Ben Greenfield: Cardiovascular Exercise: High Intensity vs. Low Intensity

Ben GreenfieldBy Ben Greenfield—Sports Science and Exercise Physiology Expert. Fitness Trainer, Coach, and Ironman Triathlete

Cardiovascular exercise is any physical activity that uses the large muscles like your legs, raises your heart rate, and makes your heart and lungs stronger. It has many health benefits, such as lowering your blood pressure and burning calories, which is of particular benefit for people who want to lose weight.

When it comes to cardiovascular exercise, the most commonly asked question I get is “Should I work out long and easy, or should I workout short and hard?” This question goes directly to the heart of the most important idea behind a good cardiovascular exercise routine. To be honest, the answer is “It depends.” Let’s start out by looking at the total amount of calories burned with each approach.

If I ask you to traverse a mile on foot, many professional trainers and exercise experts will say it doesn’t matter how you do it—walk, jog, or run you will burn the same number of calories whichever way you do it, as long as the distance is the same; in this case, a mile. This is a fallacy—it’s just not true. Numerous studies have proved that the faster you cover the distance, the more calories you will burn. This is similar to the way fuel is consumed in a car: The faster you go, the more energy you will use. You will burn the most calories by cycling, running, rowing, swimming, or doing any other cardiovascular exercise as fast as you can because you will use more muscles and require more blood flow and energy production. The bonus is that the faster you exercise, the higher your post-exercise metabolism becomes, which means that you will burn more calories throughout the day after your workout than if you had exercised at a slower pace. This adds to one’s fitness, including increased mitochondrial density, blood vessel capillarization, oxygen and nutrient delivery to tissues, etc.

However, many people become confused because the faster you “move” across that distance, the more you rely on carbohydrates for energy and the less you rely on fat.

The body depends on both fat and carbs for energy all the time but uses them in different ratios depending on the intensity of the activity and the fitness of the individual. Although burning carbohydrates is a good thing, your body should also learn how to efficiently use fat as an energy source, especially if you want to trim your waistline, lose some weight, and thereby increase athletic performance. The ideal cardiovascular workout should burn the maximal amount of calories and burn the correct ratio of carbohydrates to fat. In other words, you want your workouts to burn both fat and carbs.

The “fat-burning” zone varies from person to person, but a good rule of thumb is that when breathing becomes labored or the muscles begin to burn, you’ve crossed into the “carbohydrate-burning zone” as the primary energy source and are out of the “fat-burning zone.” The scientific fact is that it takes more oxygen to burn one calorie from fat than it takes to burn one calorie from carbohydrate. During exercise, as your body starts to work harder, your body begins to absorb less oxygen and increases the utilization of your carbohydrates as an energy source.

From an athletic performance perspective you want to condition your body to conserve energy, especially in an endurance event such as a marathon, a triathlon, or a 100-mile cycling event. Since it takes more energy to burn carbs and since your body generally has a more limited supply of carbs compared to fat, the more fit you are the more your body can rely on fat rather than carbs to go the distance. This will improve your performance over longer distances and extended time periods.

Nutrition also plays an important role in this. Magnesium, as one of the key electrolytes, is an excellent example of an energy nutrient because it activates enzymes that control digestion; absorption; and the utilization of proteins, fats, and carbohydrates. Make sure you are not low on magnesium as most people are. Natural Calm® is a great source of magnesium and can be mixed right into a water bottle for use during exercise.

That about covers the science behind this. Now let’s get to the practical application. If you are pressed for time in your workout, then exercise short and fast (i.e., 10–20 minutes, at a high intensity level). You will burn more calories, both in your workout and throughout the rest of the day. If you have the time, however—and this is more ideal—you should also incorporate a long and slow cardiovascular workout into your routine (i.e., 20–60 minutes, at an easy or low intensity), in essence, training your body to burn fat as a fuel. I often have my clients work in both zones by having them do their short and hard cardio workouts prior to weight training on their “difficult” days and then changing things up on their “easy” days by doing long and slow workouts. This also means that the long and slow cardio training day allows the body to recover more rapidly from the previous day’s difficult efforts, which translates to better results.

Here is a sample cardiovascular exercise workout that will alternate carb-burning and fat-burning zones during the same workout. This is called an “interval” routine and can be done by cycling, running, rowing, using a stair climber, or even doing uphill walking, etc. Here’s how it works:

  • 5-minute graded warm-up, gradually working up to a hard intensity by minute 5
  • 1-minute hard-fast level of effort (labored breathing)
  • 2 minutes of easy-medium level of effort (can hold conversation while exercising)
  • 2 minutes of hard-fast level of effort
  • 1 minute of easy-medium level of effort
  • repeat 1x
  • 3 minutes of hard-fast level of effort
  • 3 minutes of easy-medium level of effort
  • 4 minutes of hard-fast level of effort
  • 4 minutes of easy-medium level of effort
  • repeat 1x
  • 5-minute cool-down, gradually working down to a very easy level of effort by minute number 5

Ben Greenfield, M.A. Sports Science and Exercise Physiology—Mr. Greenfield was voted the 2008 Personal Trainer of the Year by the National Strength and Conditioning Association and is the author of Shape21—The Complete 21 Day Lean Body Manual along with several other books, including Top 12 Resistance Training Routines for Triathletes, Run With No Pain, and 100 Ways to Boost Your Metabolism. Grab Ben’s free newsletter and podcast at www.bengreenfieldfitness.com.