Human Muscle Fiber Types
Classification
and
Properties
Muscle Fiber Classification
Classification
Basis
Contractile
Properties
slow
fast
Metabolic
Properties
oxidative
glycolytic
Color
Properties
red
white
Staining
Properties
enzymes
pH
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, α2, which
belong to slow twitch
recruited first
• Largest alpha motor
neurons, α1, which
belong to fast twitch
recruited last
Motor Unit Recruitment
• Slow twitch with low • Fast twitch with
threshold nerves (α2)
higher threshold
recruited during
nerves (α1) activated
light-to-moderate
as
force
effort.
requirements
• Sustained
increase.
submaximal jogging,
• Sprint running or
cycling, lifting a
light weight at a
swimming or cyclist
slow speed.
ascends hill.
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 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.
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
Diameter
(neuron & fiber)
Force Capacity
Myosin ATPase
Activity
Myoglobin
Content
Metabolic
Properties
Fatigue
Resistance
Type I
Red
Medium
Type IIa
Red
Large
Type IIb
White
Large
Low (5g)
Moderate (20 g)
High (50 g)
Low
High
High
High
High
Low
Oxidative
Oxidative/
Glycolytic
Intermediate
Glycolytic
High
Low
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
• 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.