## 1 Introduction  
Assessing scapular position and motion is paramount in the evaluation of the clinical status and athletic performance of overhead sports athletes' shoulder function [1, 2]. Most often, the resting arms position combined with arm elevation and depression in the sagittal (flexion/extension), frontal (abduction/adduction) and scapular (\"scaption\") planes are used as movement performance tasks [2, 3]; however, it is arguable that constraining shoulder movement to the abovementioned specific planes of motion may be sufficient to compare and generalize scapular kinematics for functional hum as during work tasks or activities of daily living [4; 5; 6; 7; 8; 9], and, eventually, for an overhead throw, spike or serve (henceforth collectively termed as \"throwing\" for simplicity purposes, unless otherwise stated). The unilateral overhead throwing motion involves considerable amounts of shoulder external (late coking)/internal (follow-through) rotation to generate adequate angular momentum, most often to accelerate a ball (and decelerate the arm) [10; 11]. Scapula and humerus movements are multiplanar (3-dimensional [3D]), coupled and coordinated during arm elevation [12], and hence believed to be equally so during overhead throwing. Evaluations of 3D scapular kinematics in overhead throwers have thus evolved from simple humeral positioning in the frontal, sagittal, or scapular planes to humeral motion in the horizontal plane (shoulder internal/external rotation) with the arm at 90\({}^{\circ}\) of abduction [13; 14] and, ultimately, simulation of a throwing action [15; 16; 17; 18]. In an overhead throw, the scapular appears to externally rotate, upwardly rotate, and posteriorly tilt during the early cooking phase of the cycle, progresses into internal rotation after maximum humeral horizontal abduction and reaches the highest posterior tilting at the late cooking phase (maximum humeral external rotation). During the arm acceleration phase, the scapula seems to move toward greater internal rotation and begin to anteriorly tilt. At maximum humeral internal rotation, the scapula ends in internal rotation, upward rotation, and anterior tilting [15; 16; 17; 18]. During the rehabilitation continuum, the training of movement patterns emphasizing functional and specific sports gestures are considered valuable to improve performance before returning to sports [19]. Diagonal humeral movement pattern training, combining shoulder flexion with horizontal abduction and external rotation and shoulder extension with horizontal abduction and internal rotation, are commonly used, as they mimic the throwing motion [20]. These movement patterns increase cortical recruitment of sensorimotor areas [21], activate key shoulder muscles, such as the serratus anterior, anterior deltoid, infraspinatus, upper, middle, and lower trapezius, as well as muscles involved in the kinetic chain of the throwing motion, such as the erector spinae [22], and improve functional performance [20], offering a superior rehabilitative value than simpler humeral movements, such as shoulder flexion [21] or shoulder caption [22]. To date, however, how the scapula moves during the diagonal humeral movement patterns described above, also known as diagonal 2 pattern for flexion (PNF-D2-fix) and diagonal 2 pattern for extension (PNF-D2-ext) of proprioceptive neuromuscular facilitation concept, respectively, has not been explained. Evaluations, exercises, and manual interventions may be better designed and delivered if such knowledge were available to healthcare and sports professionals and researchers. The primary objective of this study was to provide a comprehensive description of scapular orientation during the PNF-D2-flx/ext task. Additionally, we aimed to compare the scapular orientation during this PNF task with that observed in three traditional constrained arm motion tasks, namely shoulder abduction/adduction, flexion/extension, and scaption.