354
UNIT 2
Covering, Support, and Movement of the Body
10
The many muscles in the forearm perform several basic functions.
Some cause wrist movements, some move the fingers and thumb,
and a few help pronate and supinate the forearms. In most cases,
their fleshy portions contribute to the roundness of the proximal
forearm and then they taper to long tendons distally to insert into
the hand.
At the wrist, these tendons are securely anchored by bandlike
thickenings of deep fascia called
flexor
and
extensor retinacula
(“retainers”) (Figure 10.16a). These “wrist bands” keep the tendons
from jumping outward when tensed. Crowded together in the wrist
and palm, the muscle tendons are surrounded by slippery tendon
sheaths that minimize friction as they slide against one another.
Although many forearm muscles arise from the humerus
(and thus cross both the elbow and wrist joints), their actions on
the elbow are slight. Flexion and extension are the movements
typically effected at both the wrist and finger joints. In addition,
the forearm muscles can abduct and adduct the wrist.
Fascia subdivides the forearm muscles into two main
compartments: the
anterior flexors
and
posterior extensors
. Each
has superficial and deep muscle layers. Most flexors in the anterior
compartment arise from a common tendon on the humerus
and are innervated largely by the median nerve. Two anterior
compartment muscles are not flexors but pronators, the
pronator
teres
and
pronator quadratus
(Figure 10.16a–c). Pronation is one of
the most important forearm movements.
Muscles of the posterior compartment extend the wrist and
fingers. One exception is the
supinator
muscle, which assists the
biceps brachii muscle of the arm in supinating the forearm
(Figures 10.16b, c and 10.17b). (Also residing in the posterior
compartment is the brachioradialis muscle, the weak elbow
flexor considered in Table 10.10.) Most muscles of the posterior
compartment arise from a common tendon on the humerus. The
radial nerve supplies all posterior forearm muscles.
As described above, most muscles that move the hand are
located in the forearm and “operate” the fingers via their long
tendons, like operating a puppet by strings. This design makes the
hand less bulky and enables it to perform finer movements. The
hand movements promoted by the forearm muscles are assisted
by the small
intrinsic
muscles of the hand, which control the most
delicate and precise finger movements (see Table 10.13). Table
10.12 (Parts II and III) summarizes the actions of the forearm
muscles.
MUSCLE GALLERY
Table 10.11
Muscles of the Forearm: Movements of the Wrist, Hand, and Fingers
(Figures 10.16 and 10.17)
MUSCLE
DESCRIPTION
ORIGIN (O) AND
INSERTION (I)
ACTION
NERVE
SUPPLY
PART I: ANTERIOR
MUSCLES
(Figure 10.16)
These eight muscles of the anterior fascial compartment are listed from the lateral to the medial aspect. Most
arise from a common flexor tendon attached to the medial epicondyle of the humerus and have additional
origins as well. Most of their tendons of insertion are held in place at the wrist by a thickening of deep fascia
called the
flexor retinaculum.
Superficial Muscles
Pronator teres
(pro-na
9
tor te
9
re
ˉ
z)
(
pronation
5
turning palm
posteriorly, or down;
teres
5
round)
Two-headed muscle; seen
in superficial view between
proximal margins of
brachioradialis and flexor
carpi radialis; forms medial
boundary of cubital fossa
O—medial epicondyle
of humerus; coronoid
process of ulna
I—by common tendon
into lateral radius,
midshaft
Pronates forearm
; weak
flexor of elbow
Median nerve
Flexor carpi radialis
(flek
9
sor kar
9
pe ra
0
de-al
9
is)
(
flex
5
decrease angle
between two bones;
carpi
5
wrist;
radi
5
radius)
Runs diagonally across
forearm; midway, its fleshy
belly is replaced by a flat
tendon that becomes
cordlike at wrist
O—medial epicondyle of
humerus
I—base of second and
third metacarpals;
insertion tendon easily
seen and provides guide
to position of radial
artery at wrist (used for
taking pulse)
Powerful flexor of wrist;
abducts hand
; weak
synergist of elbow flexion
Median nerve
Palmaris longus
(pahl-ma
9
ris lon
9
gus)
(
palma
5
palm;
longus
5
long)
Small fleshy muscle with
a long insertion tendon;
often absent; may be used
as guide to find median
nerve that lies lateral to it
at wrist
O—medial epicondyle of
humerus
I—palmar aponeurosis;
(fascia of palm)
Tenses skin and fascia
of palm during hand
movements
; weak wrist
flexor; weak synergist for
elbow flexion
Median nerve
Flexor carpi ulnaris
(ul-na
9
ris)
(
ulnar
5
ulna)
Most medial muscle of this
group; two-headed; ulnar
nerve lies lateral to its
tendon
O—medial epicondyle of
humerus; olecranon and
posterior surface of ulna
I—pisiform and hamate
bones and base of fifth
metacarpal
Powerful flexor of wrist;
also adducts hand
in
concert with extensor
carpi ulnaris (posterior
muscle); stabilizes wrist
during finger extension
Ulnar nerve
(C
7
and C
8
)
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