Evidence-Based Strength Training: Scapulothoracic Musculature, Part 1

In the next installment of the Evidence-Based Strength Training Series for MedBridge Education, we are going to take a look at the often-neglected scapulothoracic musculature. Typically when considering the management of painful upper quarter conditions, local exercise and manual therapy interventions are employed judiciously. However, when utilizing a proper movement assessment or regional interdependence philosophy, impairments in the scapulothoracic musculature are often found to contribute to pain in distal or proximal joints. Weakness or diminished neuromuscular control of the peri-scapular muscles has been implicated in Subacromial Impingement4,18, Lateral Epicondylalgia2,7,12, Cervicogenic Headache10, and Neck Pain3,16. Additionally, in a prospective cohort study conducted by Clarsen et al.6, the presence of scapular dyskinesis led to an 8.4x greater risk for developing a shoulder injury during the course of an elite male handball season. Furthering the support for interventions focusing on the peri-scapular musculature, Lawrence and colleagues11 found that in the presence of shoulder pain due to subacromial impingement, patients demonstrated significantly reduced scapulothoracic upward rotation at lower angles of humerothoracic elevation and significantly reduced sternoclavicular posterior rotation throughout humerothoracic elevation.

With the knowledge of the scapulothoracic musculature’s impact on potentially injurious altered biomechanics and the deficits seen in many common musculoskeletal disorders, healthcare providers need to ask themselves, “What are the best exercises to recruit these muscles?”

Before delving into specific exercises, it is necessary to understand the basic biomechanics of the scapulothoracic and glenohumeral joints. During humeral elevation, the scapula upwardly rotates 1° for every 2° of humeral elevation until 120° humeral elevation is achieved. After this point, scapular rotation contributes 1° for every 1° humeral elevation until maximal arm elevation is met. Also, the scapula typically tilts posteriorly between 20° and 40° in the sagittal plane and externally rotates between 15° and 35° in the transverse plane17. This complex movement pattern relies on coordinated and balanced contributions from the trapezius, serratus anterior, levator scapulae, rhomboid, and pectoralis minor musculature.

Trapezius Musculature

The broad posterior musculature known as the trapezius originates at the medial third of superior nuchal line, external occipital protuberance, nuchal ligament, and the spinous processes of C7-T12 vertebrae and its distal insertion is at the lateral third of clavicle, acromion process, and spine of scapula. This muscle is divided into three distinct portions with the Upper Trapezius (UT) providing scapular elevation, Lower Trapezius (LT) proving depression, and the Middle Trapezius (MT) causing scapular retraction. Additionally, the UT and LT act together to rotate the glenoid cavity superiorly, which is a very important and often dysfunctional action for individuals suffering from shoulder impingement or pain11.

The primary action of the upper trapezius musculature involves elevation of the scapula and, predictably, the exercises that provide the highest Maximal Isometric Voluntary Contraction (MVIC) are those that involve this motion. Additionally, during scapular abduction, upper trapezius activity progressively increases from 0° to 60° and from 120° to 180° of abduction1. With this knowledge in mind, researchers have found that the highest electromyographical (EMG) activity occurs during the uni-lateral shoulder shrug9, rowing14, scaption8, and shoulder abduction in the scapular plane above 120°9. Due to the infrequency of UT weakness (unless secondary to neurological involvement) strengthening of this portion of the trapezius is often not the focus during the rehabilitation of painful upper quarter conditions. Instead, clinicians have learned to focus on improving middle and lower trapezius strength and normalizing the ratio of UT to the lower two portions of the trapezius activation.

Similar to the UT, the middle trapezius, with its primary function being scapular retraction, is often activated during exercises involving this action. The highest MVIC percentages for the MT have been recorded during horizontal abduction14, prone full-can9, horizontal abduction with external rotation14, and scaption8. Additionally, as the UT often compensates for a weak MT or lower trapezius, it is likely beneficial to utilize exercises with a good upper trapezius to middle trapezius ratio (UT:MT) when attempting to strengthen this musculature. Exercises shown to provide this ratio are side-lying forward flexion, side-lying external rotation, and prone shoulder extension5.

Due to its impact on scapular upward rotation, external rotation, and posterior tilt, the lower trapezius is of more importance than the aforementioned UT and MT during rehabilitation17. There have been a multitude of studies investigating lower trapezius weakness and its association with painful conditions and most do find this connection. Due to this fact, there have also been many studies looking into maximal EMG activity of the lower trapezius during upper extremity strengthening. The results show that significantly high MVIC values have been recorded during arm raise overhead in line with the LT muscle fibers9, ER at 90° of abduction9,15, horizontal abduction with ER5, and prone shoulder abduction5. While choosing exercises with a relatively high MVIC is important, it may be more important to choose those exercises that provide an optimal upper trapezius to lower trapezius ratio (UT:LT). As these are the two primary muscle groups involved in upward rotation of the scapula, having adequate and relatively equal contributions from each is important in maintaining normal biomechanics. In a study conducted by Cools and colleagues, it was determined that side-lying forward flexion, side-lying external rotation, and horizontal abduction with external rotation had the best UT:LT ratios5. In addition to this study, McCabe et al. conducted a similar study and found that the seated press-up, uni-lateral scapular retraction, and bilateral shoulder external rotation provided UT:LT ratios that showed a preferential activation of the lower trapezius over the upper trapezius13. While these studies do give a glimpse into proper exercise selection, not every exercise has been studied to date and never will. When choosing an exercise for your patient/client, it is important to take into consideration the biomechanics of the movement, current evidence supporting/refuting, and your patient’s presentation and goals for treatment.

The trapezius musculature is a very important piece of the puzzle, but contributions from the Serratus Anterior, Rhomboids, and Levator Scapulae also play a large role and will be discussed in Part 2.

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