Slow twitch (I) fibers innervated by alpha 2 motor neurons, smaller of the two α motor neurons



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Slow twitch (I) fibers innervated by alpha 2 motor neurons, smaller of the two α motor neurons

  • Slow twitch (I) fibers innervated by alpha 2 motor neurons, smaller of the two α motor neurons

  • Fast twitch (II) fibers innervated by alpha 1 motor neurons, larger of the two α motor neurons

  • Fast twitch (II) fibers have higher excitation threshold and faster conduction velocity





Motor neurons recruited in order of size: Size Principle

  • Motor neurons recruited in order of size: Size Principle

  • Smallest alpha motor neurons, α2, which belong to slow twitch recruited first

  • Largest alpha motor neurons, α1, which belong to fast twitch recruited last



Slow twitch with low threshold nerves (α2) recruited during light-to-moderate effort.

  • Slow twitch with low threshold nerves (α2) recruited during light-to-moderate effort.

  • Sustained submaximal jogging, cycling, lifting a light weight at a slow speed.



All muscle fibers can produce energy both aerobically and anaerobically

  • All muscle fibers can produce energy both aerobically and anaerobically

  • One or the other may predominate or production may be balanced

  • SO (I) fibers rely mainly on oxidative pathways

  • FG (IIb) fibers rely mainly on glycolytic pathways

  • FOG (IIa) possess characteristics like SO and FG



FOG (IIa) and FG (IIb) have higher glycogen stores than SO (I).

  • FOG (IIa) and FG (IIb) have higher glycogen stores than SO (I).



Myoglobin contains heme (Fe) which carries O2 in muscle makes it RED.

  • Myoglobin contains heme (Fe) which carries O2 in muscle makes it RED.

  • Myoglobin cooked turns tan color.

  • Dark turkey meat (thigh) was once oxidative.

  • WHITE turkey meat (pectoralis) had no myoglobin, was not oxidative.

  • So turkeys walk all day and flap occasionally but rarely fly.





Slow twitch (type I) have myosin isoforms with low ATPase activity.

  • Slow twitch (type I) have myosin isoforms with low ATPase activity.

  • Fast twitch (type II) have myosin isoforms with high ATPase activity that promotes rapid breakdown of ATP for energy of high-speed muscle shortening.







All muscle composed of ST & FT fibers

  • All muscle composed of ST & FT fibers

  • Distribution varies from muscle to muscle within an individual

  • Most individuals possess between 45 and 55% ST



Vastus lateralis on average 52% SO, 33% FOG, 14% FG. Same for deltoid, biceps brachii.

  • Vastus lateralis on average 52% SO, 33% FOG, 14% FG. Same for deltoid, biceps brachii.

  • Soleus may have as much as 85% SO.

  • Triceps brachii may have as few as 30% SO.



Great variation between individuals

  • Great variation between individuals

  • Vastus lateralis of elite distance runners had 79% ST, untrained had 58%

  • Available evidence indicates that the distribution of slow and fast twitch fibers is genetically determined and not altered by training



McArdle, William D., Frank I. Katch, and Victor L. Katch. 2000. Essentials of Exercise Physiology 2nd ed. Image Collection. Lippincott Williams & Wilkins.

  • McArdle, William D., Frank I. Katch, and Victor L. Katch. 2000. Essentials of Exercise Physiology 2nd ed. Image Collection. Lippincott Williams & Wilkins.

  • Plowman, Sharon A. and Denise L. Smith. 1998. Digital Image Archive for Exercise Physiology. Allyn & Bacon.



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