The purpose of this study was to clarify the activities of the longissimus, multifidus, and iliocostalis muscles at different angles of shoulder joint flexion. The subjects were 14 healthy males (mean age, 23.7 ± 1.9 years). Electromyograms of the longissimus, multifidus, and iliocostalis muscles were recorded at shoulder flexion angles of 0, 30, 60, 90, 120, and 150 degrees in a seated position. The relative values of the integrated electromyograms (iEMG) of the longissimus muscle on the side of shoulder joint flexion gradually increased up to the flexion angle of 90 degrees and gradually decreased thereafter. The longissimus iEMG value was significantly higher at 90 degrees than at 150 degrees. There were no changes in the relative values of the other muscles. We conclude that the longissimus muscle on the side of shoulder joint flexion contributes to the maintenance of posture.
