The lower trapezius and pectoralis minor muscles, along with the latissimus dorsi and lower portion of the pectoralis major muscles, produce forceful depression of the scapula that is accompanied by scapular downward rotation [4, 5] (Fig. 3.1b). The balance of muscle activity between the anterior muscles (pectoralis minor and major) and posterior muscles (trapezius and latissimus dorsi) will determine whether the scapula protracts, retracts, or remains in a neutral position as the scapula moves into depression.

Scapular protraction is produced by the pectoralis minor, serratus anterior, and pectoralis major muscles [6, 7] (Fig. 3.2a). Based on the relative amount of activity in each of these muscles, other scapular movements (upward/downward rotation, internal/external rotation, and anterior/posterior tilt) may occur simultaneously with scapular protraction. For example, if the pectoralis minor and major muscles dominate the motion then based upon their attachments on the scapula, scapular protraction will be accompanied by scapular downward rotation, internal rotation, and anterior tilt. Conversely, orientation of the serratus anterior muscle allows it to upwardly rotate, externally rotate, and posteriorly tilt the scapula as it protracts the scapula.

Scapular retraction is produced by the trapezius, rhomboids, and latissimus dorsi muscles [2, 3] (Fig. 3.2b). As the rhomboids and latissimus dorsi muscles create scapular retraction, they also produce scapular downward rotation. The ability of the trapezius muscle to upwardly rotate the scapula during retraction can counter this downward rotation, thereby keeping the scapula in a neutral upwardly/downwardly rotated position.

The deltoid (anterior, middle, and posterior) along with the rotator cuff muscles (subscapularis, supraspinatus, infraspinatus, and teres minor) are the primary muscles that produce glenohumeral movements and provide glenohumeral stability. Glenohumeral elevation in the sagittal plane (flexion) occurs through activation of the anterior and middle portions of the deltoid muscle [8–10] with stabilizing contributions from the infraspinatus, supraspinatus, and subscapularis muscles [11–13]. The anterior and middle portions of the deltoid muscle [10, 14, 15] and the supraspinatus muscle [9, 11, 16] are the primary muscles responsible for producing frontal plane glenohumeral elevation (abduction) with stability being provided by the infraspinatus and subscapularis muscles [11–13, 16–18].

The muscles primarily responsible for producing glenohumeral internal and external rotation vary based upon the angle of arm elevation. The infraspinatus muscle is primarily responsible for producing glenohumeral external rotation in neutral (arm in 0° elevation) and at 90° of arm elevation [10, 18]. The supraspinatus [19] as well as the teres minor and posterior deltoid muscles assist the infraspinatus muscle during these movements [8]. Similarly, glenohumeral internal rotation is produced by the subscapularis muscle when the arm is at the side with additional contributions from the supraspinatus [11], middle deltoid [11], and pectoralis major muscles [8] when the arm is at 90° of elevation.

The anterior deltoid along with the pectoralis major and subscapularis muscles produces glenohumeral horizontal adduction [18, 20]. The posterior deltoid and infraspinatus muscles produce glenohumeral horizontal abduction [18, 20, 21].

Arm elevation can occur in many planes including the frontal, sagittal, and scapular plane. Poppen and Walker [22] suggest that scapular plane elevation, defined as 30°–45° anterior to the frontal plane, provides optimal glenohumeral joint congruity which enhances joint stability and maintains an ideal muscle length-tension relationship of the glenohumeral musculature. Thus, most overhead activities are performed in the scapular plane and require a coordinated balance of scapulothoracic and glenohumeral movements [22, 23]. These movements include scapulothoracic upward rotation, external rotation, and posterior tilt, along with glenohumeral elevation and external rotation [2, 24–27].

As the arm is elevated to an overhead position, the primary scapulothoracic motion is upward rotation. The traditionally described muscle force couple responsible for producing this movement consists of the upper and lower portions of the trapezius muscle along with the serratus anterior muscle. Contrary to this, Johnson et al. [3] purport that the middle trapezius muscle works with the serratus anterior muscle in a force couple that produces scapular upward rotation. Based on a cadaveric study, the authors proposed that once the serratus anterior muscle initiated upward rotation, the middle trapezius was optimally aligned to assist with upward rotation [3]. The serratus anterior and middle trapezius muscles then continue to work in a force couple to upwardly rotate the scapula as the arm is raised overhead (Fig. 3.3). The role of the lower trapezius was proposed to be one of scapular stabilization by offsetting scapular elevation and protraction produced by the upper trapezius and serratus anterior muscle, while the upper trapezius muscle was proposed to be one of clavicular and scapular elevation and retraction [3].

At end ranges of scapular plane arm elevation, the scapula posteriorly tilts and externally rotates [27]. The serratus anterior, rhomboids, and trapezius muscles work together as a force couple to produce these motions [14, 26, 28, 29]. It is the extensive attachment on the inferior angle of the scapula that places the lower portion of the serratus anterior muscle in an ideal orientation to produce scapular posterior tilt. The lower trapezius muscle’s attachment from the lower thoracic spinous processes to the deltoid tubercle on the scapular spine provides the ability to work with the lower portion of the serratus anterior to produce scapular posterior tilt (Fig. 3.4). Scapular external rotation results from a force couple created by the serratus anterior and rhomboid muscles. These muscle’s attachments to the vertebral border of the scapula are aligned to produce scapular external rotation (Fig. 3.5).

The contribution of the glenohumeral joint to overhead arm motions requires a balance of adequate joint mobility and stability. Glenohumeral joint stability relies heavily upon the concavity compression mechanism [30]. The concavity compression mechanism refers to the stabilizing effect of the concave glenoid fossa and compressive forces of the rotator cuff musculature on humeral head translations. Other factors that contribute to this mechanism are glenoid articular cartilage thickness and the glenoid labrum [30].

As the arm is raised, overhead glenohumeral elevation is accomplished through a force couple formed by the deltoid and the rotator cuff muscles (Fig. 3.6). During the initial phases of glenohumeral elevation, the deltoid muscle functions to elevate the humerus, while the rotator cuff muscles stabilize the glenohumeral joint by compressing the humeral head into the glenoid fossa [15, 16, 31, 32]. Additionally, the supraspinatus muscle assists the deltoid in producing arm elevation, while the infraspinatus and teres minor muscles produce humeral external rotation toward the end range of arm elevation.