MUSCLES OF FOREARM
The
forearm extends from the elbow to the wrist. It is the territory of two bones the
radius and the ulna and of many muscles. The muscles are arranged
as a flexor mass anteriorly and an extensor mass posteriorly.
There are 19 muscles in the
forearm. Eleven are classified as extensor muscles; 8 belong to the flexor
group. These designations are simply group characteristics, because certain
muscles are primarily rotators of the forearm bones. Eighteen of the muscles
can be grouped into six functional groups of 3 muscles each. The
muscle excluded from these groups is the brachioradialis,
which is actually an elbow flexor
with no action in the digits or at the wrist. All the groups, except for the
first, are composed of muscles that move the hand and digits.
1. Rotate the radius on the ulna:
• Pronator teres
• Pronator quadratus
• Supinator
2. Flex the hand at the wrist:
• Flexor carpi radialis
• Flexor carpi ulnaris
• Palmaris longus
3. Flex the digits:
• Flexor digitorum superficialis
• Flexor digitorum profundus
• Flexor pollicis longus
4. Extend the hand at the wrist:
• Extensor carpi radialis longus
• Extensor carpi radialis brevis
• Extensor carpi ulnaris
5. Extend the digits, except the thumb:
• Extensor digitorum
• Extensor indicis
• Extensor digiti minimi
6. Extend the thumb:
• Abductor pollicis longus
• Extensor pollicis brevis
• Extensor pollicis longus
The muscles of the first three
groups lie in the anterior compartment of the forearm; those of the last three groups
are located in the posterior compartment.
The muscles of the second and third
groups and the two pronator muscles of the first group comprise the anterior
antebrachial muscles. Five of these belong to a superficial layer, and three belong
to a deep layer.
Superficial Layer
The muscles of this layer are
listed in the order in which they lie from the radial to the ulnar side of the
forearm; the flexor digitorum superficialis, however, is deep
to the other four muscles:
· Pronator teres
· Flexor carpi radialis
· Palmaris longus
· Flexor carpi ulnaris
· Flexor digitorum superficialis
There is a common tendon of
origin for these muscles, which is attached to the medial epicondyle of the
humerus. The intermuscular septa and the antebrachial fascia also provide
partial origins, and certain muscles have additional bony origins.
The pronator teres muscle has
both a humeral and an ulnar head. The large humeral head arises in the medial
epicondyle via the common tendon and from the adjacent fascia and intermuscular
septa. The small, deep ulnar head takes origin from the medial side of the
coronoid process of the ulna. It joins the deep aspect of the
humeral head, the median nerve descending between them. This obliquely
descending muscle ends on the shaft of the radius at the middle of its lateral
surface. The insertion is overlaid and obscured by the brachioradialis muscle.
The flexor carpi radialis muscle
uses the common tendon of origin. Its tendon accounts for about half of the
muscle’s length, passes through the wrist in a compartment formed by a split in
the flexor retinaculum, and ends in the base of the second metacarpal
(frequently, there is an additional slip to the third metacarpal).
The palmaris longus muscle also
uses the common tendon of origin, when present (it is absent in 13% of cases).
It terminates in a slender, flattened tendon, crossing the wrist superficial to
the flexor retinaculum. It constitutes, by its
spreading tendinous fibers, the chief part of the palmar aponeurosis.
The flexor carpi ulnaris muscle has
a humeral and an ulnar head, the humeral head coming from the common flexor
tendon. The ulnar head springs from the medial border of the olecranon and the
upper two thirds of the posterior border of the ulna. The tendon of the muscle
inserts on the pisiform of the wrist and, through it by two ligaments, into the
hamulus of the hamate and the base of the fifth metacarpal.
The flexor digitorum
superficialis muscle arises by a humeroulnar and a radial head of origin;
these are connected by a fibrous band that crosses the
median nerve and the ulnar blood vessels. The larger humeroulnar head arises
from the common tendon, the intermuscular septa, the ulnar collateral ligament,
and the medial border of the coronoid process. The radial head is a thin layer
arising from the upper two thirds of the anterior border of the radius. The
muscle forms two planes; the tendons of its superficial plane pass to the
middle and ring fingers, and the deep lamina ends in tendons for digits I and
V. These tendons terminate in the palmar aspect of the shafts of the middle
phalanges of digits II to V (their relationships are described in the
discussion of the wrist and hand).
Deep Layer
The deep layer contains the
following muscles:
· Flexor digitorum profundus
· Flexor pollicis longus
· Pronator quadratus
The flexor digitorum profundus
muscle arises from the posterior border of the ulna (with the flexor carpi
ulnaris), the proximal two thirds of the medial surface of the ulna, and adjacent
areas of the interosseous membrane. The muscle produces, near the wrist, four
discrete tendons that pass side by side under the flexor retinaculum and dorsal
to the tendons of the flexor digitorum superficialis muscle. The tendons
terminate on the bases of the distal phalanges of digits II to V. In the palm, the tendons give origin to the small
lumbrical muscles.
