Cardiovascular exercise, or cardio, benefits the heart and circulatory system. The defining characteristic is aerobic exercise that extends over a period of time. High-intensity cardiovascular exercise combines traditional cardio with short periods of increased activity; any cardio exercise can be modified for higher intensity. One form is high-intensity interval training, or HIIT, which typically lasts 10 to 20 minutes.
Types of Cardio
Low-intensity cardio includes walking, jogging, swimming, cycling, treadmill and elliptical trainer use, skiing, stair climbing and jumping rope. To change any of these to high-intensity cardio, elevate the intensity periodically during the exercise. For instance, during a brisk walk, jog for a minute or two every five minutes. On a treadmill, increase the speed or the incline occasionally. Tailor the program for the individual. Elite athletes can build up to very demanding HIIT programs.
Possible Risks of Traditional Cardio
In "The Cardio-Free Diet," author Jim Karas writes that traditional cardio can lead to loss of muscle mass in the long run. This is because low-intensity exercise favors the development of oxygen-efficient slow-twitch muscle fibers, also known as Type I muscle. According to Karas, "The introduction of any cardiovascular activity promotes the use of slow-twitch muscles over fast-twitch muscles -- and subsequently causes those fast-twitch muscles to atrophy or diminish." Fitness author Lauren Shaw writes in "Done with Dieting" that traditional aerobic exercise can also lead to muscle deterioration in workout sessions over 20 to 30 minutes. These longer workouts exhaust your glycogen, the ready fuel for aerobic activity, and your body turns to burning the protein stored in muscle tissue.
Stimulating Muscle Growth
To maximize growth in muscle mass and strength, exercise must stimulate the Type II, or fast-twitch, muscle fibers. The traditional means for accomplishing this is high-intensity, low-volume resistance training. According to "Strength Training," by Lee Brown, "(T)o stimulate muscles to grow, they have to be activated -- and to activate the necessary motor units, a relatively heavy load must be used."
Anaerobic Metabolism
High-intensity exercise not only stimulates fast-twitch muscle fibers but also triggers the body to engage in anaerobic metabolism. High-intensity exercise outstrips the body's ability to provide oxygen to the muscles. As a result, protein is not available as an alternative fuel since using it requires oxidation. Instead, the body turns to fuel sources that don't require oxygen, such as creatine phosphate and glucose. The anaerobic use of glucose is known as glycolysis. Both of these anaerobic fuel sources produce acidic waste products that cause muscle discomfort.
HIIT Benefits
The goal of high-intensity cardio is to reap the benefits of traditional cardio without sacrificing muscle mass. For example, high-intensity cardio increases anaerobic capacity, allowing the body to do more work without oxygen. In a 1996 study by researcher Izumi Tabata and colleagues, high-intensity cardio trainees using a HIIT protocol showed a 28 percent increase in anaerobic capacity. According to MayoClinic.com, high-intensity cardio also improves aerobic capacity and burns more calories.
Other Factors
High-intensity cardio workouts support heart and circulatory health so muscles have better and more efficient blood supplies. According to the authors of "Science and Practice of Strength Training," hormone levels also respond positively to high-intensity cardio, with an increase in hormones that assist muscle growth. Improved nervous coordination between Type I and Type II muscles and body muscle groups also enhances performance.
References
- MayoClinic.com: Interval Training: Can It Boost Your Calorie-Burning Power?
- "The Cardio-Free Diet"; Jim Karas; 2008
- "Done With Dieting"; Lauren Shaw; 2004
- "Strength Training"; Lee Brown; 2007
- Medicine & Science in Sports & Exercise: Effects of Moderate-Intensity Endurance and High-Intensity Intermittent Training on Anaerobic Capacity and VO2-Max
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