Types of Muscle Fibre
Learning Objectives:
1. To know the structural and functional
differences between different types of muscle
fibre.
2. To know how muscle fibre type affects sporting
performance.
Slow Twitch Fibres (Type 1)
 Contract relatively slowly.
 Small in size.
 Produce less force.
 Greater number of blood
capillaries delivering O2.
 Therefore, greater aerobic
capacity and do not fatigue
quickly.
 Produce energy from aerobic
pathways.
 Recruited during low
intensity endurance events.
Fast Twitch Fibres (Type II)
 Contract with greater speed.
 Produce more force due to more
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myosin filaments which are also
thicker.
More developed sarcoplasmic
reticulum (releases and removes Ca
ions more quickly for faster
contractions.
Contain different myosin ATPase
which releases energy more
quickly.
Produce energy from anaerobic
pathways.
Fatigue quickly.
Suited to short, intense burst of
effort (sprinting activities).
Characteristics of
Muscle Fibres
Slow twitch fibres Fast oxidative
glycotic fibres
Fast glycotic
fibres
Structural
Fibres per motor
neurone
(10-180 )
(300-800)
(300-800)
Motor neurone size
Large
Small
Large
Type of Myosin
ATPase
Fast
Fast
Slow
Sarcoplasmic
reticulum
development
low
high
high
Aerobic capacity
low
high
moderate
Anaerobic capacity
high
very high
low
Contractile speed
slow
fast
fast
Functional
Fatigue resistance
moderate
low
high
Motor unit strength
high
low
high
Characteristics of
Muscle Fibres
Slow twitch fibres
Fast oxidative
glycotic fibres
Fast glycotic fibres
Fibres per motor
neurone
10-180
300-800
300-800
Motor neurone size
small
large
large
Type of Myosin
ATPase
slow
fast
fast
Sarcoplasmic
reticulum
development
low
high
high
Aerobic capacity
high
moderate
low
Anaerobic capacity
low
high
very high
Contractile speed
slow
fast
fast
Fatigue resistance
high
moderate
low
Motor unit strength
low
high
high
Structural
Functional
Muscle Fibre Percentages
Athlete
Muscle
% Slow twitch
% Fast twitch
Sprinter
Gastrocnemius
25
75
Marathon runner
Gastrocnemius
75
25
Swimmer
Deltoid
67
33
Shot-Putter
Gastrocnemius
38
62
Non-athlete
Quadriceps
47
53
Sporting Performance
- Slow twitch fibres suit endurance athletes.
- Fast twitch fibres suit sprint activities.
- An individual with a high percentage of fast twitch fibres
therefore has a natural advantage.
- However, other factors play a key role, as the table below
shows.
Range of % slow twitch
fibres
Average % slow twitch
fibres
Marathon Runners
50-95
85
800m Runners
50-80
55
Sprinters
20-55
35
Responses to Training
 % of fast/slow twitch muscle fibres is genetically
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•
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determined.
endurance training results in type IIb being converted to type
IIa.
explains why long steady training results in loss of speed.
endurance training increases the aerobic capacity of ST
fibres.
high intensity anaerobic training causes increase in size of FT
fibres (hypertrophy), and number of FT fibres (hyperplasia).
lack of training causes atrophy.
Motor Units
 A motor unit consists of a motor neurone and all of the fibres that are supplied by that
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motor unit
A neurone does not activate a single muscle fibre, but a group of muscle fibres (the ‘all
or none law’)
All the muscle fibres in a motor unit will be the same type (Type IIa)
Therefore it is more accurate to talk of fast and slow twitch motor units rather than
muscle fibres
The number of muscle fibres supplied by a neurone in a motor unit dependent on the
degree of control required by the muscle.
In muscles that require a high degree of fine control there may be as few as 5 muscle
fibres supplied by the neurone e.g. eyelids
In large muscles that do not require a high degree of control there may be 1000 muscle
fibres supplied by a motor nerve in a motor unit e.g. the hamstrings
FIBRE TYPE and EXERCISE
RECRUITMENT
based on intensity of exercise
•at low intensity ST motor units recruited first
•at higher intensity FOG FT IIa motor units recruited
•at greatest intensity FG FT IIb motor units recruited to produce powerful
fast muscle contractions
•all available fibres are recruited for all power activities
Spatial Summation
 The response of a motor unit follows the ‘all or nothing law’
 However, the strength of the response of the whole muscle is
determined by the number of motor units involved.
 For greater force the brain recruits more motor units.
 This is called spatial summation.
Muscle Fibre Exam Questions
1.
2.
3.
4.
5.
During a race, a swimmer has to dive off the starting
blocks as quickly as possible. Identify the ‘muscle fibre
type’ used to complete this action and justify your answer.
(3)
What are the main characteristics of the main type of
motor unit used in marathon running? (4)
How are motor units involved in spatial summation? (3)
What do you understand by the term motor units? (3)
The table below shows the percentage of slow twitch fibres
in elite sprinters. Discuss whether the sampling of muscle
is a good indicator of sprinting performance. (3)
Male Sprinters
Range of % of ST fibres
Avg % of ST fibres
20-55
35
During a race, a swimmer has to dive off the
starting blocks as quickly as possible. Identify the
‘muscle fibre type’ used to complete this action
and justify your answer. (3)
 Fast twitch fibres / type 2 fibres.
 Fast speed of contraction.
 High force of contraction / powerful contraction.
What are the main characteristics of the main type
of motor unit used in marathon running? (4)
 Slow twitch fibres.
 Contract less powerfully.
 Contract slowly.
 High fatigue resistance.
How are motor units involved in spatial summation? (3)
 Motor units follow the all or nothing law – all fibres within a
motor unit either contract or do not contract.
 The strength of contraction is determined by the number of
motor units recruited.
 To produce greater force of contraction more motor units
are recruited.
 For movements that require more fine control fewer motor
units are involved.
What do you understand by the term motor units?
(3)
 A motor unit consists of a motor neurone and all of the
fibres that are supplied by that motor unit
 A neurone does not activate a single muscle fibre, but a
group of muscle fibres (the ‘all or none law’)
 All the muscle fibres in a motor unit will be the same
type (Type IIa)
 The number of muscle fibres supplied by a neurone in a
motor unit dependent on the degree of control required
by the muscle.
The table below shows the percentage of slow twitch
fibres in elite sprinters. Discuss whether the sampling of
muscle is a good indicator of sprinting performance. (3)
Male Sprinters
Range of % of ST
fibres
Avg % of ST fibres
20-55
35
 A higher percentage of fast twitch fibres gives a natural
advantage for sprint events.
 However, this does not mean that an individual with a high
percentage of FT fibres will show good sprinting
performance.
 Other factors such as motivation, lever length, physique, VO2
max also play an important role.