text: Physiology
  Figure 4-1. Fiber type training
  Type I
  4-20. Muscle contractions during low-intensity endurance efforts are mainly assigned to Type I or slow
  oxidative (known as SO) fibers. These fibers are smaller with more mitochondria (parts of the cell that assist
  with energy production) and capillaries (blood vessels) surrounding them to deliver more oxygen to the fiber.
  Type I fibers are more resistant to fatigue than the others, but Type I fibers have a slower contractile speed
  and generate less force.
  Type IIa
  4-21. During moderate-intensity effort such as one mile runs, Type IIa or fast oxidative-glycolytic (known
  as FOG) fibers are the main determinants of performance. These fibers’ size, contractile ability, and
  resistance to fatigue make them the main ones Soldiers should target in their training. Training to build
  muscle size (hypertrophy) and strength or using moderate to moderate hard intensity (RPE 8/10) when
  performing endurance training recruits more Type IIa fibers. This type of training also encourages the
  conversion of Type IIx fibers to Type IIa.
  Type IIx
  4-22. During brief, maximal intensity bouts of exercise (RPE 10/10), Type IIx or as fast glycolytic (known
  as FG) fibers are recruited. These larger muscle fibers with fast contractile speeds depend on glycogen and
  have poor resistance to fatigue. Once Type IIx fibers are recruited, lactate can no longer be cleared from the
  blood and the lactate threshold is reached.
  MUSCLE STRENGTHENING
  4-23. Heavy resistance training is included in physical readiness training so that a Soldier is ready to perform
  baseline occupational tasks. This type of training prepares Soldiers for the unplanned extreme load and
  intensity tasks that he or she might encounter in training, deployment, or everyday life such as responding to
  a motor vehicle accident or moving furniture. A specific real-life example is the case of Army medics at the
  Pentagon who had no notice of the physical and nonphysical demands that would be placed on them on
  September 11, 2001.
  4-24. Physical readiness training that stimulates Type II fiber development is necessary for the strength and
  power development that supports anaerobic tasks—such as heavy lifting, heavy load carriage, fast sprints