320
UNIT 2
Covering, Support, and Movement of the Body
10
Muscles can be classified into four
functional
groups: prime
movers, antagonists, synergists, and fixators. A muscle that has the
major responsibility for producing a specific movement is a
prime
mover
, or
agonist
(ag
9
o-nist; “leader”), of that movement. Te
pectoralis major muscle, which fleshes out the anterior chest (and
inserts on the humerus), is a prime mover of arm flexion.
Muscles that oppose, or reverse, a particular movement are
antagonists
(an-tag
9
o-nists; “against the leaders”). When a prime
mover is active, the antagonist muscles may stretch or remain
relaxed. Usually, however, antagonists help to regulate the action
of a prime mover by contracting slightly to provide some resis-
tance, thus helping to prevent overshooting the mark or to slow or
stop the movement. As you might expect, a prime mover and its
antagonist are located on opposite sides of the joint across which
they act, as shown in
Focus on Muscle Action
(Figure 10.1)
.
Antagonists can also be prime movers in their own right.
For example, flexion of the arm by the pectoralis major muscle
is antagonized by the latissimus dorsi, the prime mover for ex-
tending the arm.
In addition to agonists and antagonists, most movements in-
volve the action of one or more
synergists
(sin
9
er-jists;
syn
5
together,
erg
5
work). Synergists help prime movers in one of
these ways:
Adding a little extra force to the same movement
Reducing undesirable or unnecessary movements that might
occur as the prime mover contracts
Let’s look at the second function more closely. When a muscle
crosses two or more joints, its contraction causes movement at
all of the spanned joints unless other muscles act as joint sta-
bilizers. For example, the finger flexor muscles cross both the
wrist and the interphalangeal joints, but you can make a fist
without bending your wrist because synergistic muscles stabi-
lize the wrist. Additionally, as some flexors act, they may cause
several other (undesirable) movements at the same joint. Syner-
gists can prevent this, allowing all of the prime mover’s force to
be exerted in the desired direction.
When synergists immobilize a bone, or a muscle’s origin so
that the prime mover has a stable base on which to act, they
are called
fixators
(fik
9
sa-terz). Recall from Chapter 7 that the
scapula is held to the axial skeleton only by muscles and is quite
freely movable. Te fixator muscles that run from the axial skel-
eton to the scapula can immobilize the scapula so that only the
desired movements occur at the mobile shoulder joint. Addi-
tionally, muscles that help maintain upright posture are fixators.
In summary, although prime movers seem to get all the
credit for causing certain movements, antagonistic and syner-
gistic muscles are also important in producing smooth, coordi-
nated, and precise movements. Furthermore, a muscle may act
as a prime mover in one movement, an antagonist for another
movement, a synergist for a third movement, and so on.
Naming Skeletal Muscles
List the criteria used in naming muscles. Provide an example
to illustrate the use of each criterion.
Skeletal muscles are named according to a number of criteria.
±o simplify the task of learning muscle names and actions, pay
attention to the following cues.
Muscle location.
Some muscle names indicate the bone or
body region with which the muscle is associated.
Examples: Te temporalis (tem
0
por-a
ˇ
9
lis) muscle overlies
the temporal bone, and intercostal (
costal
5
rib) muscles run
between the ribs.
Muscle shape.
Some muscles are named for their distinctive
shapes.
Examples: Te deltoid (del
9
toid) muscle is roughly trian-
gular (
deltoid
5
triangle), and together the right and le² tra-
pezius (trah-pe
9
ze-us) muscles form a trapezoid.
Muscle size.
±erms such as
maximus
(largest),
minimus
(smallest),
longus
(long), and
brevis
(short) are o²en used in
muscle names.
Examples: Te gluteus maximus and gluteus minimus are
the large and small gluteus muscles, respectively.
Direction of muscle fibers.
Te names of some muscles re-
veal the direction in which their fibers (and fascicles) run in
reference to some imaginary line, usually the midline of the
body or the longitudinal axis of a limb bone. In muscles with
the term
rectus
(straight) in their names, the fibers run par-
allel to that imaginary line (axis).
Transversus
indicates that
the muscle fibers run at right angles to that line, and
oblique
indicates that the fibers run obliquely to it.
Examples: Te rectus femoris (straight muscle of the thigh,
or femur) and transversus abdominis (transverse muscle of
the abdomen).
Number of origins.
When
biceps
,
triceps
, or
quadriceps
forms
part of a muscle’s name, you can assume that the muscle has
two, three, or four origins, respectively.
Example: Te biceps brachii (bra
9
ke-i) muscle of the arm
has two origins, or
heads
.
Location of the attachments.
Some muscles are named ac-
cording to their points of origin and insertion. Te origin is
always named first.
Example: Te sternocleidomastoid (ster
0
no-kli
0
do-mas
9
toid) muscle of the neck has a dual origin on the sternum
(
sterno
) and clavicle (
cleido
), and it inserts on the mastoid
process of the temporal bone.
Muscle action.
When muscles are named for the movement
they produce, action words such as
flexor
,
extensor
, or
adduc-
tor
appear in the muscle’s name.
Example: Te adductor longus, located on the medial
thigh, brings about thigh adduction. (±o review the termi-
nology for various actions, see Chapter 8, Figures 8.5, 8.6,
pp. 256–259.)
O²en, several criteria are combined in naming a muscle.
For instance, the name
extensor carpi radialis longus
tells us the
muscle’s action (extensor), what joint it acts on (
carpi
5
wrist),
and that it lies close to the radius of the forearm (radialis). It also
hints at the size (longus) relative to other wrist extensor mus-
cles. Unfortunately, not all muscle names are this descriptive.
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