Type I muscle fibers are slow-twitch fibers suited for endurance activities. They are smaller in size, have many mitochondria and myoglobin which give them a red appearance. Type II muscle fibers are fast-twitch fibers suited for bursts of strength or speed. They are larger in size, have fewer mitochondria and myoglobin which give them a white appearance. Type I fibers are fatigue-resistant while Type II fibers are fast-fatigable.