Chapter 5 The Muscular System 167
Parallel Fiber Architecture
With parallel fi ber architecture, the fi bers run
largely parallel to each other along the length of
the muscle. As Figure 5.11 shows, these parallel
fi ber arrangements may result in muscle shapes
that are fusiform (wide in the middle and tapering
on both ends), bundled, or triangular. Examples
of muscles with this type of architecture are
the biceps brachii (fusiform), rectus abdominis
(bundled), and pectoralis major (triangular).
The individual fi bers in the parallel
architecture typically do not run the entire length
of the muscle. Instead, the individual parallel
fi bers have interconnections with neighboring
fi bers. These interconnections promote
contraction when the muscle is stimulated. This
fi ber arrangement enables shortening of the
muscle and the ability to move body segments
through large ranges of motion.
Pennate Fiber Architecture
In a pennate fi ber arrangement, each fi ber
attaches obliquely to a central tendon, and
sometimes attaches to more than one tendon. As
Figure 5.11 shows,
• fi bers that are aligned in one direction to a
central tendon are unipennate;
• fi bers that attach to a central tendon are
bipennate; and
• fi bers that attach to a central tendon in
more than two directions are referred to as
multipennate.
Certain muscles of the hand are unipennate,
the rectus femoris (a member of the quadriceps
group in the thigh) is bipennate, and the deltoid
is multipennate.
With a pennate fi ber arrangement, the
muscle does not shorten as much upon
contraction as a muscle with a parallel fi ber
arrangement. However, the pennate arrangement
makes it possible to pack more fi bers into the
muscle. This means that the muscle can generate
more force.
Muscular Strength,
Power, and Endurance
In everyday conversation we sometimes use
strength and power interchangeably. However,
as we will discuss in this section of the lesson,
muscular strength and power are quite different
concepts. We will examine what it means to have
muscle fatigue, along with the related concept
of muscular endurance, which is a little more
complicated.
1. What is the difference between fast-
twitch and slow-twitch fi bers?
2. Which fi ber type helps a sprinter get out
of the blocks fast?
3. Why can pennate-arranged fi bers
generate more force than parallel-
arranged fi bers?
Check Your Understanding
Fusiform Bundled Triangular
Unipennate Bipennate Multipennate
A. Parallel fiber arrangements
B. Pennate fiber arrangements
Figure 5.11 Fibers within a muscle may be
arranged so that they are largely parallel or pennate
(feathered). Can you identify a muscle with parallel
fiber arrangement? With pennate fiber arrangement ?