The flexor pollicis longus
muscle arises principally from the anterior surface of the radius (just
below its tuberosity nearly to the upper border of the pronator quadratus) and
from the adjacent interosseous membrane. Its tendon, passing between the two
sesamoids of the metacarpophalangeal joint of the thumb, inserts on the base of
the distal phalanx of the thumb.
The pronator quadratus muscle is
a quadrilateral muscle located just above the wrist and deep to the flexor
digitorum profundus and flexor pollicis longus tendons. It arises from the
anterior surface of the distal one fourth of the ulna, its fibers running
transversely across the wrist and inserting into the anterior surface of the
distal fourth of the shaft of the radius.
EXTENSOR MUSCLES
The muscles of the fourth, fifth,
and sixth groups, the supinator muscle of the first group, and the
brachioradialis muscle make up the posterior antebrachial muscles. Six of these
comprise the superficial layer, and five lie in the deep layer.
Superficial Layer
The six muscles of the superficial
layer are listed in the order in which they lie across the back of the forearm, from
the radial to the ulnar side:
· Brachioradialis
· Extensor carpi radialis longus
· Extensor carpi radialis brevis
· Extensor digitorum
· Extensor digiti minimi
· Extensor carpi ulnaris
As for the flexor muscles, there is
a common tendon of origin from the lateral epicondyle for all muscles arising
below the lateral epicondyle.
The brachioradialis muscle arises
from the upper two thirds of the supracondylar ridge of the humerus. Its tendon
appears at about the middle of the forearm and descends to insert into the
lateral side of the base of the styloid process of the radius.
The extensor carpi radialis
longus muscle arises from the lower third of the supracondylar ridge of the
humerus.
It has a flat tendon that reaches
into the hand to insert on the dorsum of the second metacarpal.
The extensor carpi radialis
brevis muscle uses the common tendon of origin for the extensors. Its
tendon appears in the lower third of the forearm, closely applied to the
overlying tendon of the extensor carpi radialis longus, and inserts on the
dorsum of the base of the third metacarpal.
The extensor digitorum muscle also
uses the common tendon of origin for the extensors. Above the wrist, it
provides four tendons that spread out on the dorsum of the hand, joined side to
side in a variable manner by intertendinous connections. Participating in
the rather complex “extensor expansion” described in the section on the wrist
and hand, these tendons terminate on the bases of the middle and distal
phalanges of digits 2 to 5. The extensor digiti minimi muscle is a slender
muscle that is sometimes only incompletely separated from the extensor
digitorum muscle. Its tendon joins the ulnar side of the tendon of the extensor
digitorum muscle to the fifth digit for which it provides independent extensor
action.
The extensor carpi ulnaris
muscle arises by the common tendon from the lateral epicondyle but also
from the middle one half of the posterior border of the ulna. It inserts on the
ulnar side of the base of the fifth metacarpal.
Deep Layer
The muscles of the deep layer are
generally submerged under those of the superficial group, although certain of
their tendons and parts of their fleshy bellies outcrop just above the wrist:
· Supinator
· Abductor pollicis longus
· Extensor pollicis brevis
· Extensor pollicis longus
The supinator muscle has a
complex origin from the lateral epicondyle of the humerus, the radial
collateral ligament, the annular ligament of the radius, and the supinator
crest and fossa of the ulna. Its fibers form a flat sheet, directed downward
and lateralward, which wraps almost completely around the radius and inserts on
the lateral surface of the upper third of this bone. As the muscle courses into
the posterior compartment of the forearm, it is separated into superficial and
deep laminae by the deep branch of the radial nerve.
The abductor pollicis longus
muscle lies immediately distal to the supinator. It arises from the middle
third of the posterior surface of the radius and the lateral part of the
posterior surface of the ulna below the anconeus muscle. The fibers of the
muscle converge onto its tendon, which, with the tendon of extensor pollicis
brevis closely applied to its medial side, crosses the tendons of the extensor
carpi radialis longus and brevis muscles and inserts on the radial side of the base
of the metacarpal of the thumb.
The extensor pollicis brevis
muscle, with origins from the radius and the interosseous membrane distal
to that of the abductor pollicis longus muscle, inserts on the base of the
proximal phalanx of the thumb. It is a specialization of the distal part of
the abductor pollicis longus muscle.
The extensor pollicis longus
muscle arises from the ulna and the interosseous membrane distal to the
abductor pollicis longus muscle. Its tendon passes to the ulnar side of the dorsal
tubercle of the radius, then obliquely across the tendons of both radial carpal
extensors, terminating on the base of the distal phalanx of the thumb.
The extensor indicis muscle arises
just below the extensor pollicis longus from the ulna and from the interosseous
membrane. In the hand, the tendon joins the ulnar side of the tendon of the
digital extensor muscle for the index finger and participates with it in
forming the extensor expansion over that digit.
Knowledge of the cross-sectional
anatomy of the forearm is critical to understanding advanced imaging views of
the forearm as well as surgical anatomy. Knowing the origins and insertions of
the muscles and tendons of the forearm is helpful during operative dissection
and exploration of the forearm and wrist and provides the framework to
understandi g the functional anatomy of the forearm and wrist.
