MUSCLES OF UPPER ARM AND
ELBOW
The arm, or brachium, is
the region between the shoulder joint and the elbow. The arm muscles are few,
and they are served by certain of the terminal branches of the brachial plexus
and portions of the great vascular channels of the limb (see Plates
2-7 to 2-11).
Brachial Fascia
A strong tubular investment of the
deeper parts of the arm, the brachial fascia is continuous above with the
pectoral and axillary fasciae and with the fascial covering of the deltoid and
latissimus dorsi muscles. Below, the brachial fascia is attached to the
epicondyles of the humerus and to the olecranon and then is continuous with the
antebrachial fascia. It is perforated for the passage of the basilic vein, for
the medial antebrachial cutaneous nerve, and for many lesser nerves and
vessels.
Two intermuscular septa are
prolonged upward from the epicondylar attachments of the brachial fascia. These
blend with the periosteum of the humerus along its supracondylar ridges and
borders and fuse peripherally with the brachial fascia to form the anterior and
posterior compartments of the arm. Above, the lateral intermuscular septum ends
at the insertion of the deltoid muscle; the medial intermuscular septum ends
in continuity with the fascia of the coracobrachialis muscle. The medial
intermuscular septum has an additional, weaker anterior lamina, and the
anterior and posterior laminae together with the brachial fascia form the
neurovascular compartment of the arm (see Plate 2-10).
Muscles
The muscles of the arm are separated
both positionally and functionally by the humerus and the intermuscular septa
into an anterior and a posterior group or compartments (see Plates 2-8 and
2-9). The anterior group comprises the coracobrachialis, biceps brachii, and
brachialis muscles. The posterior group includes the triceps brachii and
anconeus muscles. Important neurovascular structures in the anterior
compartment include the musculocutaneous nerve, median nerve, ulnar nerve
(proximally), radial nerve (distally), and brachial artery. Important
neurovascular structures in the posterior compartment include the ulnar nerve (distally),
radial nerve (proximally), and radial recurrent arteries. The median nerve,
ulnar nerve, and brachial artery run along the medial aspect of the upper arm,
while the radial nerve runs laterally. The musculocuta- neous nerve travels
more in the midline, between the biceps brachii and brachialis muscles. Key
origin and insertion points of the muscles of the arm are pictured on Plate
2-7.
Coracobrachialis Muscle
A short, bandlike muscle of the upper
arm, the coracobrachialis, arises from the tip of the coracoid process and acts
to flex and adduct the arm. The short head of the biceps brachii muscle
originates from the lateral side of the coracoid process and runs side by side
with the coracobrachialis to form the conjoined tendon. The coracobrachialis
inserts by a flat tendon into the medial surface of the humerus just proximal
to its midlength (see Plate 2-8). The musculocutaneous nerve supplies the
coracobrachialis muscle and passes diagonally through the muscle at its
midlength.
Biceps Brachii Muscle
The biceps brachii is a long, fusiform
muscle of the anterior aspect of the arm (see Plate 2-8). Its long head arises as a rounded tendon
from the supraglenoid tubercle of the scapula, crosses the head of the
humerus within the capsule of the shoulder joint, and emerges from that capsule
to travel down the bicipital groove of the proximal humerus, between the
greater and lesser tuberosities. It is covered by the intertubercular synovial
sheath. The short head of the biceps brachii muscle arises by a thick,
flattened tendon from the tip of the coracoid process, in common with the
coracobrachialis muscle and, unlike the long head, never has an intra-articular
course.
The two bellies of the biceps brachii
muscle unite at about the middle of the arm to form the most prominent muscle
of the anterior compartment. The tendon of insertion is a strong, vertical cord
palpable down the center of the cubital fossa. Here, its deeper part turns its
anterior surface lateralward to end on the tuberosity of the radius, separated
from the anterior part of the tuberosity by the small bicipitoradial bursa. The
variable interosseous cubital bursa may separate the tendon from the
ulna and its covering muscles. The musculocutaneous nerve supplies both heads
of the biceps brachii, whose function is unique in that it acts across both the
shoulder and elbow joints. At the shoulder, the muscle assists in forward
flexion, joint stabilization (long head), and adduction (short head). At the
elbow, its main actions are to flex the elbow and supinate the forearm. The
biceps acts as an elbow flexor particularly when the forearm is supinated and
is a strong supinator with the elbow at least partially flexed and with the
forearm in a more pronated position.
The bicipital aponeurosis, or
lacertus fibrosis, formed from the more anterior and medial tendon fibers of
the muscle, arises at the bend of the elbow and passes obliquely over the
brachial artery and median nerve to blend with the antebrachial fascia over the
flexor group of the forearm (see Plate 2-8). The pull of the bicipital
aponeurosis is largely exerted on the ulna.
