Which statement accurately describes the general differences between Type I and Type II skeletal muscle fibers?

• A. Type I fibers are slow-twitch, red with high oxidative capacity and fatigue resistance; Type II fibers are fast-twitch with IIa and IIx/IIb subtypes that differ in oxidative and glycolytic capacity.
• B. Type I fibers are fast-twitch and white.
• C. Type I fibers rely mainly on anaerobic glycolysis.
• D. Type IIa fibers are slow-twitch and oxidative.

Answer: (A)

Understanding these fiber types starts with how they meet energy needs. Type I fibers contract slowly, fatigue less easily, and appear red because they are rich in myoglobin. They rely mainly on oxidative metabolism, with many mitochondria and capillaries to support sustained activity. Type II fibers contract quickly and fatigue more rapidly and come in subtypes: IIa fibers are fast oxidative-glycolytic, using both oxygen- and non-oxygen-dependent pathways, while IIx/IIb fibers are more glycolytic, geared toward rapid, powerful movements. This combination is why the statement is accurate: it describes Type I as slow-twitch, red with high oxidative capacity and fatigue resistance, and Type II as fast-twitch with subtypes that differ in oxidative and glycolytic capacity. The other descriptions don’t fit because Type I are not fast-twitch or white, they do not rely mainly on anaerobic glycolysis, and IIa is not slow-twitch.