Pathologic deltoid activation in rotator cuff tear patients: normalization after cuff repair?

Rotator cuff (RC) tears have a high prevalence, and RC repair surgery is frequently performed. Evaluation of deltoid activation has been reported as an easy to measure proxy for RC functionality. Our goal was to test the success of RC repair in restoring muscle function, by assessing deltoid activation with varying arm abduction moment loading tasks in controls and in RC tear patients before and 1 year after RC repair. Averaged rectified electromyography recordings (rEMG) of the deltoid during 2-s isometric arm abduction tasks were assessed in 22 controls and 33 patients before and after RC repair. Changes in deltoid activation as a response to increased arm abduction moment loading (large vs. small moment), without changing task force magnitude, were expressed as: R = (rEMG(Large) - rEMG(Small))/(rEMG(Large) + rEMG(Small)), where R > 0 indicates an increase in muscle activation with larger moment loading. In controls, a significant increase in deltoid activation was observed with large abduction moment loading: R = 0.11 (95 % CI 0.06-0.16). In patients, R was larger: 0.20 (95 % CI 0.13-0.27) preoperatively and 0.16 (95 % CI 0.09-0.22) postoperatively. Increased compensatory deltoid activation was found in pre-operative RC tear patients. The post-operative decrease in compensatory deltoid activation, although not significant, could indicate (partially) restored RC function in at least some patients.