Red fibers

The red fibers are made up of few thick myosin filaments, so they are highly vascularized. They are capable of storing lipids and glucose , which is why they are indicated for a good performance doing aerobic exercises. These are also called “type I fibers,” or slow twitch, or oxidative metabolism (SO) fibers, or S fibers.


  • They are characterized by slower contraction, great resistance to fatigue and good for medium or long-term work because they stop a large number of mitochondria (cells responsible for generating energy for aerobic work).
  • They are small in diameter and contain large amounts of myoglobin and numerous mitochondria, which are arranged in rows between the myofibrils and in clusters below the sarcolemma.
  • These fibers predominate in the postural muscles (trunk muscles) whose activity is continuous. These red muscles contract more slowly, so it has been assumed that the red fiber is a slow fiber.
  • They are rich in myoglobin, a substance that is precisely red in color and that, inside the fiber, transports oxygen to the corpuscles in which it is used, the mitochondria. Oxygen is essential for these fibers since they bring energy almost exclusively from the “aerobic mechanism”, the one in which oxygen mixes with sugars and fats.
  • Each red fiber is surrounded by a greater number of capillaries, precisely to favor the supply of oxygen . These have slightly less force and less speed of contraction (that is, of shortening), but they are endowed with great resistance to fatigue.

Red fibers in atletlas

Athletes who excel in cross-country disciplines (running and marathon runners, road cyclists, cross-country skiers, etc.) normally have muscles in which these types of fibers prevail.


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