Passive and Active Glenohumeral Joint Stiffness

1. Introduction It is not clear how different portions of the capsuloligaments and muscles contribute to GH stability. Since GH instabilities occur frequently in various shoulder injuries, it is important to understand the restraining forces provided by various structures in the GH-joint. A better understanding of the GH stiffness will also help improve existing shoulder models by including GH translations that have not been incorporated and the joint is assumed to have only three rotational degrees of freedoms. GH-joint stiffness and laxity are assessed clinically by evaluating the resistance and range of motion during passive manual movement [1]. Using a servomotor to perturb the GH-joint under various levels of voluntary muscle contraction, it was also found that GH-joint stiffness in the scapular plane could increase linearly with muscle contraction by several folds [2]. However, to date, the passive and active GH-joint stiffness along anterior, posterior, superior and inferior directions has not been evaluated to characterize its roles in maintaining GH-joint stability in 3-D space. The purpose of this study was to investigate contributions of both passive and active structures crossing the GH-joint to GH stability along multiple anatomical axes. Study of passive and active control of GH-joint will help us better understand neuromuscular control and biomechanics of the shoulder in 3-D space and evaluate shoulder injuries, surgical outcome, and rehabilitation progress more accurately.