Covering, Support, and Movement of the Body
How can the deltoid muscles both extend and ﬂex the arm?
Aren’t these antagonistic movements?
Which of the thenar muscles does not have an insertion on
bones of the thumb?
For answers, see Appendix H.
Check Your Understanding
As John listened to Roger’s account of how he ﬂirted with his
neighbor, he raised his eyebrows and then winked at Sarah.
What facial muscles was he using?
What muscles would you contract to make a “sad clown’s face”?
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Actions and Interactions of Skeletal Muscles
Skeletal muscles are arranged in opposing groups across body joints
so that one group can reverse or modify the action of the other.
Muscles are classiﬁed as prime movers (agonists), antagonists,
synergists, and ﬁxators.
Naming Skeletal Muscles
Criteria used to name muscles include a muscle’s location,
shape, relative size, ﬁber (fascicle) direction, number of origins,
attachment sites (origin/insertion), and action. Several criteria are
combined to name some muscles.
Muscle Mechanics: Importance of Fascicle Arrangement
Common patterns of fascicle arrangement are circular, convergent,
parallel, fusiform, and pennate. Muscles with ﬁbers that run parallel
to the long axis of the muscle shorten most; stocky pennate muscles
shorten little but are the most powerful muscles.
A lever is a bar that moves on a fulcrum. When an eﬀort is
applied to the lever, a load is moved. In the body, bones are the
levers, joints are the fulcrums, and skeletal muscles exert the
eﬀort at their insertions.
When the eﬀort is farther from the fulcrum than is the load, the
lever operates at a mechanical advantage (it’s slow and strong).
When the eﬀort is exerted closer to the fulcrum than is the load,
the lever operates at a mechanical disadvantage (it’s fast and
promotes a large degree of movement).
First-class levers (eﬀort-fulcrum-load) may operate at a
mechanical advantage or disadvantage. Second-class levers
(fulcrum-load-eﬀort) all operate at a mechanical advantage.
Tird-class levers (fulcrum-eﬀort-load) always operate at a
mechanical disadvantage. Most skeletal muscles of the body act in
third-class lever systems.
Major Skeletal Muscles of the Body
Muscles of the head that produce facial expression tend to be
small and to insert into so± tissue (skin and other muscles)
rather than into bone. Tese muscles open and close the eyes and
mouth, compress the cheeks, allow smiling and other types of
facial language (see ²able 10.1*).
Muscles of the head involved in mastication include the masseter
and temporalis that elevate the mandible and two deep muscle
pairs that promote grinding and sliding jaw movements (see
²able 10.2*). Extrinsic muscles of the tongue anchor the tongue
and control its movements.
Deep muscles of the anterior neck promote swallowing
movements, including elevation/depression of the hyoid bone,
closing oﬀ the respiratory passages, and peristalsis of the pharynx
(see ²able 10.3*).
Neck muscles and deep muscles of the vertebral column promote
head and trunk movements (see ²able 10.4*). Te deep muscles
of the posterior trunk can extend large regions of the vertebral
column (and head) simultaneously. Te anteriorly located
sternocleidomastoid and scalene muscles eﬀect head ﬂexion and
Te diaphragm and external intercostal muscles of the thorax
promote movements of quiet breathing (see ²able 10.5*).
Downward movement of the diaphragm increases intra-
Te four muscle pairs forming the abdominal wall are layered
like plywood to form a natural muscular girdle that protects,
supports, and compresses abdominal contents. Tese muscles
also ﬂex and laterally rotate the trunk (see ²able 10.6*).
Muscles of the pelvic ﬂoor and perineum (see ²able 10.7*)
support the pelvic viscera, resist increases in intra-abdominal
pressure, inhibit urination and defecation, and aid erection.
Except for the pectoralis major and the latissimus dorsi, the
superﬁcial muscles of the thorax ﬁx or promote movements of
the scapula (see ²able 10.8*). Scapular movements are eﬀected
primarily by posterior thoracic muscles.
Nine muscles cross the shoulder joint to move the humerus (see
²able 10.9*). Of these, seven originate on the scapula and two
arise from the axial skeleton. Four muscles contribute to the
“rotator cuﬀ” helping to stabilize the multiaxial shoulder joint.
Generally speaking, muscles located anteriorly ﬂex, rotate, and
adduct the arm. Tose located posteriorly extend, rotate, and
adduct the arm. Te deltoid muscle of the shoulder is the prime
mover of shoulder abduction.
Muscles that move the forearm form the ﬂesh of the arm (see
²able 10.10*). Anterior arm muscles are forearm ﬂexors; posterior
muscles are forearm extensors.
Muscles originating on the forearm mainly move the wrist, hand,
and ﬁngers (see ²able 10.11*). Except for the two pronator muscles,
the anterior forearm muscles are wrist and/or ﬁnger ﬂexors; those of
the posterior compartment are wrist and ﬁnger extensors.
Te intrinsic muscles of the hand aid in precise movements of
the ﬁngers (²able 10.13*) and in opposition, which helps us grip
things. Tese small muscles are divided into thenar, hypothenar,
and midpalmar groups.