Overhead throwing athletes have been shown to develop adaptive changes in humeral rotation to allow for higher throwing velocities. This manifests as an increase in humeral external rotation and a decrease in internal rotation, which is called glenohumeral internal rotation deficit (GIRD). The percentage of GIRD that significantly affects glenohumeral joint kinematics is not known. The objective of the study was to create a throwers shoulder model with fixed percentages of GIRD to determine at which point kinematic changes start occurring.
The results showed that there was a significant decrease in posterior translation starting at 10% GIRD. With inferior translational loads, significantly less inferior translation starts occurring at 20% GIRD. The humeral head apex position at maximum external rotation moves superiorly, posteriorly and laterally, with significant changes in the superior direction occurring with 10% GIRD onwards. Overall, significant kinematic changes begin at 10% GIRD and this should be taken into account for clinical decision-making as to when intervention is necessary.