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 elevation, and to determine any association of kinematic and electromyographic (EMG) responses. Twenty-five subjects, 19-37 years old and without any recent history of shoulder or neck symptoms, were evaluated. Three-dimensional scapular coordinate data and surface EMG signals from the trapezius (upper and lower), levator scapulae, and serratus anterior were collected at static positions (0°, 90°, and 140°) of humeral elevation in the scapular plane and head positions (0°, 25° and 50°) of sagittal plane flexion. Scapular upward rotation and tipping were significantly decreased in flexed head positions. The effect of head position on scapular tipping increased as humeral elevation increased. Mean EMG activity of the levator scapula, upper trapezius, and serratus anterior was unchanged across head positions. The lower trapezius demonstrated small but statistically significant increases in mean activity at the 0° and 140° arm positions when the head was flexed to 50°. Significant correlations were found between some EMG and kinematic responses in flexed head positions, however, no consistent patterns were apparent across muscles or positions.