1. Types of Muscle Fibre Learning Objectives:
To know the structural and functional differences between different types of muscle fibre.
To know how muscle fibre type affects sporting performance.
To understand the function of motor units in muscle contraction.

2. Muscle Fibre Revision Team 1 – Gary, Luke, Alex.
Team 2 – Callum, Matty.
Team 3 – Michael, Archie, Rich.
Team 4 - James, Dan.
Give two structural and two functional characteristics of slow twitch fibres.
Fast twitch fibres are able to contract more quickly. Explain why this is.
Fast twitch fibres are able to contract more powerfully. Explain why this is.
Explain why slow twitch fibres do not fatigue as easily as fast twitch fibres.

3. Characteristics of Muscle Fibres
Slow twitch fibres
Fast oxidative glycotic fibres
Fast glycotic fibres
Structural
Fibres per motor neurone
10-180
Motor neurone size
small
large
Type of Myosin ATPase
slow
fast
Sarcoplasmic reticulum development
low
high
Functional
Aerobic capacity
moderate
Anaerobic capacity
very high
Contractile speed
Fatigue resistance
Motor unit strength

4. Muscle Fibre Percentages
Athlete
Muscle
% Slow twitch
% Fast twitch
Sprinter
Gastrocnemius 25 75
Marathon runner
Swimmer
Deltoid 67 33
Shot-Putter 38 62
Non-athlete
Quadriceps 47 53

5. Sporting Performance - Slow twitch fibres suit endurance athletes
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

6. Responses to Training
% of fast/slow twitch muscle fibres is genetically 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.

7. Motor Units
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)
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

8. 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.

9. Muscle Fibre Exam Questions
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)
Range of % of ST fibres
Avg % of ST fibres
Male Sprinters
20-55 35

10. 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.

11. 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.

12. 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.

13. 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.

14. 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)
Range of % of ST fibres
Avg % of ST fibres
Male Sprinters
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.