section_path: Fiber types/Twitch speed/Type distribution
section_name: Type distribution
paragraph_text: Type distribution
  
  Individual muscles tend to be a mixture of various fiber types, but their proportions vary depending on the actions of that muscle. For instance, in humans, the quadriceps muscles contain ~52% type I fibers, while the soleus is ~80% type I. The orbicularis oculi muscle of the eye is only ~15% type I. Motor units within the muscle, however, have minimal variation between the fibers of that unit. It is this fact that makes the size principal of motor unit recruitment viable.
  
  The total number of skeletal muscle fibers has traditionally been thought not to change.It is believed there are no sex or age differences in fiber distribution; however, proportions of fiber types vary considerably from muscle to muscle and person to person. Among different species there is much variation in the proportions of muscle fiber types.
  
  Sedentary men and women (as well as young children) have 45% type II and 55% type I fibers. People at the higher end of any sport tend to demonstrate patterns of fiber distribution e.g. endurance athletes show a higher level of type I fibers.Sprint athletes, on the other hand, require large numbers of type IIX fibers.Middle-distance event athletes show approximately equal distribution of the two types. This is also often the case for power athletes such as throwers and jumpers.It has been suggested that various types of exercise can induce changes in the fibers of a skeletal muscle.
  
  It is thought that by performing endurance type events for a sustained period of time, some of the type IIX fibers transform into type IIA fibers. However, there is no consensus on the subject. It may well be that the type IIX fibers show enhancements of the oxidative capacity after high intensity endurance training which brings them to a level at which they are able to perform oxidative metabolism as effectively as slow twitch fibers of untrained subjects. This would be brought about by an increase in mitochondrial size and number and the associated related changes, not a change in fiber type.