The inferior glenohumeral ligament serves as a critical restraint to anteroinferior dislocation. This study utilizes a biomechanical model to determine the strain field in the inferior glenohumeral ligament (IGHL) in a subluxed shoulder. Spherical lead object markers were carefully attached to the IGHL surface in a grid pattern. Two capsular configurations were used for strain field calculations: a “nominal” state and a “strained” state. For the nominal state, the capsule was inflated in neutral rotation with low pressure air. For the strained state, the humerus was externally rotated and anteroinferiorly subluxed. Stereoradiographs were taken of both configurations and markers digitized. Three-dimensional coordinates of object markers were reconstructed and strain fields calculated. Maximum principal strains on the glenoid side of the IGHL were significantly greater than the humeral side, and strain fields were highly variable over the region studied. This study reports an accurate method for measuring planar strains in a three-dimensional membrane.