Types of Muscle Fibre

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.

2. 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.

3. Fast Twitch Fibres (Type II) Contract with greater speed. Produce more force due to more 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).

6. Muscle Fibre Percentages

7. 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.

8. 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.

9. 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

10. 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

11. 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.

12. 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)

13. 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.

14. 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.

15. 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.

16. 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.

17. 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) • 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.