The lower trapezius is a muscle which has been suggested to function
ideally with the mechanics of the scapula. It has been acknowledged that improper
mobilization of the scapula, also referred to as scapula dyskinesis, during
overhead activities may lead athletes to experience sports injuries in the form
of impingement, subacromial bursitis and instability. Due to its essential role
in scapula function and subsequent athletic shoulder pain, the lower trapezius
has become a topic of interest regarding both its activation ratios against the
other trapezius as well as its timing during movement.
Anatomy of the Lower Trapezius
Throughout the years, only a small amount of research has been conducted
to study the anatomy of the lower trapezius. However, there is plenty of
research regarding the role of the lower trapezius in scapula function and the
association between lower trapezius dysfunction and shoulder pain. A notable
study conducted in 1994 on the anatomy of the lower trapezius found that the
lower trapezius begins on the spine and lengthens from the T2 to T12 vertebrae,
inserting onto the spine of the scapula from the acromion process to its root.
According to the anatomy study, the lower trapezius closely follows the middle
trapezius which attaches to the C7 and T1 vertebrae, also attaching to the
spine of the scapula. The lower trapezius is a multipennate muscle which is
innervated by the accessory nerve, or the eleventh cranial nerve, and the
ventral rami of the third and fourth cervical nerves along the cervical plexus.
The Function of the Lower Trapezius
The scapula is the foundation of all upper limb kinetic chain movements.
To function effectively, it must have enough flexibility and mobility to achieve
the necessary positions to allow the humerus to move smoothly, without
impingement. It also needs to remain strong and stable during upper limb
movements, specifically overhead sports activities to provide the proper
transmission of force from the body to the hand, most important in athletic
activities such as swimming, tennis and throwing sports.
The lower trapezius is a muscle that plays an essential role during
scapula movement and positioning as well as dynamic scapula stability. The upward
rotation, posterior tilt, and external rotation movements of the scapula
function to increase the width of the subacromial space during humeral
elevation. Nonetheless, a lack of proper scapula function, or scapula
dyskinesis, can increase the adaptation of the humeral head, altering the position
and motion of the scapula, often leading to injury.
The lower trapezius is one of the various muscles of the body that
function during upward rotation, posterior tilt and external rotation of the
scapula along with the middle trapezius and serratus anterior. The role the
lower trapezius plays in scapula function cannot be discussed on its own
because it works together with the other muscles that surround it to create a force
couple at the scapula. The contribution of other competing factors in scapula
dysfunction including pectoralis minor tightness, posterior shoulder capsule
tightness and thoracic spine stiffness should also be considered when
diagnosing scapula complications.
The exact role of the trapezius in shoulder mobility has been previously
researched. The researchers of the study first calculated the anatomical lines
of action of the fibres of the trapezius and, considering these lines of action
together with the changing scapulothoracic axis of rotation, they found that
the middle and lower trapezius are perfectly suited to stabilize the scapula and
its external rotation. Because of the immediate center of rotation of the
scapula on the thorax, it is designed to move from the root of the spine
towards the AC joint along the line of trapezius insertion.
The lower trapezius is the only muscle of the trapezius that can
considerably rotate the scapula upwards. Although, its length between each
joint and the force acting on it will change across the range of motion to
elevate the arm. As the scapula moves through upward rotation, a movement that
shortens the lower trapezius, it also protracts and elevates, movements that
elongate the lower trapezius. As a matter of fact, the actual change in muscle
fibre length can remain unchanged, making the lower trapezius contraction
almost exclusively isometric.
The lower trapezius takes part in several essential functions as
follows:
First, the scapula stabilizes the shoulder as it moves into abduction.
The initial movement and inertia of the humerus in abduction causes a drag
effect on the scapula and pulls it into a downward rotation position. The lower
trapezius functions as a feedforward muscle before its abduction to contract
and hold the scapula stable in order to counteract the downward rotation drag
effect. As a result, it counteracts the scapula at the start of abduction.
During the first 30 degrees of abduction, the scapula remains immobile but it is
held steady by the lower trapezius.
