Which type of muscle fiber is a 'reservoir' fiber and upon consistent activation changes into a more oxidative form along the continuum?

Which type of muscle fiber is a 'reservoir' fiber and upon consistent activation changes into a more oxidative form along the continuum?

• A. Type I
• B. Type IIa
• C. Type IIx
• D. Type IIb

Answer: (C)

Muscle fibers aren’t fixed; they can adapt along a spectrum from more oxidative to more glycolytic with training. The so‑called reservoir fiber is the fast‑twitch glycolytic type that sits at the glycolytic end of the spectrum and has the capacity to become more oxidative with consistent activation that emphasizes endurance-style, oxidative demand. In humans, this is the Type IIx fiber. With training, IIx fibers can increase mitochondrial content, oxidative enzyme activity, and capillarization, shifting toward a more oxidative phenotype (more like Type IIa). That plasticity is what makes IIx the reservoir: it’s the fiber most capable of transforming to meet oxidative requirements over time. Type I fibers are already oxidative and tailored for endurance, Type IIa are already intermediate-oxidative fast-twitch fibers, and Type IIb (where present in humans) are highly glycolytic and least prone to becoming more oxidative.

Muscle fibers aren’t fixed; they can adapt along a spectrum from more oxidative to more glycolytic with training. The so‑called reservoir fiber is the fast‑twitch glycolytic type that sits at the glycolytic end of the spectrum and has the capacity to become more oxidative with consistent activation that emphasizes endurance-style, oxidative demand. In humans, this is the Type IIx fiber. With training, IIx fibers can increase mitochondrial content, oxidative enzyme activity, and capillarization, shifting toward a more oxidative phenotype (more like Type IIa). That plasticity is what makes IIx the reservoir: it’s the fiber most capable of transforming to meet oxidative requirements over time.

Type I fibers are already oxidative and tailored for endurance, Type IIa are already intermediate-oxidative fast-twitch fibers, and Type IIb (where present in humans) are highly glycolytic and least prone to becoming more oxidative.