THE EFFECT OF HEAD POSITION ON SCAPULAR ORIENTATION AND MUSCLE-ACTIVITY DURING SHOULDER ELEVATION

The purposes of this study were to compare the effects of flexed head positions on scapular orientation and muscle activity during humeral e levation, and to determine any association of kinematic and electromyo graphic (EMG) responses. Twenty-five subjects, 19-37 years old and wit hout any recent history of shoulder or neck symptoms, were evaluated. Three-dimensional scapular coordinate data and surface EMG signals fro m the trapezius (upper and lower), levator scapulae, and serratus ante rior were collected at static positions (0 degrees, 90 degrees, and 14 0 degrees) of humeral elevation in the scapular plane and head positio ns (0 degrees, 25 degrees, and 50 degrees) of sagittal plane flexion. Scapular upward rotation and tipping were significantly decreased in f lexed head positions. The effect of head position on scapular tipping increased as humeral elevation increased. Mean EMG activity of the lev ator scapula, upper trapezius, and serratus anterior was unchanged acr oss head positions. The lower trapezius demonstrated small but statist ically significant increases in mean activity at the 0 degrees and 140 degrees arm positions when the head was flexed to 50 degrees. Signifi cant correlations were found between some EMG and kinematic responses in flexed head positions, however, no consistent patterns were apparen t across muscles or positions.