Increased co-contraction of arm adductors is associated with a favorable course in subacromial pain syndrome

Background: Enhancement of arm adductor activity during abduction (ie, adductor co-contraction), may be effective in the treatment of subacromial pain syndrome (SAPS). We assessed whether an increase of adductor co-contraction is associated with a favorable course of SAPS. Methods: At baseline and after nearly 4 years of follow-up, electromyography of the latissimus dorsi (LD), teres major (TM), pectoralis major, and deltoid muscle was obtained during isometric abduction and adduction tasks in 26 patients with SAPS. Changes in co-contraction were assessed with change in the activation ratio (ΔAR). The AR ranges between −1 and 1, where lower values indicate more co-contraction. Clinical course was determined from an anchor question (reduced, persistent, or increased complaints), the visual analog scale for pain (VAS), and the Western Ontario Rotator Cuff score (WORC). Results: In patients indicating persistent complaints (31%), the VAS and WORC remained stable. In patients who indicated reduced complaints (69%), the VAS reduced (z score, −3.4; P =.001) and WORC increased (z score, 3.6; P <.001). Unchanged ARs associated with complaints persistence, whereas decreased AR of the LD (ΔAR_LD, −0.21; 95% confidence interval [CI], −0.36 to −0.06) and TM (ΔAR_TM, −0.17; 95% CI, −0.34 to −0.00) coincided with reduced complaints. There was a significant between-group difference in ΔAR_LD (−0.35; 95% CI, −0.60 to 0.10) and ΔAR_TM (−0.36; 95% CI, −0.66 to –0.05). Conclusions: Increased co-contraction of the LD and TM is associated with a favorable course of SAPS. This may be explained by widening of the subacromial space accomplished by adductor co-contraction.