Final answer:
Type I muscle fibers, also known as slow oxidative fibers, have high concentrations of capillaries, mitochondria, myoglobin, and oxidative enzymes, which allow them to sustain aerobic activity for long periods and are used primarily in endurance activities.
Step-by-step explanation:
The muscle fiber type that has high concentrations of capillaries, mitochondria, myoglobin, and oxidative enzymes, giving them superior endurance capabilities, is the Type I muscle fibers. These fibers are also referred to as slow oxidative (SO), slow twitch, or 'red' muscle. They are dense with capillaries and are rich in mitochondria and myoglobin, enabling them to carry oxygen efficiently and sustain aerobic activity over extended periods. This high oxidative capacity allows them to generate large amounts of ATP, which sustains muscle activity without fatiguing for long durations. Therefore, Type I muscle fibers are especially suited for endurance activities, such as distance running or cycling, where prolonged muscle contractions are required.
On the other hand, Type II muscle fibers are known as fast twitch muscle fibers. Subdivisions within Type II include: Type IIa, which are also rich in mitochondria and capillaries and can perform aerobic activities; Type IIx fibers, which are the fastest but have less mitochondrial and myoglobin density, thus they fatigue quickly; and Type IIb fibers, which are primarily anaerobic, 'white' muscle with even fewer mitochondria and myoglobin, used mostly in short, high-intensity activities that require quick bursts of power.