Electromyographic activities of the biceps during arm elevation in shoulders with rotator cuff tears

We investigated electromyographic activities of the biceps in 40 shoulders with full-thickness tears of the rotator cuff and 40 asymptomatic shoulders, with a normal rotator cuff on MRI, to determine the role of the biceps in cuff-deficient shoulders. Using surface electrodes, biceps activities were recorded during arm elevation in the scapular plane with and without a 1-kg load. The percentages of integrated electromyograms to the maximum voluntary contraction (%MVC) were obtained at 30 degrees, 60 degrees, 90 degrees, and 120 degrees of elevation. In the normal shoulders, %MVC of the biceps was always less than 10% through the are of elevation both with and without load. Among 40 shoulders with rotator cuff tears, 14 showed increased activities of the biceps more than 10% in %MVC (p < 0.0001), whereas the remaining 26 shoulders had activities similar to the normal shoulders. The biceps activities in these 14 shoulders increased with load application and at higher angles of elevation. The muscle strength tended to be weaker in shoulders with increased biceps activities than in those without. Our findings suggest a potential supplemental function of the biceps in shoulders with rotator cuff tears.