Type I muscle fiber is also known as "slow twitch oxidative" fibers. Muscle fiber types can be broken down into two main types: slow twitch (Type I) muscle fibers and fast twitch (Type II) muscle fibers. Type I fibers are used in lower-intensity exercises such as very light resistance work aimed at muscular endurance and long-duration aerobic activities such as 5K and 10K runs. Type I fibers are identified by slow contraction times and a high resistance to fatigue. Structurally, they have a small motor neuron and fiber diameter, a high mitochondrial and capillary density, and a high myoglobin content. ST fibers also have a low supply of creatine phosphate, low glycogen content, and a high store of triglycerides (the stored form of fat). ST fibers contain few of the enzymes involved in glyc