Brachialis Muscle
This muscle arises from the lower half
of the anterior surface of the humerus and the two intermuscular septa and lies
deep to the biceps. Its upper extent has two pointed processes positioned on
either side of the insertion of the deltoid muscle (see Plate 2-8). The
muscular fibers converge to a thick tendon, which adheres to the capsule of the
elbow joint and inserts on the tuberosity of the ulna and on the anterior
surface of its coronoid process. Its major attachment is to the coronoid
process about 2 mm distal from the articular margin. This muscle bulges beyond
the biceps brachii muscle on either side, and anterior to its medial border lie
the brachial vessels and the median nerve. The medial half of this muscle is
supplied by the musculocutaneous nerve, whereas the lateral portion is supplied
by the radial nerve. The muscle’s main action is to flex the elbow. The
brachialis has the largest cross-sectional area of any of the muscles that flex
the elbow, but it has a poor mechanical advantage owing to its close proximity
to the axis of rotation. This natural internervous plane within the muscle
allows it to be split during a routine anterolateral surgical approach to the
humerus to come down on the anterior surface of the humerus, typically for
fixation of a humeral shaft fracture.
Triceps Brachii Muscle
This large muscle with three heads
occupies the entire dorsum of the arm (see Plate 2-9). Each head of the triceps (long, lateral,
medial) originates distal to the other, with a progressively larger area of
origin. The long head arises by a strong tendon from the infraglenoid
tubercle of the scapula. Its belly descends between the teres major and teres
minor muscles and joins the lateral and medial heads of the triceps in a common
insertion on the olecranon. The long head is a defining border of the
triangular space (lateral border), which contains the circumflex scapular
artery; the quadrangular space (medial border), which contains the axillary
nerve and the posterior humeral circumflex artery; and triangular interval
(medial border), which contains the radial nerve and the profunda brachii
artery. The lateral head takes origin from the posterior surface and
lateral border of the humerus above and lateral to the radial groove and from
the lateral intermuscular septum. Crossing the groove and concealing the radial
nerve and deep brachial vessels, its fibers join in the common tendon insertion
on the olecranon. The lateral head is the lateral border of both the
quadrangular space and triangular interval. The medial head arises from
the humerus entirely medial and below the radial groove from as high as the
insertion of the teres major muscle to as low as the olecranon fossa of the
humerus (see Plate 2-9). It also takes origin from the entire length of the
medial intermuscular septum and from the lateral septum below the radial nerve
groove. The medial head is deep to the other heads and is hidden by them. The tendon
of the muscle appears as a flat band covering its distal two fifths. It inserts
on the posterior part of the olecranon and into the deep fascia of the forearm
on either side of it.
All three heads of the triceps brachii
are innervated by the radial nerve and have a primary action of elbow
extension. The long and lateral heads are innervated by branches of the radial
nerve that arise proximal to the radial groove, whereas the branch to the
medial head originates distal to the radial groove and also innervates the
anconeus. A radial nerve injury at the midshaft of the humerus, therefore,
usually does not disrupt function of the more proximally innervated long and
lateral heads of the triceps. The humeral shaft can be surgically approached
posteriorly by splitting the triceps muscle to come down onto the posterior
cortex or by elevating
the muscle along its lateral border and reflecting all three heads of the
muscle medially. This approach may afford more exposure than an anterolateral
approach in more distal fractures of the humeral shaft.
Anconeus Muscle
This is a small, triangular muscle
that arises from a broad site on the posterior aspect of the lateral epicondyle
of the humerus (see Plate 2-9). Its fibers diverge from this origin and insert
into the side of the olecranon and the adjacent one fourth of the posterior
surface of the ulna. The muscle is deep to the dorsal antebrachial fascia and
extends across the elbow and the superior radioulnar joints. It is innervated
by the terminal branch of the radial nerve that also innervates the medial head
of the triceps. The function of this muscle has been the subject of some debate
and includes assisting in elbow extension and stabilizing the elbow joint. Joint
stabilization may be its primary role. The anconeus serves as a key landmark in
the lateral or Kocher approach to the elbow, in which the interval between the
extensor carpi ulnaris anteriorly and the anconeus posteriorly is uti- lized to
approach the lateral or posterolateral aspect of the elbow.
Muscle Actions
The principal movements produced by
the muscles of the arm are flexion and extension of the forearm at the elbow.
The brachialis and biceps brachii muscles are the principal flexors. In this action,
the brachialis muscle is always active; the biceps brachii muscle becomes
active against resistance and is most effective when flexion of the forearm is
combined with supination. It is a powerful supinator of the forearm and one of
the primary muscles for producing this movement. Extension of the forearm is
produced by the triceps brachii muscle and assisted by the anconeus muscle. The
medial head of the
triceps brachii muscle is
usually active, and the lateral and long heads are recruited for extra
powers.
Certain heads of these muscles are
active at the shoulder joint. The long head of the biceps brachii muscle flexes
the arm at the shoulder, and its tendon aids in stabilization of the joint. The
long head of the triceps brac ii muscle assists in extension and adduction of
the arm.
