise performance. A recent study measured the VO2Max responses among men and women who participated in an 8 week HIIT program and a continuous cardiovascular training program. VO2Max increases were higher in the HIIT program (15%) than they were in the continuous training program (9%) (Daussin et al. 2008).
I have limited time to exercise, which is my best bang for the buck?
One of the benefits of a HIIT program is that you can burn the same number of calories in less time. In this case, the intensity matters more than the duration. To get the most benefits from the “interval” component of the program you must get your body over the Anaerobic Threshold (AT) and the Lactate Threshold (LT). A simple test is that you should not be able to hold a conversation during the “on” phase. You really do have to push your body so it is uncomfortable, and you can’t sustain it for long. Then you get the release of the “off – or active recovery” phase to recover.
With HIIT, you are burning calories at the moment of exertion but you actually change the muscles metabolism. You get a boost in metabolism because you increase the mitochondria (energy producing units) density of your muscle, so you increase the muscles oxidative capacity and you burn more calories. With Steady-State cardio you only burn the calories at that precise moment (no boost in metabolism) and over time your body adjusts to low intensity exercise as the new norm.
Another metabolic benefit of HIIT is Excess Post-exercise Oxygen Consumption (EPOC). After a HIIT exercise session, oxygen consumption (and thus caloric expenditure) remains elevated as the working muscle cells restore physiological and metabolic factors in the cell to pre-exercise levels. This translates into higher and longer calorie burning after exercise has stopped. Studies have shown higher EPOC values with HIIT programs than with steady-state training (LaForgia, Withers and Gore (2006)). The end result is that you will have more calorie burn during and post exercise.
I want to lose fat, not muscle. Which is better?
Steady-state cardio is more catabolic (muscle wasting) than HIIT cardio. Your metabolism becomes accustomed to the demands of steady state cardio more rapidly. During long periods of steady state cardio you become glycogen depleted (the stored carbs in muscle) and your body will look for other energy sources in the form of protein in your muscles which leads to the cycle of catabolism (muscle wasting). Think of the differences in marathon runners bodies compared to a sprinters body. HIIT actually builds muscle by overloading them (for short periods) stimulating muscle fibers, which in turn means more muscle growth.
A word of caution abut HIIT
If you are a beginner, start slowly. You must master steady-state cardio first. Build up to 30-40 minutes at a stretch 2 times a week, for a period of 2-3 months. If your resting heart rate is below 60, feel free to experiment with HIIT. If it is above 65, then stick with the steady-state cardio a little longer.
The Bottom Line
Both styles are beneficial and useful, but focus on the type of cardio you prefer. You will be more committed and work harder if you enjoy it. HIIT takes less time, proves to be more effective for fat loss, creates metabolic changes and helps with muscle retention. Steady-state gives you more aerobic benefits, still helps with fat loss, good for beginners, less mentally taxing, and sometimes the perfect antidote to a high intensity day. Choose your cardio workouts to meet your goals and bring enjoyment to your fitness.
Alicia Guerra, MSPH, CPT, RYT200