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”?
Chapter Summary
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Actions and Interactions of Skeletal Muscles
(pp. 319–320)
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
(pp. 320–322)
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
and Leverage
(pp. 322–324)
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
(pp. 324–382)
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-
abdominal pressure.
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.
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