Oxidative fibers, loaded with mitochondria and rich in myglobin (the reason why they are red in colour), that undergo slow, but long-lasting, contractions typical of tonically active postural muscles. Thus, they are resistant to fatigue. The main reason these slow-twitch fibers are slow to fatigue is that they contain more mitochondria than fast-twitch fibers (Type 2), and hence are able to produce more energy. They are also smaller in diameter than fast-twitch fibers and have increased capillary blood flow around them. Because they have a smaller diameter and an increased blood flow, the slow-twitch fibers are able to deliver more oxygen and remove more waste products from the muscle fibers, which decreases their ‘fatigability’. Humans are born with about 40% Type 1 muscle fibers, and during the next two years fiber-type composition changes (e.g., to 60% Type 1 fibers in the deltoid muscle).
See Mitochrondria, Muscle fibers, Myoglobin, Type 2 muscle fibers