Chapter 10
The Muscular System
353
10
Muscles fleshing out the arm cross the elbow joint to insert on
the forearm bones. Since the elbow is a hinge joint, movements
promoted by these arm muscles are limited almost entirely to flexing
and extending the forearm. Walls of fascia divide the arm into two
muscle compartments—the
posterior extensors
and
anterior flexors
.
The main forearm extensor is the bulky
triceps brachii
muscle, which
forms nearly the entire musculature of the posterior compartment
(Figure 10.15a, b).
All anterior arm muscles flex the forearm (elbow). In order of
decreasing strength, these are the
brachialis
,
biceps brachii
, and
brachioradialis
(Figure 10.15a, c, d). The brachialis and biceps insert
(respectively) into the ulna and radius and contract simultaneously
during flexion; they are the chief forearm flexors. The biceps
brachii, a muscle that bulges when the forearm is flexed, is familiar
to almost everyone. The brachialis, which lies deep to the biceps, is
less known but is equally important in flexing the elbow. Because
the brachioradialis arises from the distal humerus and inserts
on the distal forearm, it resides mainly in the forearm. Its force
is exerted far from the fulcrum, so the brachioradialis is a weak
forearm flexor. The biceps muscle also supinates the forearm and
is ineffective in flexing the elbow when the forearm
must
stay
pronated. (This is why doing chin-ups with palms facing anteriorly
is harder than with palms facing posteriorly.)
Table 10.12 (Part II) summarizes the actions of the muscles
described here.
MUSCLE GALLERY
Table 10.10
Muscles Crossing the Elbow Joint: Flexion and Extension of the Forearm
(Figure 10.15)
MUSCLE
DESCRIPTION
ORIGIN (O) AND
INSERTION (I)
ACTION
NERVE
SUPPLY
POSTERIOR MUSCLES
Triceps brachii
(tri
9
seps bra
9
ke-i)
(
triceps
5
three heads;
brachi
5
arm)
Large fleshy muscle; the
only muscle of posterior
compartment of arm;
three-headed origin;
long and lateral heads lie
superficial to medial head
O—long head:
infraglenoid tubercle
of scapula; lateral
head: posterior shaft of
humerus; medial head:
posterior humeral shaft
distal to radial groove
I—by common tendon
into olecranon of ulna
Powerful forearm
extensor
(prime mover,
particularly medial head);
antagonist of forearm
flexors; long and lateral
heads mainly active in
extending the forearm
against resistance;
long head tendon may
help stabilize shoulder
joint and assist in arm
adduction
Radial nerve (C
6
–C
8
)
Anconeus
(an-ko
9
ne-us)
(
ancon
5
elbow)
(see Figure 10.17)
Short triangular muscle;
partially blended with
distal end of triceps on
posterior humerus
O—lateral epicondyle of
humerus
I—lateral aspect of
olecranon of ulna
May control ulnar
abduction during forearm
pronation
; synergist of
triceps brachii in elbow
extension
Radial nerve
ANTERIOR MUSCLES
Biceps brachii
(bi
9
seps)
(
biceps
5
two heads)
Two-headed fusiform
muscle; bellies unite
as insertion point is
approached; tendon of
long head helps stabilize
shoulder joint
O—short head: coracoid
process; long head:
supraglenoid tubercle
and lip of glenoid cavity;
tendon of long head runs
within capsule and into
intertubercular sulcus of
humerus
I—by common tendon
into radial tuberosity
Flexes elbow joint and
supinates forearm
; these
actions usually occur at
same time (e.g., when you
open a bottle of wine,
it turns the corkscrew
and pulls the cork); weak
flexor of arm at shoulder
Musculocutaneous nerve
(C
5
and C
6
)
Brachialis
(bra
9
ke-al-is)
Strong muscle that is
immediately deep to
biceps brachii on distal
humerus
O—front of distal
humerus; embraces
insertion of deltoid
muscle
I—coronoid process
of ulna and capsule of
elbow joint
A major forearm flexor
(lifts ulna as biceps lifts
the radius)
Musculocutaneous nerve
Brachioradialis
(bra
0
ke-o-ra
0
de-al
9
is)
(
radi
5
radius, ray)
(also see Figure 10.16)
Superficial muscle of
lateral forearm; forms
lateral boundary of
cubital fossa; extends
from distal humerus to
distal forearm
O—lateral supracondylar
ridge at distal end of
humerus
I—base of radial styloid
process
Synergist in flexing
forearm
; acts to best
advantage when forearm
is partially flexed and
semipronated; stabilizes
elbow during rapid
flexion
and
extension
Radial nerve (an
important exception:
the radial nerve typically
serves extensor muscles)
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