Fundamentals of Biomechanics

(Lieber & Friden, 2000). Anatomically, this
fiber arrangement has been classified as either
parallel or pennate. A parallel arrangement
means that the muscle fascicles
are aligned parallel to the long axis or line
of pull of the muscle. Muscles like the rectus
abdominis, sartorius, and biceps brachii
have predominantly a parallel architecture
(Figure 3.7a). Pennate muscles have fibers
aligned at a small angle (usually less than
15º) to a tendon or aponeurosis running
along the long axis of the muscle. An
aponeurosis is a distinct connective tissue
band within a muscle. This arrangement is
called pennate because of the feathered appearance.
The tibialis posterior and semimembranosus
are primarily unipennate,
while rectus femoris and gastrocnemius are
bipennate (Figure 3.7b). An example of a
multipennate muscle is the deltoid.
Muscles with parallel architecture favor
range of motion over force development.
The greater muscle excursion and velocity
of parallel muscles comes from the
greater number of sarcomeres aligned in se-
CHAPTER 3:ANATOMICAL DESCRIPTION AND ITS LIMITATIONS 47
(a) (b)
Figure 3.7. (a) Parallel arrangement of muscle fibers with the tendon favors range of motion over force. (b) Pennate
arrangement of fibers are angled into the tendon and create greater force but less range of motion.
ries. The rectus abdominis can shorten from
1/3 to 1/2 of its length because of the parallel
arrangement of fibers and fascicles.
Small muscles may have a simple parallel
design with fibers that run the length of the
muscle, while larger parallel muscles have
fibers aligned in series or end to end. These
end-to-end connections and transverse connections
within muscles make force transmission
in muscle quite complex (Patel &
Lieber, 1997; Sheard, 2000). Fiber architecture
also interacts with the connective tissue
within muscle to affect force or fiber
shortening. The fibers in the center of the
biceps do not shorten uniformly due to differences
in the distal and proximal aponeurosis
(Pappas, Asakawa, Delp, Zajac, &
Draceet, 2002). The amount of tendon a
muscle has and the ratio of tendon to fibers
also affects the force and range-of-motion
potential of a muscle.
In essence, pennate muscles can create
a greater tension because of a greater physiological
cross-sectional area per anatomical
cross-sectional area, but have less range