Glenohumeral stability during a hand-positioning task in previously injured shoulders

This study aimed to compare glenohumeral stability during functional tasks in subjects with previous dislocation injury against non-injured controls. Six subjects with previous injury and six controls were asked to complete hand-positioning tasks against external forces applied in six directions. Arm kinematics, muscle activations and hand forces were measured and used as input to an inverse-dynamic model of the shoulder that optimised muscle forces to solve the load-sharing problem. Glenohumeral stability was calculated using the direction of the joint reaction force vector in the glenoid. The simulations showed that GH stability was significantly lower in the previously injured group compared to the controls, and that the direction of exerted forces had a significant effect on GH stability, with the hand pushing away from the body and medially producing significantly lower stability. GH stability was significantly lower in the previously injured group for all six force directions, even though all participants were back to normal activities and reported no symptoms from their injuries.