White muscle f bers can contract rapidly because oF these Fac-
tors, although they Fatigue as lactic acid accumulates and as
the ATP and the biochemicals to regenerate ATP are depleted.
A type oF white f ber, the Fast-twitch Fatigue-resistant f bers
(type IIa), are also called
These f bers have
the Fast-twitch speed associated with white f bers with a sub-
stantial oxidative capacity more characteristic oF red f bers.
While some muscles may have mostly one f ber type or
another, all muscles include a combination oF f ber types. The
speed oF contraction and aerobic capacities oF the f bers reﬂ ect
the specialized Functions oF the muscle. ±or example, muscles
that move the eyes contract about ten times Faster than those
that maintain posture, and the muscles that move the limbs
contract at intermediate rates. Slowing oF eye movements is an
early sign oF certain neurological diseases. Clinical Application
9.2 discusses noticeable eFFects oF muscle use and disuse.
Birds that migrate long distances have abundant dark, slow-twitch
muscles—this is why their f
esh is dark. In contrast, chickens that can
only Flap around the barnyard have abundant Fast-twitch muscles
and mostly white f
World-class distance runners are the human equivalent oF the
migrating bird. Their muscles may have more than 90% slow-twitch
Fibers! In some European nations, athletic coaches measure slow-
twitch to Fast-twitch muscle ±
ber ratios to predict who will excel at
long-distance events and who will Fare better in sprints.
Distinguish between a twitch and a sustained contraction.
Explain the di²
erences between isometric and isotonic
Distinguish between Fast-twitch and slow-twitch muscles ±
in postural muscles that stabilize skeletal parts and hold the
illustrates isotonic and isometric
Most body actions require both isotonic and isometric
contractions. In walking, For instance, certain leg and thigh
muscles contract isometrically and keep the limb stiFF as it
touches the ground, while other muscles contract isotoni-
cally, bending and liFting the limb. Similarly, walking down
stairs requires eccentric contraction oF certain thigh muscles.
Fast- and Slow-Twitch Muscle Fibers
bers vary in contraction speed (slow-twitch or Fast-
twitch) and in whether they produce ATP oxidatively or
glycolytically. At least three combinations oF these charac-
teristics are Found in humans. Slow-twitch f
bers (type I) are
always oxidative and are thereFore resistant to Fatigue. ±ast-
twitch f bers (type II) may be primarily glycolytic (Fatigable)
or primarily oxidative (Fatigue resistant).
Slow-twitch (type I) Fibers, such as those in the long
muscles oF the back, are oFten called
contain the red, oxygen-storing pigment myoglobin. These
f bers are well supplied with oxygen-carrying blood. In addi-
tion, red f bers contain many mitochondria, an adaptation
For the aerobic reactions oF cellular respiration. These f bers
have a high respiratory capacity and can generate ATP Fast
enough to keep up with the ATP breakdown that occurs
when they contract. ±or this reason, these f
bers can contract
For long periods without Fatiguing.
±ast-twitch glycolytic Fibers (type IIb) are also called
because they have less myoglobin and have a
poorer blood supply than red f bers. They include f bers in
certain hand muscles as well as in muscles that move the
eye. These Fibers have Fewer mitochondria and thus have
a reduced respiratory capacity. However, they have a more
extensive sarcoplasmic reticulum to store and reabsorb cal-
cium ions, and their ATPase is Faster than that oF red f bers.
Types oF muscle contractions. (
) Isotonic contractions include concentric and eccentric contractions. (
contractions occur when a muscle contracts but does not shorten or lengthen.
Muscle contracts with
force greater than
with force less than
Muscle contracts but
does not change length