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 glycolysis, but contain many of the enzymes involved in the oxidative pathways (Krebs cycle, electron transport chain).
ST fibers are predominantly used for aerobic activities requiring low-level force production, such as walking and maintaining posture, but are also the primary fiber type found in endurance athletes. Most activities of daily living use ST fibers.
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