This document discusses muscle fiber types and their relationship to athletic performance. It summarizes that type 1 fibers contract slower than type 2A or 2X fibers. While male fibers are larger than female fibers and therefore develop higher force, the specific tension is not different between sexes when corrected for size. The proportion of fast fiber types in a muscle can vary between individuals, from 20% to 80% of type 1 fibers, and positively correlates with the ability to produce dynamic force and power. The number of fibers in a muscle can differ more than two-fold between individuals but is not affected by resistance training, though the cross-sectional area of fibers does increase with training.

1. Muscle fibre types
and athletic
performance
SR2002
Oct 31, 2013
Dr. Arimantas Lionikas

2. Outline
1) Functional properties of type 1, 2A and 2X
fibres
2) Male and female fibres
3) Variability in proportion of fibre types in
humans
4) Does variation in the fibre types have
functional consequences?

3. 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

4. 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
X
X

5. 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
IIX
IIA

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

7. In vitro motility speed, um/s
IVM: difference in speed between types of
MyHC declines at physiological temperature
7
6
5
I
IIA
IIA/X
4
3
2
1
0
15
25
Temperature, C
35
Lionikas, Li, Larsson 2006

8. Are male fibres stronger than female fibres?
2000
Force, microN
Yu et al. 2007
1500
1000
M
F
500
0
I
IIA
IIX
Male fibres are stronger
because they are larger
6000
CSA, um2
5000
4000
3000
M
2000
F
1000
0
I
IIA
IIX

9. Male and female fibres (cont)
60
Yu et al. 2007
Specific tension, uN/um2
50
40
30
M
F
20
10
0
I
IIA
IIX
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

10. Relative occurrence in %
How variable is proportion of fibre
types in humans?
Simoneau & Bouchard 1989

11. Does variation in the fibre types
have functional consequences?
Thorstensson et al. 1976

12. 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

13. Average fibre area (um2 x 1000)
Number of fibres x 1000
MacDougall et al 1984
Area of biceps brachii (mm2 x102)
Hypertrophy of the fibres in
body builders contribute to a
larger cross-sectional area
(CSA) of the muscle (average
CSA in body builders nearly 2fold larger than in untrained
subjects).
Area of biceps brachii (mm2 x102)
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.

14. 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