What type of muscle fibers are characterized by low resistance to fatigue and high glycolytic ability?

The correct choice identifies Type IIb fibers, which are primarily characterized by their low resistance to fatigue and high glycolytic ability. These muscle fibers are also known as fast-twitch fibers and are specially designed for rapid and powerful contractions. They rely predominantly on anaerobic metabolism, utilizing glycolysis for energy production, which leads to quick bursts of strength and speed but is not sustainable over longer periods.

Due to their reliance on quick energy sources and their metabolic pathways, Type IIb fibers lack endurance, resulting in quicker fatigue during prolonged or sustained activities, aligning with the question's emphasis on low fatigue resistance. Additionally, they are less efficient at oxygen usage and have fewer mitochondria compared to other fiber types, which further supports their classification as fibers that fatigue easily.

In contrast, Type I fibers, known for their endurance, have a high resistance to fatigue and rely on aerobic metabolism, while Type IIa fibers have intermediate properties—offering a balance of power and endurance. Intermediate fibers, often a blend of characteristics from both Type IIa and Type IIb, do not match the specific description of having low fatigue resistance and high glycolytic capacity as clearly as Type IIb fibers do.