Electromyographic analysis of internal rotational motion of the shoulder in various arm positions.

The electromyographic activity of 9 muscles of the rotator cuff and shoulder girdle was measured in 8 volunteers during active motion and isometric contraction during internal rotation of the shoulder in four positions as follows: neutral rotation with the arm at the side, neutral rotation at 90 degrees abduction in the frontal plane, the zero-position as described by Saha, and the position of the liftoff test from the midlumbar spine as described by Gerber and Krushell. The subscapularis was active at levels of about 44% maximal manual muscle test (MMT) during active internal rotation in the liftoff test position, whereas the activity of the subscapularis decreased remarkably to about 2% to 12% MMT in the other positions. The subscapular is exceptionally important as an internal rotator in the liftoff test position. During resistive motion at the zero-position, the activity level of the subscapularis (81.7%) was higher than that of other muscles. These results indicate that internal rotation at the zero-position may selectively exercise the subscapularis muscle.