Jason R. Karp, Ph.D. November 18, 2011 TWEET COMMENTS 6

Physiology Lessons - Page 3

LESSON 2: There are different muscle-fiber types.

Some runners have superior speed, while others have superior endurance. Much of this difference is due to the types of fibers that make up different muscles, which also dictate how you respond to training.

Humans have three different types of muscle fibers, with gradations between them. Slow-twitch (ST) fibers are recruited for all of your aerobic runs, while fast-twitch B (FT-B) fibers are recruited for short, high-intensity sprints. Fast-twitch A (FT-A) fibers, which represent a transition between the two extremes of ST and FT-B fibers, are recruited for prolonged anaerobic activities, like racing 400 meters. Some distance runners may have 90-percent ST and 10-percent FT fibers (marathoners), while others may have 60-percent ST and 40-percent FT fibers (milers).

Understanding your fiber type (see sidebar on next page) helps you train smarter. While most runners do the same workouts to focus on a specific race, your training and racing should reflect your physiology. For example, the more slow-twitch fibers you have, the more you'll benefit from aerobic training; the more fast-twitch fibers you have, the more you'll benefit from speedwork.

If both runners want to race a 5K or 10K, the former should initially do longer intervals, trying to get faster with training, such as three-quarter-mile repeats at 5K race pace, increasing speed to 3K race pace or decreasing the recovery as training progresses.

The latter runner should do shorter intervals, trying to hold the pace for longer with training, such as half-mile repeats at 3K race pace, increasing distance to three quarters of a mile or increasing the number of repeats as training progresses.

How to Efficiently Train According to Muscle-Fiber Type

:: More ST fibers: Emphasize mileage and LT runs.

:: More FT fibers: Emphasize interval training.

LESSON 3: A larger, stronger heart can pump more blood and oxygen to your muscles.

Stroke volume is the amount of blood the heart pumps with each contraction. Multiply stroke volume by your heart rate, and you get the amount of blood your heart pumps each minute. This is the cardiac output. The larger your left ventricle, the more blood it can hold; the more blood it can hold, the more blood it can pump.

A large heart is a physiological condition of genetically gifted and highly trained runners, known in the scientific community as Athlete's Heart. While you may never attain the heart size and associated cardiac output of an Olympic champion, specific training can make your heart larger and improve your performance.

Intervals provide the heaviest load on the cardiovascular system because of the repeated attainment of the heart's maximum stroke volume and cardiac output (VO2max). In response, the left ventricle enlarges so that more blood and oxygen can be sent to the working skeletal muscles.


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