Slow twitch fibers are those used primarily in endurance activities such as distance running. Weight training with high repetitions of relatively light weights will stimulate and develop these muscle fibers.
Slow twitch fibers
Slow-twitch fibers are muscle fibers that are used more when compared to the rest. Your legs are full of slow-twitch fiber because you are on your feet much of the time.
However, some people inherit a high percentage of slow twitch fibers that enhance their performance in endurance activities, such as long distance runners. Most world class marathon runners have a very high amount of slow twitch fibers.
In general, slow twitch fibers are activated first followed by larger fast twitch fibers. Now when muscles begin to fatigue the asynchronous firing of fibers become more and more synchronized (Butchal, 1950). This allows for greater force production.
The diagram above shows the structure of the muscles. The inset picture (brown colored) shows you the fast and slow twitch fibers. Lighter = fast twitch / darker = slow twitch.
Characteristics of Different Muscle Fibers
Fibre Type ...
Marathon runners generally have a higher percentage of slow twitch fibers.
Muscles that primarily maintain posture against gravity require more endurance and generally have a higher percentage of slow-twitch fibers.
Slow twitch fibers have fewer contractile elements and stored ATP but many more mitochondria. Because they have lots of mitochondria they can sustain lower levels of contractile activity for very long periods of time.
For endurance athletes, more slow twitch fibers is optimal. This test does not replace an actual muscle fiber composition test, which would be more accurate but involves an invasive muscle biopsy and more sophisticated analysis.
As for your muscle physiology, the relative proportion of fast twitch and slow twitch fibers in your muscles is also genetically predetermined, and slow twitch muscle fibers are able to consume more oxygen than fast twitch muscle fibers.
Marathon runners tend to have more slow twitch fibers. And the rest of us tend to have an equal distribution of both fiber types. It is not clear whether training can change the distribution of fiber types within an individual.
 See also: Fiber, Muscle fiber, Slow, Aerobic, Endurance
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