Also, during progressive shoulder abduction, approximately from 30
degrees to 120 degrees, the lower trapezius functions to establish the upward
rotation of the scapula together with the serratus anterior. The lower
trapezius muscle stabilizes the scapula against the protraction effect produced
by the serratus anterior.
Then, at the uppermost levels of abduction, about 120+ degrees, it works
to also create a posterior tilt of the scapula. It neutralizes the elevation
effect of the upper trapezius and levator scapulae during the end of range
abduction.
Moreover, while the upper trapezius doesn’t seem to have a line of
action for being an upward rotator in healthy individuals, the lower trapezius helps
to produce scapulothoracic upward rotation. Further evidence has demonstrated
that the lower trapezius is the primary upward rotator of the scapula along
with serratus anterior.
Furthermore, the lower trapezius also retracts and pushes down the
scapula during horizontal pulling movements, such as rowing, and works with
other scapular retractors in postural positions to counteract the effect of
scapular protraction when sitting.
And last but not least, the activity of the lower trapezius has been
found to be relatively low during scapular abduction and flexion angles of less
than 90 degrees with rapid increases during 90 to 180 degrees. This emphasizes
the increasing role it plays in upward rotation and posterior tilt as the
shoulder abducts above 90 degrees.
Shoulder Injuries and Conditions
Research
studies have evaluated the cause and effect relationship between lower
trapezius complications and consecutive shoulder pain. Theories have before
mentioned that the muscle may be dysfunctional, leading to poor scapula
movement and symptoms of pain. Other theories explain that pathology in the
joint may develop first, inhibiting the lower trapezius. Regardless of its
cause or effect, the presence of a dysfunctional lower trapezius demonstrates that
the muscle will often require medical interventions to improve its function.
Numerous studies have been conducted on the role the periscapular
muscles play in scapula function/dysfunction and their associated symptoms. It
has been recognized that the scapula muscles, including the lower trapezius, functions
to enhance the rotator cuff’s ability to perform properly. These create a
resistant scapula that allows the rotator cuff to function more efficiently.
Various findings have been recorded below, relating to lower trapezius
dysfunction and pain syndromes:
Foremost, a lack of activity in the lower trapezius has been recognized
along with overhead movements that cause impingement, often together with an
excessive upper trapezius activation.
Also, structures often associated with secondary subacromial impingement
are identified as low levels of serratus anterior and lower trapezius muscle
activation which cause a distinction of the medial border and inferior angle of
the scapula, along with its excessive internal rotation.
The strength of the lower trapezius also decreases among individuals
with unilateral neck pain.
Then, considerably delayed middle and lower trapezius activation has
been displayed in overhead athletes with shoulder impingement, in response to
an unexpected drop of the arm from an abducted position. The lower trapezius is
believed to react slower when compared to the upper trapezius, which may become
overactive, leading to scapular elevation rather than upward rotation.
Researchers found a decrease in lower trapezius activity during
isokinetic scapula protraction in 19 overhead athletes with subacromial impingement.
Additionally, the researchers reported that athletes with impingement
complications have a significantly higher upper trapezius activation compared
to healthy individuals, a considerable decrease in lower and middle trapezius
activation and altered trapezius muscle balance.
Activity of the Lower Trapezius in Exercise
When an
athlete has suffered damage or injury on the lower trapezius, there are several
rehabilitation exercises the individual can follow. Literary conflict exists
where some authors have argued about the proper treatment plans for injury.
According to several sources, recovery for individuals with lower trapezius
imbalances must be performed with reduced activation to avoid fatigue,
approximately 20 to 40 percent of maximum voluntary contraction. Furthermore,
high levels of activity may be associated to other scapular muscles, such as
the upper trapezius and even the latissimus dorsi.
Other researchers argue that the exercises for the rehabilitation of the
lower trapezius need to be performed in weight bearing and in kinetic chain
patterns to fully resemble what the muscle does in gross kinetic chain function.
They’ve noted that in average sports specific movements, early upper trapezius
activity is normal, and thus, rehabilitation for athletes should encourage early
upper trapezius activation.
