Final answer:
The highly resistant to fatigue and injury muscle fiber is the slow-twitch muscle fiber (SO fiber), which is adapted for sustained aerobic activity and resists fatigue by utilizing a high density of capillaries, mitochondria, and myoglobin.
Step-by-step explanation:
The muscle fiber that is highly resistant to fatigue and injury is the slow-twitch muscle fiber (SO fiber). This type of muscle fiber is specialized for endurance and is able to sustain aerobic activities such as long-distance running due to its high density of capillaries, abundance of mitochondria, and rich content of myoglobin. These properties allow slow-twitch fibers to carry more oxygen, maximize ATP production during the metabolic cycle, and thus resist fatigue over extended periods.
Slow-twitch fibers are less suited for short, powerful bursts of activity that fast-twitch fibers (especially the fast glycolytic or FG fibers) are known for. FG fibers have fewer capillaries and mitochondria compared to SO fibers and rely on anaerobic glycolysis to create ATP, leading to a quicker fatigue rate. However, they are essential for powerful muscle contractions needed in resistance exercises like weight lifting, where there is a high rate of ATP hydrolysis and cross-bridge formation resulting in muscle hypertrophy.