What is aerobic (cardiovascular) exercise?

Many people engage in regular aerobic exercise such as running, cross country skiing, bicycling, rowing, fast walking, roller blading (in-line skating), distance swimming, and health club aerobic programs. During these activities the resistance against movement is not as great as weight training and the activity is sustained for 15 minutes or longer. Because muscle energy is generated by burning fat and carbohydrate in oxygen required processes mostly, these forms of exercise are termed aerobic. And, because the heart and blood vessels are responsible for delivering the oxygen endowed blood to muscle these types of activity are also called cardiovascular exercise.

 

What are the training adaptations occur from aerobic exercise?

Because the resistance to muscle movement is much lower than resistance exercise, muscle enlargement (hypertrophy) is much less pronounced, if at all. However, muscle will adapt in another amazing way. Here the adaptation allows the trained muscle to have greater endurance by increasing its aerobic ATP generative capacity. In doing so there is an increase in the number of mitochondria in the trained muscle cells.

 

Furthermore, the trained muscle develops more capillaries to deliver blood. The increase in the number of capillaries provides more O2 and energy nutrients during exercise. The heart grows a little as well to provide a more powerful stroke and greater cardiac output (blood delivery) to working muscles. A greater heart stroke is often reflected by a slower heart rate when not exercising. Some top endurance athletes have resting heart rates as low as 40 to 45 beats per minute whereas in active people tend to have heart rates between 60 and 75 beats per minute.

 

Which type of muscle fibers are used in aerobic exercise?

During sustained lower intensity efforts (e.g., brisk walking, slow swim) the brain will call upon primarily Type I muscle fibers. Here the intensity is low so epinephrine levels will only be slightly elevated. Working muscle cells will be primarily fueled by fatty acids, with the majority coming from the blood. However, as the intensity of the effort increases so too will epinephrine in the blood and as a result the breakdown of glycogen in working muscle. As this occurs, glucose from glycogen stores starts to become a bigger contributor of fuel.

 

As the intensity level continues to increase, so too will the reliance on glucose. One reason for this is that as the intensity level is increased the brain will support Type I muscle fiber efforts with more and more Type II muscle fibers. Type II muscle fibers tend to use more glucose.