Complex sentence: They contain large and numerous mitochondria and with the high levels of myoglobin that gives them a red pigmentation they have been demonstrated to have high concentration of mitochondrial enzymes, thus they are fatigue resistant.
Split into simpler sentences: They contain large and numerous mitochondria and with the high levels of myoglobin that gives them a red pigmentation. They have been demonstrated to have high concentration of mitochondrial enzymes, thus they are fatigue resistant.