The most important determinant of the proportions of muscle fiber types a person has is

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The most important determinant of the proportions of muscle fiber types a person has is their genetic heritage. This determines whether they have a higher percentage of slow-twitch (Type 1) or fast-twitch (Type 2) muscle fibers. While exercise and type of exercise can influence the development and adaptation of muscle fibers, genetics plays a primary role in determining the initial makeup of muscle fiber types in an individual. Body size and age can also have some influence, but are not as significant as genetic factors.

The Most Important Determinant of the Proportions of Muscle Fiber