Oct 31, 2013

Muscle fibre types and athletic performance

SR2002

Dr. Arimantas Lionikas

Outline

1) Functional properties of type 1, 2A and 2X fibres

2) Male and female fibres3) Variability in proportion of fibre types in

humans4) Does variation in the fibre types have

functional consequences?

Isolated muscle fibres• Skeletal muscle is a mix of

different fibre types• To understand the functional

characteristics of each type it is important to study fibres in isolation

• “Skinned” single fibre preparation provides a model to study the fundamental properties of the contractile machinery

• Also, properties of the same type of fibres can be compared between males and females, young and old, etc.

Larsson et al. 1995

XX

Contraction speed (shortening velocity)

• “Unloaded shortening velocity” experiments demonstrate differences in contractile speed between muscle fibre types

• Type I fibres are ≈3 times slower than IIA and ≈10* times than IIX

• * large between-type differences are observed at sub-physiological temperatures…

Larsson & Moss 1993

X

IIA

IIX

Lessons from the fibres co-expressing different MyHC

• Slower isoform of myosin heavy chains (MyHC) determines the contraction speed of a fibre unless faster isoform is appreciably abundant (>50%)

Larsson & Moss 1993

In vitro motility (IVM) assay: myosin function at molecular level

IVM: difference in speed between types of MyHC declines at physiological temperature

Lionikas, Li, Larsson 2006

15 25 350

1

2

3

4

5

6

7

IIIAIIA/X

Temperature, C

In v

itro

moti

lity

spee

d, u

m/s

Are male fibres stronger than female fibres?

I IIA IIX0

200

400

600

800

1000

1200

1400

MF

Forc

e, m

icro

N

I IIA IIX0

500100015002000250030003500400045005000

MF

CSA,

um

2

Yu et al. 2007

Male fibres are stronger because they are larger

Male and female fibres (cont)

I IIA IIX0

5

10

15

20

25

30

35

40

45

MF

Spec

ific

tens

ion,

uN

/um

2

When corrected for the size difference, female fibres develop the same specific tension (amount of force per unit of cross sectional area) as male fibres.Type IIX fibres generates greater specific force compared to type I and IIA fibres

Yu et al. 2007

Rela

tive

occu

rren

ce in

%

Simoneau & Bouchard 1989

How variable is proportion of fibre types in humans?

Does variation in the fibre types have functional consequences?

Thorstensson et al. 1976

Fibre number, size and athletic performance• Fibres in human biceps brachii extend from origin to

insertion of the muscle• Therefore, it is possible to estimate the number of fibres

in the biceps if cross-sectional area of the muscle and its fibres are known (can be obtained by CT scan and needle biopsy, respectively)

• Study by MacDougall et al 1984. Number of fibres ranged between 172,085 and 418,884 (>2-fold difference)

• Importantly, there was no difference in mean number of fibres between elite body builders and untrained controls

Num

ber

of fi

bres

x 1

000

Area of biceps brachii (mm2 x102)

Ave

rage

fibr

e ar

ea (

um2

x 1

000)

Area of biceps brachii (mm2 x102)

MacDougall et al 1984

Hypertrophy of the fibres in body builders contribute to a larger cross-sectional area (CSA) of the muscle (average CSA in body builders nearly 2-fold larger than in untrained subjects).

Greater number of fibres is positively associated with muscle CSA. However, the data argue against the role of training-induced hyperplasia - similar numbers present both in elite body builders and in control subjects.

Summary• Type 1 fibres contract slower than type 2A or 2X fibres• Males fibres are large than females fibres and

therefore develop higher force, however, specific tension is not different

• Some individuals have 20% while others can have 80% of type 1 fibres in the same muscle

• Proportion of fast fibres in the muscle positively correlates with the ability to develop dynamic force and power

• Number of fibres in a muscle can differ >2-fold between individuals. However, that is not affected by training (no hyperplasia). CSA of the fibres increases in response to resistance training