The muscles of the shoulder joint are listed according to their
position in relation to the joint. This position is not always apparent,
as a look at the illustrations will show. All muscles in this classification
pass either from the trunk or the scapula to the arm.
|Triceps brachii, long head|
This large fan-shaped muscle of the chest (Figure 5.11) converges
to a flat tendon, which twists on itself so that the lowest fibers
become the uppermost at its point of attachment. The muscle is divided
functionally into two parts, clavicular and sternal (or sternocostal).
The clavicular portion lies close to the anterior deltoid muscle
and acts with it in flexion, horizontal adduction, and inward rotation
of the humerus.
Ordinarily the line of pull of the clavicular portion of the
pectoralis major lies below the axis of the shoulder joint. However,
when the arm is raised sideward well above the horizontal, the line of
pull of the upper clavicular fibers shifts above the center of the
shoulder joint, and these fibers then cease to adduct and become
abductors of the humerus (see Figure 3.4). The clavicular fibers of
the pectoralis major are significantly active in abduction at the
level of 110 degrees. The sternocostal portion is generally antagonistic
to the clavicular portion in its actions in the sagittal plane.
It acts in downward and forward movements of the arm and in inward
rotation when accompanied by adduction. The pectoralis major as
a whole is most powerful for actions in the sagittal plane and is
particularly important in all pushing, throwing, and punching activities.
The clavicular portion may be palpated just below the medial two-thirds
of the clavicle, the sternal portion just lateral to the sternum
and below the clavicular part, and the muscle as a whole at the anterior
border of the axilla.
The muscle’s line of pull passes in front of the shoulder
joint (Figure 5.12), which suggests that it participates in forward movements of the humerus. Because
the proximal attachment and line of pull of the coracobrachialis
and the long head of the biceps are so similar, it has been difficult
to determine the specific actions of the coracobrachialis. However,
Stevens et al. (1976) were successful in isolating and recording
activity in the coracobrachialis, confirming that it serves as a
main force in horizontal adduction movements. Their research also
verified the belief that this muscle, the middle deltoid, and the
long head of the triceps, acting like guy wires on a mast, serve
to stabilize the shoulder joint. It may be palpated on the front
of the upper arm between the anterior deltoid and the pectoralis
major, but it is a difficult muscle to identify.
Although essentially a muscle of the elbow joint, the biceps
(see Figure 5.12) crosses the shoulder joint and is active in some
of the movements of the humerus. Both heads are always active in
flexion and in abduction with resistance when the elbow is straight.
The muscle is also active in horizontal adduction, and the short
head sometimes participates in adduction against resistance and
in medial rotation. The biceps also stabilizes the shoulder joint.
The complex structure of the deltoid (Figures 5.11 and 5.14),
with the multipenniform arrangement of the bundles making up the
middle portion, gives it a potential for great strength without undue
bulk. The middle portion of the muscle is a powerful
abductor of the humerus, its greatest activity occurring when
the humerus is raised between 90 and 120 degrees. It is capable
of supporting the weight of the upper extremity for long periods
while the hand is working at a height. The multipenniform arrangement
of fibers compensates for the middle deltoid’s rather poor
angle of pull. The angle of pull, however, serves the useful purpose
of providing the muscle with a strong stabilizing componentforce.
This is fortunate because, in the raised position, the shoulder
joint depends more on its muscles than on its ligaments for holding
the head of the humerus on the glenoid fossa. The middle portion
has also been found to be active in horizontal abduction.
The anterior portion of the deltoid aids in all forward movements of the arm and in inward rotation of the humerus. It
is also active in abduction. There is disagreement in the literature
concerning the movements effected by the posterior deltoid, but
there seems to be sufficient evidence to conclude that, in addition
to extension and lateral rotation, the lowest fibers, being situated
below the axis of motion, assist in forceful adduction of the humerus
from an overhead position. On the other hand, some of the upper
fibers (those closest to the middle deltoid) probably act with the
latter in contributing to abduction.
The muscles of the rotator cuff (subscapularis, supraspinatus,
infraspinatus, and teres minor) serve dual roles as stabilizers
and primary movers. Although the rotator cuff is frequently treated as
a single entity in research studies, each of the muscles of the
rotator cuff will be dealt with separately in this text.
As one of the rotator cuff muscles,
the subscapularis (Figures 5.12 and 5.13a) contributes significantly
to stabilization of the glenohumeral joint, especially in the prevention
of dislocation during forced lateral rotation of the abducted arm.
It is also one of the depressors of the humeral head during abduction
and flexion of the arm. Its chief action as a mover is inward rotation,
which it performs most effectively when the arm is at the side or
is elevated posteriorly. It has also shown significantly more electrical
activity in horizontal abduction than in horizontal adduction.
This muscle (Figures 5.13 and 5.15) acts together with the deltoid
in abduction of the arm throughout the entire range. It also acts
in flexion and horizontal extension. Action potentials reach their
maximum when the arm is at 100 degrees of flexion. As part of the
rotator cuff, it plays a significant part in the stability of the
shoulder joint and is important in preventing downward dislocation.
It may be palpated above the spine of the scapula, provided that
the scapula is supported, as when the armpit rests over the back
of a chair.
and Teres Minor
In addition to their outward rotary action, these two muscles
(see Figures 5.13b, 5.14, and 5.15), which seem to act as one, have
two additional noteworthy functions. Together with the subscapularis,
they depress the head of the humerus and thus prevent it from jamming
against the acromion process during flexion and abduction of the
arm. They are also part of the rotator cuff muscles (infraspinatus,
teres minor, subscapularis, and supraspinatus), whose important
function it is to aid materially in holding the head of the humerus
in the glenoid fossa. Their important function in this capacity
is to prevent dislocation of the shoulder joint, especially when
the humerus is in the abducted position. They may be palpated on
the posterior surface of the scapula, medial to and below the posterior
This is a broad sheet of muscle (see Figure 5.14) that covers
the lower and middle portions of the back. Coming mainly from the
lower half of the thoracic spine and the entire lumbar spine, the fibers
gradually converge as they pass upward and laterally toward the
axilla. Here the fibers twist on themselves in such a way that the
lowest fibers become the uppermost. They end in the narrow flat
tendon of the distal attachment. The muscle has a favorable angle
of pull for extension and adduction of the arm, particularly when
the latter is raised between 30 and 90 degrees. EMG has confirmed
the action of the latissimus dorsi in extension and adduction during
static and dynamic, resisted and unresisted movements. Electromyographic
evidence confirms that the latissimus dorsi muscle is also very
active during inward rotation of the humerus (Basmajian and DeLuca 1985;
Suenaga et al. 2003). The latissimus dorsi is also an active spinal
extensor and rotator muscle. Action of the muscle on the vertebral
column is covered in Chapter 9. The muscle may be palpated on the
posterior border of the axilla just below the teres major.
Structurally this muscle (see Figures 5.14 and 5.15) appears
to be in a favorable position to work with the latissimus dorsi
in downward and backward movements of
the humerus and also in inward rotation, but
Basmajian and DeLuca (1985) detected no sign of activity in this
muscle during these movements unless external resistance was applied.
Against active resistance, activity was evident during internal
rotation, adduction, and extension. No added resistance is needed,
however, for the teres major to be active during hyperextension
and adduction when the arm is behind the back.
Although primarily a muscle of the elbow joint, the triceps (see
Figures 5.13 and 5.15) is active in movements of the humerus because
its long head crosses the shoulder joint. It assists in adduction,
extension, and hyperextension of the humerus and acts to stabilize