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