Many studies acknowledge the significance of glenohumeral external
rotation in activating greater lower trapezius activation. Exercises such as
the scaption, robbery exercise, the lawn mower and the shoulder horizontal with
external rotation, all draw out greater levels of lower trapezius activation. Some
researchers state that the rotator cuff and scapular stabilizers function
conjointly to maintain optimal length-tension relationships within the rotator
cuff. They postulated that with shoulder external rotation, as the humeral
attachment of the infraspinatus and posterior deltoid approximates the scapula,
the muscle would lose optimal length-tension. Accordingly, if the scapula was
to retract simultaneously to humeral external rotation, the medial scapula
would distance itself from the humeral attachment, thus maintaining the
length-tension relationship.
The position of arm elevation may also be crucial. Abduction angles
around 130 degrees appear to obtain the greatest lower trapezius activation
while minimizing the upper trapezius. A research study, for instance, determined
that the position in which the individuals elevated the humerus above the head
in line with the lower trapezius muscle fibres, activated the lower trapezius
up to 97 percent MVIC, or market value of invested capital.
Scapular Resetting in the Neutral Position
A
healthcare professional can assess scapula muscle imbalances by evaluating the
position of the scapula in a standing position. If the scapula appears to be
downwardly rotated/ anterior tilted and protracted, an imbalance may exist in
the pectoralis minor and the lower trapezius.
A
relatively safe exercise known as an active scapula setting drill can be guided
by a healthcare professional to gently activate posterior tilt, upwardly rotate
and retract. Also, the individual can actively externally rotate their humerus
gently. The healthcare specialist can palpate the lower trapezius for
activation and this position can be held for 10 second holds. Resistance can
then be added in the form of tubing to force the scapula into a more downward
rotation.
Scapula Re-Setting in the Horizontal Plane
This
is an exercise progression aimed to encourage the retraction
and depression role of the lower trapezius. Weight is needed to
create a drag effect on the scapula into protraction and elevation to
create the necessary length-tension curve for the lower trapezius.
First,
the individual is positioned in one hand supported prone, or one-arm row
posture, with a 2.5kg weight for women or 5kg for men. The individual is
encouraged to allow the scapula to ‘hang’. The healthcare professional then
guides the appropriate movements of retraction and depression while the
individual slightly externally rotates the humerus during the movement to
keep the lift of the dumbbell only subtle. This position can be held for a 5
second contraction. Care must be taken to avoid excessive upper trapezius use,
where the individual will lift the shoulder towards the ear, excessive
latissimus dorsi use, where the arm will move slightly into extension and
rhomboids use, where the muscles will bunch and the scapula will be seen to
downwardly rotate.
Scapula Setting in Elevated Plane
This
is a further progression that is appropriate for those with minimal pain on
shoulder elevation. If the individual is suffering current shoulder pain during
elevation then this would be inappropriate.
First,
the client sits and holds a lat pulldown bar. The weight needs to be sufficient
to create an elevation drag effect. Then, the individual sits back slightly so
that the trunk angle is approximately 70-80°. This allows the humerus to follow
the ‘scaption’ plane. Third, the individual is guided to gently retract and
depress the scapula using the lower trapezius. Similar to the above exercise,
they are encouraged to gently externally rotate the humerus. As the bar is a
solid object, they are encouraged to simply and gently ‘bend the bar’. Again
care must be taken to keep the movement subtle to avoid excessive latissimus
dorsi and/or rhomboid activation.
High Level Activation
This
is an exercise best suited to end stage rehabilitation or as a pre-rehab
setting exercise prior to training. The individual will need a pain-free
shoulder to perform this movement. First, the individual must slowly reach the
hand along the floor to create scapula upward rotation.
Now,
the individual must slowly externally rotate the humerus.
Finally,
the individual should slowly lift the arm off the floor to encourage retraction
and posterior tilt of the scapula. 5. Hold for 5 seconds and repeat.
Shoulder Stretches
Furthermore, the individual may also perform a series of shoulder
stretches, to strengthen the shoulder and the surrounding muscles of the neck,
shoulder and back. It’s important to keep in mind to consult a healthcare
professional before attempting any of the above exercises and stretches, to
avoid further injury for the athlete.
By Dr. Alex Jimenez