Human Muscle Fiber Types Classification and Properties Muscle

Human Muscle Fiber Types Classification and Properties

Muscle Fiber Classification

Contractile Properties

Contractile Properties • 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

Contractile Properties

Motor Unit Recruitment • Motor neurons recruited in order of size: Size Principle • Smallest alpha motor neurons, α 2, which belong to slow twitch recruited first • Largest alpha motor neurons, α 1, which belong to fast twitch recruited last

Motor Unit Recruitment • Slow twitch with low • Fast twitch with threshold nerves (α 2) higher threshold recruited during light nerves (α 1) activated -to-moderate effort. as force requirements • Sustained increase. submaximal jogging, cycling, lifting a light • Sprint running or swimming or cyclist weight at a slow ascends hill. speed.

Metabolic Properties • 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

Metabolic Properties • FOG (IIa) and FG (IIb) have higher glycogen stores than SO (I). • SO (I) has more triglyceride than either FG or FOG.

Color Properties • Myoglobin contains heme (Fe) which carries O 2 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.

Color Properties Pheasants fly far to survive. Pheasant pectoralis muscle has myoglobin which is red or dark.

Staining Properties • 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 highspeed muscle shortening.

Muscle Fiber Types

Distribution of Fiber Types • All muscle composed of ST & FT fibers • Distribution varies from muscle to muscle within an individual • Most individuals possess between 45 and 55% ST

Distribution of Fiber Types • 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.

Distribution of Fiber Types • 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

Illustration References • Mc. Ardle, William D. , Frank I. Katch, and Victor L. Katch. 2000. Essentials of Exercise Physiology 2 nd ed. Image Collection. Lippincott Williams & Wilkins. • Plowman, Sharon A. and Denise L. Smith. 1998. Digital Image Archive for Exercise Physiology. Allyn & Bacon.