Kinematics and Electromyographic Analysis of Three Types of Internal Rotation in the Shoulder

Introduction Kinematic and electromyography (EMG) studies concerning internal rotation (IR) of the shoulder are limited. This study investigated the kinematic components and EMG activity of the rotator cuff muscles during IR at the side (IRs), IR at 90 ° of abduction (IRa), and IR to the posterior (IRp). Methods Ten healthy young men attended this study. The angles of IR, extension, and abduction of the humerothoracic joint, and angles of scapular IR, anterior tilt, and upward rotation were measured using a three-dimensional (3-D) motion analyzer. EMG activities in the 4 rotator cuff muscles were simultaneously recorded using synchronized 3-D motion analysis. Results The maximum IR angles were 56.6°, 72.4°, and 31.6° in IRs, IRa, and IRp, respectively. The maximum extension angle was 34.1° in IRp, and the maximum abduction angles were 1.4°, 87.4°, and 24.1° in IRs, IRa, and IRp, respectively. The EMG activity patterns of the rotator cuff muscles differed across the 3 IR types. %IEMGs of the subscapularis in IRs were approximately 20%, gradually increased up to 18.2% in IRa, and greatly increased to 29.8% at maximum IRp. %IEMGs of the supraspinatus were constant during the 3 movements, and EMG activities of the infraspinatus and teres minor decreased preceding IRa, whereas those increased up to maximum IRp. Discussion The rotation axes of IRs and IRa are constant, but that of IRp always changes. Our results show that the internal rotation angles for IRs and IRa are nearly identical to those reported previously. In contrast, the rotation angle for IRp differs from those in the current reports. EMG analysis of the 3 types of IR movements reveals different muscle activity patterns. Conclusion Kinematic and EMG analyses revealed that the three types of IR completely differ. The total IR angles in the humerothoracic joint increased in the order IRa > IRs > IRp. The highest activity of the subscapularis was observed in IRp. The internal rotation angle is the smallest, and the internal rotation movement of the IRp is completed in the early phase of the movement. The joint structure itself is presumed to limit the internal rotation movement of the IRp.