Shoulder pain is a common complaint in overhead athletes involved in sports
such as swimming, tennis and the throwing sports. Overhead arm movements
place high demands on the shoulder complex, and require muscular activation
around both the scapula-thoracic joint and the glenohumeral joint.
Researchers report that abnormal biomechanics of the shoulder girdle and
repeated overhead movements can lead to injuries in overhead throwing
athletes(1).
In particular, muscular imbalances around the shoulder complex in the form of
altered activation patterns and inherent myofascial restrictions, may lead to
diminished scapular control and dyskinesis resulting in glenohumeral joint
injuries, such as instability and impingement(2).
The serratus anterior (SA) is one of the muscles that provides a link between
the shoulder girdle and the trunk. Its dysfunction likely plays a role in shoulder
pathologies(3,4). The SA is a prime mover of the scapula, and contributes to
normal scapulohumeral rhythm and motion(4). It has large moment arm which
produces upward rotation and posterior tilting as a result of its insertion on the
inferior and medial border of the scapula. Poor activation of the SA muscle
may result in reduced scapular rotation and protraction. This dyskinesia may
trigger a relative anterior-superior translation of the humeral head in relation to
its glenoid articulation, causing subacromial impingement and rotator cuff
tears(5).
Anatomy and biomechanics
The SA is a flat sheet of muscle originating from the lateral surface of the first
nine ribs (see figure 1). It passes posteriorly around the thoracic wall before
inserting into the anterior surface of the medial border of the scapula(6). Overall,
the main function of the SA is to protract and rotate the scapula. This
movement provides optimal positioning of the glenoid fossa for maximum
efficiency for upper extremity motion(7). The SA consists of three functional
anatomical components(8,9):
1. The superior component – Originates from the first and second ribs
and inserts into the superior medial angle of the This component
serves as the anchor that allows the scapula to rotate when the arm is
lifted overhead. These fibers run parallel to the 1st and 2nd rib;
2. The middle component – Originates from the second, third and
fourth ribs and inserts onto the medial border of the scapula anteriorly
(sandwiched between the scapula and ribs). This component is the
prime protraction muscle of the scapula;
3. The inferior component – originates from the fifth to ninth ribs and
inserts on the inferior angle of the scapula. The fibers form a ‘quarter
fan’ arrangement, inserting onto the inferior border of the scapula.
This third portion serves to protract the scapula and rotate the inferior
angle upward and laterally. Inman (1944) proposed that the lower part
of the serratus anterior is the stabilizer of the inferior border of the
scapula, and works with the lower trapezius to create a force couple
to upwardly rotate the scapula during overhead movement(10).
Figure 1: Serratus anterior overview
The functional roles of the SA are to(9):
1. Upwardly rotate the scapula during shoulder abduction, particularly
from 30 degrees of shoulder abduction onwards;
2. Stabilize and protract the scapula during shoulder flexion movements;
3. Rotate the inferior angle anteriorly (posterior tilt of the scapula);
4. Stabilize the scapula against the thorax during forward pushing
movements in order to prevent the scapula ‘winging’ (see below);
5. Hold the medial border of the scapula firmly against the thorax so that
with the hand fixed, it can displace the thorax posteriorly during a
push up.
In the athlete, specific movements require precise function of the SA to
achieve either full scapular protraction and/or upward rotation. Examples of
athletic endeavors requiring this SA function include:
1. Throwing a punch in boxing – The SA helps to achieve maximum
reach of the arm. Hence the SA is often referred to as the ‘boxers
muscle’.
2. Absorb the impact of a punch in boxing – The SA braces the
scapula on impact with the punch. This allows maximum transfer of
force from the lower limbs through the torso to the punching arm. If
the scapula was to ‘collapse’ into retraction upon impact of the punch,
the boxer would lose power in the punch.
3. Maximum reach for hand entry in swimming – The SA again
lengthens the arm to enable the athlete to take the greatest stroke
possible.
4. A tennis player serving – The overhead athlete such as a tennis
player needs full upward rotation in the act of serving.
5. Extend reach during the catch phase of the rowing stroke – The
sweep style rower needs full protraction on the ‘long’ side to achieve
necessary reach.
6. Baseball pitcher’s follow through – In baseball, the pitcher needs
high levels of protraction during the follow through of the baseball
pitch. Similarly in other throwing events in athletics.
The SA is innervated by the long thoracic nerve, which originates from the
anterior rami of the fifth, sixth, and seventh cervical nerves (see figure 2)(7,8).
