Slow Twitch and Fast Twitch
Muscle Fibres
Did you eat white meat or dark meat
over the holidays?
Slow-Twitch
Most active during: long-distance running,
swimming, and cycling
Red or dark in colour
Generate and relax tension slowly; able to
maintain a lower level of tension for long
durations
Low levels of myosin ATPase and glycolytic
enzymes
High levels of oxidative enzymes
Fast-Twitch
Ideal for: short sprints, power lifting, and
explosive jumping
Pale in colour
Ability to tense and relax quickly; generate large
amounts of tension with low endurance levels
High levels of myosin ATPase and glycolytic
enzymes
Fast vs. Slow
Approximate Distribution of Muscle Fibre Types for
Different Sports
The Importance of Myoglobin
• The difference in _____________________ is
determined by the amount of ____________
used in energy production
• Myoglobin: ____________________________
Myoglobin
• Slow fibres vs. Fast fibres
– Slow twitch fibres are __________ in myoglobin
– Fast Twitch Fibres have ______________ amounts
of myoglobin
3 Fibre Types
 Type I or Slow-Oxidative (SO)
 Type IIA or Fast-Oxidative Glycolytic (FOG)
 Type IIB of Fast-Glycolytic (FG)
Why the Split of Type II Fibres?
• Studies have shown that there is evidence for
changing the type ________to Type ______
muscle fibres with endurance training
• THERE IS NO WAY ANY TYPE ___ MUSCLE
FIBRES CAN BECOME TYPE ____
• In other words a fast twitch muscle can
_________become a slow twitch muscle
Characteristics of Different Muscle Fibre Types
So how are the fibres distributed?
• How a muscle works is determined by its fibre
type and vice versa
• TWO categories for basic muscle functions:
– Tonic
– Phasic
Tonic Muscles
• These are used on a daily basis for
• High concentration of _________ twitch fibres
(Type I)
• Great for endurance activities
• Core and back muscles: constantly contracted
Phasic Muscle
• Higher percentage of ________ twitch fibres
• Example: Biceps have a lower amount of Type
I fibres and a higher amount of type II because
they are used for explosive movement (lifts)
So how do you find where these fibres
are?
• The only true way is to do a _____________to
find the percentage of each type of fibre.
• Muscle biopsy: tissue sample using a needle
to take a small amount from the muscle belly
Athletic Performance
• Elite athletes may have __________% of a
specific fibre type in their body
Why does Kenya keep winning in long
distance?
• You are going to read a short article on page
91
• Answer the following questions
– What has been found regarding the Kenyan’s
muscle fibre type?
– What social factors have played a large role in
Kenyan runners?