Contribution of the supraspinatus to the external rotator lag sign: Kinematic and electromyographic pattern in an in vivo model

Abstract

The external rotation lag sign (ERLS) is a test designed to assess the integrity of the supraspinatus (SSP) and infraspinatus tendons. This study intends to determine the electromyographic pattern of shoulder girdle muscles during a series of ERLS tasks conducted at full adduction and 20 degrees of elevation to figure out the better way to perform the test. The second aim is to assess the final contribution of the SSP to the ERLS by measuring the amount of lag after an SSP block induced by botulinum toxin. Ten subjects with healthy shoulders were examined by a series of five ERLS trials at full adduction and 20 degrees of elevation in the scapular plane. Surface and intramuscular electromyographic activity of the shoulder girdle muscles was recorded and normalized against either the mean activity of all the muscles or the peak activity. The lag was simultaneously measured by an infrared optoelectronic system before and after the selective block of the SSP muscle. The SSP contributed 20% of the electrical activities during the ERLS, which was found to be significantly greater than the contributions of the other shoulder girdle muscles, except for the infraspinatus. The selective block of the SSP caused a lag of 4 degrees in all 10 shoulders at 20 degrees but no increase in lag at 0 degrees of elevation. The ERLS is potentially able to detect an isolated SSP tear if the test is performed correctly (20 degrees of abduction). The deltoid and biceps muscles are almost silent during the test, limiting confounding factors.