Branches from the fifth and sixth cervical nerves pass anteriorly through the
scalenus medius muscle before joining the seventh cervical nerve branch that
courses anteriorly to the scalenus medius. The long thoracic nerve then dives
deep to the brachial plexus and the clavicle to pass over the first rib. Here, the
nerve enters a fascial sheath and continues to descend along the lateral
aspect of the thoracic wall to innervate the SA muscle.
Figure 2: Long thoracic nerve (from Safran et al
2004)(11)
SA dysfunction associated with scapula dyskinesis
Proper positioning of the humerus in the glenoid cavity during movement,
known as scapulohumeral rhythm, is critical to the proper function of the
glenohumeral joint during overhead motion. A disturbance in normal scapula
movement may cause inappropriate positioning of the glenoid relative to the
humeral head, resulting in an impingement or instability(2,12,13). Small changes in
activation in the muscles around the scapula can affect its alignment, as well
as the forces involved in upper limb movement(14). One of the primary muscles
responsible for maintaining normal rhythm and shoulder motion is the SA(15).
Actively assisting a patient’s scapula into an ‘ideal’ posture by reducing the
anterior tilt, often reduces pain and increases strength in the shoulder during
overhead activities(16). Since the SA actively positions the scapula into a
posterior tilt during overhead activities, it is assumed that an anteriorly tilted
scapula is a result of SA dysfunction. A weak SA positions the scapula in a
downwardly rotated and anteriorly tilted position, making the inferior border
more prominent or winged. Pathological inhibition of the SA from nerve
damage or an imbalance between the SA and the other protracting muscle,
the pectoralis minor, may also result in a winged scapula. Scapular winging
may precipitate or contribute to persistent symptoms in patients with
orthopedic shoulder abnormalities(17,18).
This scapular winging is best appreciated when watching the scapular position
during a push-up exercise. Often, if the winging is due to a muscle imbalance
and the primary scapula stabilizer is the pectoralis minor, it usually corrects if
the patient is asked to ‘plus’ and protract the scapula. To cue the athlete to
perform this plus maneuver, once they are in the plank position, ask them to
push the floor away. This is also referred to as a scapular pushup. If the wing
disappears then the cause is most likely muscle imbalance, if it remains then it
may be a pathological inhibition of the SA due to an injury to the cervical nerve
root or long thoracic nerves (see figures 3-6).
Figure 3: Scapular winging on push up bilaterally
Figure 4: Winging corrects on execution of a ‘plus’
Figure 5: Scapular winging on push up bilaterally
(right greater than left)
Figure 6: Left scapula corrects with ‘plus’ however
note the right is still winged
Research review
1. A comparison between the strength of contraction of the trapezius
and the SA in people with and without shoulder pathology found that
the upper trapezius shows increased activity during arm elevation and
lowering, and the SA shows decreased activation at some elevation
angles (usually 70-100 degrees) in people with an injury(19).
2. When the muscle activation patterns of swimmers with shoulder pain
is compared to those without, the middle and lower SA show
decreased activity in all phases of swimming motion in the painful
shoulders. Is this the cause of the shoulder pain or a consequence of
a painful shoulder whereby the swimmer uses compensatory muscle
activation patterns(20)? Studies could not determine.
3. Similarly, other researchers have found a ‘latency’ or activation delay
in the SA in the painful shoulders of swimmers as they raise their
arms in the scapular plane(21).
4. Ludewig and Cook (2000) hypothesized that patients with decreased
SA activation suffer from shoulder pain or instability, and that an
increase in lower trapezius activity is an attempt to compensate for
the decreased serratus anterior activation(2).
5. Lin et al (2005) studied subjects with various types of shoulder
dysfunction and found decreased serratus anterior activity and
increased upper trapezius activity, without a change in lower trapezius
activity, in injured shoulders when compared to normal subjects(22).
Scapular position also impacts the ability of the rotator cuff to function.
Excessive anterior tilt, internal rotation, or excessive elevation decrease
rotator cuff activation and cause an unequal distribution of tension along the
tendons. Such situations impair the optimum length-to-tension ratio of these
muscles, leading to a loss of stabilization and increasing the chance of
muscular disruption or degeneration(23).
A strong and conditioned serratus anterior muscle improves performance in
sports such as swimming, throwing, and tennis. A fatigued serratus anterior
muscle reduces scapular rotation and protraction. The dyskinesia likely allows
the humeral head to translate anteriorly and superiorly, and possibly leads to
secondary impingement and rotator cuff tears. Exercises to strengthen the SA
is the topic for the second part of this series.