Assertion: muscle fiber | /r/IsA | type
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Source sentences:
text: Initially, muscle fibers are classified as three types, including type I fibers (also called slow-twitch oxidative fibers or red fibers), type IIa fibers (also known as fast-twitch oxidative glycolytic fibers or intermediate fibers), and type IIb fibers (also referred to as fast-twitch glycolytic fibers or white fibers).