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.

Types of Muscle Fibre

Muscle Fibre RevisionTeam 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.Characteristics of Muscle Fibres

Slow twitch fibres

Fast oxidative glycotic fibres

Fast glycotic fibres

StructuralFibres per motor neurone10-180300-800300-800Motor neurone sizesmalllargelargeType of Myosin ATPaseslowfastfastSarcoplasmic reticulum developmentlowhighhighFunctionalAerobic capacityhighmoderatelowAnaerobic capacitylowhighvery highContractile speedslowfastfastFatigue resistancehighmoderatelowMotor unit strengthlowhighhighMuscle Fibre PercentagesAthleteMuscle% Slow twitch% Fast twitchSprinterGastrocnemius2575Marathon runnerGastrocnemius

7525SwimmerDeltoid6733Shot-PutterGastrocnemius

3862Non-athleteQuadriceps4753 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 fibresAverage % slow twitch fibresMarathon Runners50-9585800m Runners50-8055Sprinters20-5535Responses 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.

Motor Units

A motor unit consists of a motor neurone and all of the fibres that are supplied by that motor unitA 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 fibresThe 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. eyelidsIn 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

Spatial SummationThe response of a motor unit follows the all or nothing lawHowever, 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 QuestionsDuring 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 fibresAvg % of ST fibresMale Sprinters20-5535During 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 unitA 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)

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.

Range of % of ST fibresAvg % of ST fibresMale Sprinters20-5535