Stiffness Regulation and Muscle-Recruitment Strategies of the Shoulder in Response to External Rotation Perturbations

Abstract

Moderate levels of muscle contraction can significantly increase glenohumeral joint stiffness and stability. Preactivation of the subscapularis appears to be the primary dynamic stabilizer with the arm in 0 degrees of external rotation. However, with the arm in 90% of maximum external rotation (the apprehension position), less subscapularis activity is observed and the maintenance of stability may shift toward other musculoskeletal structures because joint stiffness does not change. A relationship between generalized joint laxity, glenohumeral laxity, and stiffness was not observed in healthy subjects.