166 Introduction to Anatomy and Physiology
What Research Tells Us
...about
Fast- and Slow-Twitch Muscles
Researchers have taken muscle
biopsies (small, needle-sized plugs
of muscle tissue) from elite athletes
in a variety of sports. They have
found that individuals specializing
in events that require explosive
strength or speed have unusually
high proportions of fast-twitch (FT)
fi bers, and that elite endurance
athletes tend to have very high
proportions of slow-twitch (ST) fi bers.
It may be the case that many
of those who are able to achieve
athletic success at the highest
levels are simply born with high
percentages of either FT or ST
fi bers. Once these individuals have
experienced success in a particular
sport or event, it is likely that they
gravitate toward that sport or event
(Figure 5.9).
Of course, certain individuals
within the general population of
untrained people also have high
percentages of FT or ST muscles.
The distribution of FT/ST ratios
among the general population is
represented in the normal, bell-
shaped curve (Figure 5.10).
We also know from research
that FT-fi ber types can change over
time. FT fi bers can be converted to
ST fi bers with years of endurance
training. No evidence exists,
however, that any form of training
can convert ST fi bers to FT fi bers.
We do know that a progressive loss
of FT motor units and fi bers occurs
as we age, although this loss can
be minimized by regular, high-
intensity exercise throughout life.
Taking It Further
1. Why do FT muscle fi bers affect
strength and speed? Why
do ST muscle fi bers provide
increased endurance?
2. Why might regular exercise
minimize the natural loss of FT
muscle fi bers with age?
Rena Schild/Shutterstock.com
Figure 5.9 Elite sprint cyclists tend to
have high percentages of fast-twitch
muscle fibers. Aside from sprint
cycling, what are some sports in
which athletes with high percentages
of fast-twitch muscle fibers would be
particularly successful?
Figure 5.10 The percentages
of fast-twitch (FT) and slow-
twitch (ST) fibers in the general
population are normally distributed.
FT=ST Very high
% FT
Very high
% ST
Number
of
people
the fast-twitch fi bers contract much faster than
slow-twitch fi bers.
Because suffi cient variation exists among
the fast-twitch fi bers, they too have been divided
into two categories—Type IIa and Type IIb. The
Type IIa fi bers are intermediate in contraction
speed between the slow-twitch fi bers and the
classic fast-twitch fi bers, which are Type IIb. The
Type IIb fi bers contract very rapidly, in about
one-seventh the time required for slow-twitch
fi bers to contract. As a result, the Type IIb fi bers
also fatigue rapidly. Although all of the muscle
fi bers in a motor unit are of the same type, most
skeletal muscles include motor units of both
fast-twitch and slow-twitch fi bers. The fast-
twitch/slow-twitch ratio varies from muscle to
muscle and from person to person.
Fiber Architecture
Another factor that affects the ways in which
skeletal muscles function is fi ber architecture.
Fiber architecture refers to the ways in which
fi bers are arranged within the muscle. The two
major categories of muscle fi ber arrangement are
parallel and pennate.
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