Motion of the shoulder complex in individuals with isolated acromioclavicular osteoarthritis and associated with rotator cuff dysfunction: part 2 - muscle activity.

This study aimed to compare muscle activity in individuals with isolated acromioclavicular osteoarthritis (ACO), ACO associated with rotator cuff disease (ACO+RCD), and controls. Seventy-four participants (23 isolated ACO, 25 ACO+RCD, 26 controls) took part in this study. Disability was assessed with the Disabilities of the Arm, Shoulder and Hand (DASH) questionnaire. Muscle activity of the upper trapezius (UT), lower trapezius (LT), serratus anterior (SA), and anterior deltoid (AD) was collected during arm elevation in the sagittal and scapular planes. Pain during motion was assessed with the numerical pain rating scale. Analysis of the DASH, pain and kinematics were reported in part 1 of this study. For each muscle, separate 2-way linear mixed-model ANOVAs were performed to compare groups. ACO+RCD group had more UT and AD activity than the the isolated ACO and control other groups, more AD activity than the isolated ACO group during the ascending phase, and more AD activity than the ACO and control groups during the descending phase in both planes. Isolated ACO group had less SA activity than the control group only in the sagittal plane. Alterations in shoulder muscle activity are present in individuals with isolated ACO and with ACO+RCD and should be considered in rehabilitation.