The pitching motion is an asymmetric action by which coordination of scapular rotation in the dominant arm might be affected in time and in comparison with the non-dominant arm. The study aimed to compare asymmetry and the evolution of scapular upward rotation over a one-year period. Data were collected twice, before and after a one-year period, from 92 participants (age = 15.1 SD 1.4 years, body height = 177.3 SD 10.9 cm, body weight 69.2 SD 14.5 kg). Scapular motion was tracked at different glenohumeral angles of elevation in the scapular plane: anatomical position (0°), 45°, 90° and 135°. Scapular upward rotation was calculated as the angle between the spinae scapula and the spine. Scapular upward rotation of the dominant arm was 5.1° (95% CI: 2.1°−8.1°) more compared to the non-dominant arm. Age group or glenohumeral angles of elevation did not affect this difference. Scapular upward rotation of the dominant arm decreased 1.9° (95% CI: −0.5° to 4.3°) after a one-year period, however, neither this observation, nor the interaction with age group or elevation angle was significant. These findings may indicate that pitchers could be at risk to develop shoulder injuries especially those that have been associated with scapular asymmetry.

In baseball pitching, biomechanical parameters have been linked to ball velocity and potential injury risk. However, although the features of a biomechanical model have a significant influence on the kinematics and kinetics of a motion, this influence have not been assessed for pitching. The aim of this study was to evaluate the choice of the trunk and shoulder features, by comparing two models using the same input. The models differed in thoraco-humeral joint definition (moving or fixed with the thorax), joint centre estimation, values of the inertial parameters and computational framework. One professional pitcher participated in the study. We found that the different features of the biomechanical models have a substantial influence on the kinematics and kinetics of the pitchers. With a fixed thoraco-humeral joint the peak average thorax angular velocity was delayed and underestimated by 17% and the shoulder internal rotation velocity was overestimated by 7%. The use of a thoraco-humeral joint fixed to the thorax will lead to an overestimation of the rotational power at the shoulder and will neglect the power produced by the forward and upward translation of the shoulder girdle. These findings have direct implications for the interpretation of shoulder muscle contributions to the pitch.

In baseball, pitchers are the players that are most prone to injury. Most injuries occur at the elbow and shoulder of the throwing upper limb. It is widely accepted that understanding the loading in the joints during pitching is a key factor to prevent injuries. To deepen the understanding of the joint actions this study proposes to split the net joint actions into two part: the motor actions and the stability actions representing respectively the actions generating the joint motion and the actions maintaining the joint integrity. The actions represent the actions applied on the distal segment of the joint. Eight youth elite pitchers participated the study and performed 5 fastball pitches while equipped with skin markers. Three pitches per pitchers were used to compute the joint actions with an inverse dynamics method. The results indicate at the elbow a maximal elbow stability moment in adduction (52±5Nm) on the lower arm at maximal external rotation and a motor action in flexion (38±10Nm) during the acceleration phase. At maximal internal rotation the maximal stability shoulder loading occurred, with a pulling force of 520±80N, a downward force of -290±95N and a depression moment of 65±17Nm. The motor actions at the shoulder were mainly a forward force (93±46N) and an exorotation moment (24±12Nm) during the arm acceleration phase. This study suggest that the main action of the shoulder is to stabilise the joint, with a maximal load at maximal internal rotation, and that the main action at the elbow is avoiding hyperextension, with a critical phase at maximal external rotation. Further study is needed to link the stability actions to injury risk.